ICES Journal of Marine Science, 59: 679–710. 2002
doi:10.1006/jmsc.2002.1249, available online at http://www.idealibrary.com on
Vertical density distributions of fish: a balance between
environmental and physiological limitation
Boonchai K. Stensholt, Asgeir Aglen, Sigbjørn Mehl,
and Eivind Stensholt
Stensholt, B. K., Aglen, A., Mehl, S., and Stensholt, E. 2002. Vertical density
distributions of fish: a balance between environmental and physiological limitation. –
ICES Journal of Marine Science, 59: 679–710.
Data (trawl, acoustic, CTD) from scientific surveys along the Norwegian coast, in the
North Sea, in the Barents Sea, west of the British Isles, and in the Irminger Sea are
used. The vertical density distributions of blue whiting, cod, haddock, redfish, saithe,
capelin, and herring are described in relation to environmental conditions and
physiological limitations. The first four surveys mainly cover banks and shelf areas
shallower than 500 m. The last two surveys, aimed at blue whiting and redfish, mainly
cover shelf edge and deep-sea areas with depths from 200 to 1300 m and from 440 to
3000 m. In regard to cod some information from data-storage tags is used.
For physoclists the relative vertical profile of each acoustic sample i.e. acoustic-area
backscattering coefficient (sA), is expressed in terms of the relative pressure reduction
level from seabed up to surface. Thus relative vertical profiles with different bottom
depths are normalized and are made compatible for a discussion in terms of the free
vertical range (FVR). This restriction to rapid vertical movement is evident in the
physoclist species studied. For samples in the shelf area, the profiles show that blue
whiting, haddock, saithe, cod, and redfish are mainly distributed within the bottom
half of the water column. Some fish adapt to pelagic living especially in areas with high
acoustic density and where the bottom is deep. Here a pelagically living fish is defined
as an individual fish having a current free vertical range that does not include the
seabed.
For demersal fish, day and night relative vertical profiles are corrected for unequal
day and night losses in the bottom acoustic dead zone, which is the zone near the
seabed where echoes from fish cannot be discriminated from the sea bottom echo. Day
and night samples are separated by the sun’s passing 5 below the horizon. In most
years evidence of diurnal vertical migration is found for all investigated species. In
many cases of demersal fish there is a higher relative acoustic density (sA-values) in the
mid-range of the bottom half of the water column in the daytime as opposed to the
night-time. At night there is a degree of separation, one group of fish descends to
aggregate near the seabed and another ascends. Inter-annual variations in the diel
movement from different parts of the stock are discussed in relation to the inter-annual
variations in age composition of the stock.
2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.
All rights reserved.
Keywords: area backscattering coefficient (sA), acoustic bottom dead zone loss,
demersal, free vertical range, fish vertical distribution, hydrostatic pressure, pelagic,
physostomes, physoclists, swimbladder.
Received 29 August 2001; accepted 8 April 2002.
Boonchai K. Stensholt, Asgeir Aglen, and Sigbjørn Mehl: Institute of Marine Research,
PO Box 1870, N-5817, Bergen, Norway; tel: 47 55 23 8667; fax: 47 55 23 8687; e-mail:
[email protected]. Eivind Stensholt: The Norwegian School of Economics and Business
Administration, Helleveien 30, N-5045 Bergen, Norway.
Introduction
In marine teleosts with a swimbladder, that feature
occupies 3.1–5.7% of the body volume (Alexander, 1966;
Marshall, 1972). One of its functions is to be a buoyancy
1054–3139/02/080679+32 $35.00/0
organ (Blaxter and Tytler, 1978). However, its physiological limitation in adaptation to pressure changes
imposes restrictions on vertical migration (Harden
Jones, 1949, 1951, 1952; Brawn, 1962; Kutchai and
Steen, 1971; Blaxter and Tytler, 1972, 1978; Ross, 1979;
2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.
680
B. K. Stensholt et al.
Harden Jones and Scholes, 1981, 1985; Arnold and
Greer Walker, 1992). There are metabolic costs of
swimbladder adaptation and of compensatory swimming when the buoyancy is not neutral (Alexander,
1970, 1972; Blaxter and Tytler, 1978).
During pressure reduction fish with open swimbladders (physostomes, e.g. herring and capelin), may
release gas through a pneumatic duct. Gas secretion is
known to be slow and insignificant in physostomes.
Brawn’s experiment indicates that herring take in air at
the surface. The amount of air determines the level of
neutral buoyancy, which may be no more than 50 to
100 m. In experiments physostomes respond to rapid
pressure increases by ascending (Brawn, 1962; Blaxter
and Tytler, 1978; Sundnes and Bratland, 1972).
Fish with closed swimbladders (physoclists, e.g. blue
whiting, cod, haddock, redfish and saithe) produce gas
by secretion in response to under-buoyancy and remove
gas by resorption in response to over-buoyancy but
these processes take time (Blaxter and Tytler, 1978;
Harden Jones and Scholes, 1985). At any point in time
the gas content determines a depth level of neutral
buoyancy. For rapid vertical movement around that
depth is a zone of free vertical range (FVR). Within this
range the fish can swim freely and compensate for the
deviation from neutral buoyancy by means of swimming
movements (Harden Jones, 1949, 1951, 1952; Harden
Jones and Scholes, 1981, 1985; Tytler and Blaxter, 1973,
1977; Blaxter and Tytler, 1972, 1978; Arnold and Greer
Walker, 1992). By ascending rapidly too far above the
neutral buoyancy plane, a physoclist gets too overbuoyant, risking an uncontrollable rise to the surface or
a swimbladder rupture (Harden Jones, 1951, 1952;
Tytler and Blaxter, 1973; Harden Jones and Scholes,
1985), while physostomes get rid of surplus gas
through their duct (Brawn, 1962). On the other hand,
during deep dives physoclists have an advantage over
physostomes because their gas secretion reduces underbuoyancy. Thus physoclists and physostomes are
naturally disposed towards different patterns of vertical
distribution and vertical migration.
Given the current gas content of the swimbladder, the
current free vertical range (FVR) allows rapid vertical
movement between two planes, at M and m metres, the
limiting depth levels for under- and over-buoyancy. As
one atmospheric pressure is about the pressure of a 10 m
water column, the pressure ratio (m+10)/(M+10) can be
considered a ‘‘theoretical’’ physiological limit to pressure
changes within a free vertical range. Experiments
(Harden Jones and Scholes, 1985) have determined that
the ratio is ca. 50% in the case of cod. Blaxter and Tytler
(1972) concluded that the sensitivity to pressure decrease
was about the same for cod and saithe, and that haddock
was slightly more sensitive. We expect that saithe,
haddock, redfish and blue whiting have a free vertical
range which is not very different from that of cod.
Several studies conclude that a physoclist, which
undertakes extensive diel vertical migrations, generally
maintains neutral buoyancy only near the top of its daily
vertical range and otherwise is negatively buoyant.
Descending some distance below the current free vertical
range has no serious consequences however. It may
adapt at the bottom by gas secretion and, perhaps with
an extra effort, ascend into its FVR (Alexander, 1966;
Harden Jones and Scholes, 1981, 1985; Arnold and
Greer Walker, 1992; Rose and Porter, 1996; Righton
et al., 2001).
Because of gas loss by passive diffusion due to
permeability of the swimbladder wall, there may be a
maximal depth where neutral buoyancy maintenance is
possible (Kutchai and Steen, 1971; Ross, 1976, 1979;
Blaxter and Tytler, 1978).
Temperature and salinity impose environmental
barriers to fish distribution. In cod a physiological
adaptation process starts when it enters waters colder
than 2C that enables it to live in colder temperatures. It
also tolerates cold shocks from 4C to 0C in normal
seawater. High temperatures (above 10C) with low
salinity may be lethal for large cod (Woodhead and
Woodhead 1959, 1965; Harden Jones and Scholes,
1974). Björnsson et al. (2001) found that large Icelandic
cod (d2 kg) cannot tolerate prolonged exposure to 16C
with salinity 32‰. The optimal temperature for cod’s
growth found in laboratory experiments is higher than
its natural ambient temperature (Björnsson et al., 2001).
In the natural environment preferred temperature is just
one of many concerns, like availability of food and
avoidance of predation (Kristiansen et al., 2001).
Time series of depth and temperature from data
storage tags (DST) attached to adult cod show interplay
between the individual cod’s migration behaviour and
season, depth, temperature, tidal current and its physiological limitation (Stensholt, 2001; Righton et al. 2001).
Combined information from different sources helps the
interpretation of conventional fisheries survey data
(Righton et al., 2001).
In the present study information from acoustic
records, i.e. the area backscattering coefficient (sA) as
defined in MacLennan et al. (2001), trawl sampling, and
CTD, are combined to investigate the distribution of
cod, haddock, saithe, redfish, blue whiting, herring, and
capelin in relation to environmental and physiological
factors.
Besides the swimbladder physiology described above,
daylight, food availability, predator avoidance, tidal
currents, temperature, salinity, depth, and vessel noise
are among the factors that influence vertical migration
(Neilson and Perry, 1990; Ona and Godø, 1990; Godø
et al., 1999; Aglen, 1994). The observed large-scale
vertical distribution is expressed in physiological terms,
thus all samples with various depths are normalized,
and on that basis the influence of some environmental
Vertical density distributions of fish
factors is discussed. The structure of the spatial density
distribution of fish in relation to environment and
physiological limitation can be established. This can be
used in the estimation of total stock abundance.
Vertical migration behaviour affects the accuracy of
acoustic stock estimates of demersal physoclists, mainly
through the loss of acoustic fish information in the
bottom dead zone, i.e. fish very close to the bottom
cannot be distinguished from the bottom itself
(Aglen, 1994; Ona and Mitson, 1996; Aglen et al., 1999).
Moreover, the extent of losses due to negative buoyancy
near the bottom, body orientation, and avoidance
reactions to vessel passage among cod, haddock and
saithe is still unclear (Ona and Godø, 1990; Aglen, 1994;
Godø et al., 1999; Totland, pers. comm.). The accuracy
can be improved when the amount that is lost varies
according to predictable environmental conditions, e.g.
daylight and tidal current. How should one interpret
vertical profiles, which are based on the observed
acoustic abundance of the variable proportion of fish
above the dead zone? The difference between the losses
in acoustic density for day and night is estimated and the
vertical profiles are adjusted accordingly, under the
assumption that the dead zone is the main reason
for day–night differences in acoustic observations for
demersal fish during winter in the Barents Sea (Aglen
et al., 1999). In the case of cod the results are discussed
in the light of information from data-storage tags, which
provides details of the daily vertical ranges and the time
spent at each depth level.
Materials and methods
The Institute of Marine Research, Bergen, performs
several annual acoustic surveys for fish stock assessment.
In this study observations of acoustic backscattering
(sA-values) from several acoustic surveys aimed at
demersal fish and blue whiting have been used. Data
from the demersal fish surveys contain acoustic values
that are allocated to the demersal species cod, haddock,
saithe and redfish, and, in addition, values allocated to
the pelagic species blue whiting, capelin, and herring. In
the demersal fish surveys the latter are non-target species
and not completely covered.
The data considered in this paper are from annual
surveys in the main. Firstly those for saithe, haddock,
blue whiting, and herring from the saithe surveys along
the Norwegian coast from 62N to 72N during
October–November, 1992–2000 (Nedreaas, 1995;
Korsbrekke and Mehl, 2000) and from demersal fish
surveys in the North Sea from 55N to 62N during
October–November, 1992–1998 (Anon., 1992–1996) and
during August–September in 1999–2000 (Anon., 1999)
(Figure 1). Secondly those for cod, haddock, redfish,
capelin, and herring from demersal fish surveys in the
681
Barents Sea, one during February–March 1996–2002
(Mehl and Nakken, 1996; Mehl, 1997, 1998, 1999;
Aglen, et al., 2001; Aglen, 2001b, 2002) (Figure 2) and
one during July–August 1995–2000 (Aglen and Nakken,
1996; Aglen, 1999, 2001a; Michalsen et al., 2000)
(Figure 3). Thirdly from surveys of the spawning stock
of blue whiting found west of the British Isles during
March–April 1995–1996 (Monstad et al., 1995, 1996)
and finally a survey of oceanic redfish in the Irminger
Sea, southeast of Greenland, during June–July 2001
(Dalen and Nedreaas, 2001).
The saithe survey is a pure acoustic survey, which
means that the decision regarding when and where to
make trawl hauls (both demersal and pelagic) is largely
based on the acoustic observations. This survey covers
areas with highly variable topography, and the survey
design therefore varies between regions (Figure 1). The
other demersal fish surveys are combined bottom-trawl
and acoustic surveys, where there is a fixed grid of
predetermined bottom-trawl positions used both for a
swept-area estimate and for the interpretation of the
acoustic data. Some additional hauls (both demersal and
pelagic) are made for identifying acoustic recordings and
these are only used for the acoustic estimate. These
surveys mainly follow parallel transects with regular
spacing. In all the surveys CTD measurements (vertical
profiles of salinity and temperature) are made at nearly
all trawl positions. The exceptions to this are the Barents
Sea summer surveys in 1996 and 1997, and the Barents
Sea winter survey in 2000, where parts of the area were
covered by hired fishing vessels not equipped for CTD
measurements. Typical survey-line transects and trawl
stations for each of the demersal fish surveys are shown
in bathymetric maps in Figures 1–3. They cover mainly
the banks and shelf areas of depth <600 m. The blue
whiting survey is a combined pelagic trawl, bottom
trawl, and acoustic survey (Monstad et al., 1995, 1996),
which cover mainly the banks and shelf edge of depth
from 200 to 1400 m. The oceanic redfish (Sebastes
mentella) survey is a pelagic trawl and acoustic survey of
depths from 500 to 3000 m. The sA-values are split into
the redfish within and above the deep scattering layer
and, at the same time, above 500 m, and the rest of the
redfish because redfish biologists suspect that there are
two separate stocks. The sA-values within and below the
deep scattering layer are underestimated by 34% and
55% respectively according to data from a deep towed
vehicle (Magnússon et al., 1996; Dalen and Nedreaas,
2001).
In order to investigate the relationship between
fish and environmental variables satisfactorily the
respective data must be located at the same locations
over the entire studied area. The values at un-sampled
locations of temperature, salinity, sea bottom depth,
acoustic sA-values, and density of the studied species,
are estimated on the basis of observed values from
682
B. K. Stensholt et al.
69
71
67
5
65
10
63
Lo
15
61
ng
itu
de
E
20
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25
0m
100
30
way
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Norwegian Trench
20
0m
1996
Figure 1. 1996 acoustic surveys in the North Sea south of 62N, and along the Norwegian coast north of 62N. Trawl stations are
marked with . Depth isolines are in metres.
neighbouring locations by application of geostatistical
methods (Cressie, 1991; Stensholt and Sunnanå, 1996).
They are available in the software ISATIS (ISATIS,
1997), e.g. kriging with a moving window.
Acoustic data and vertical density profile
Area backscattering (sA-values) were sampled with echointegrators (Knudsen, 1990; Foote et al., 1991). During
the surveys the acoustic data were scrutinized and the
sA-values were allocated to a number of categories
(species or groups of species). For each survey series
there is a list of standard categories for the target
species and then several non-target species are defined
as separate categories usually. Cod, haddock, redfish,
herring, and capelin are defined as separate categories in
the Barents Sea surveys, while values representing saithe
are merged with several non-target species in the
category ‘‘other demersal’’. Saithe, haddock and blue
whiting are defined as separate categories in the coastal
saithe survey, the North Sea survey, and the blue
whiting survey.
During daily scrutinizing onboard the survey vessel,
the post-processing system (BEI, Knudsen, 1990)
presents the echogram data (from surface to bottom) in
blocks of 5 nautical miles (nmi) horizontal. Within such
a block the user can define a number of sub-blocks
(regular or irregular depth intervals or rectangular
boxes). Within each sub-block the operator partitions
the sA-values (averaged over 5 nmi) into the separate
categories by using the trawl catch species composition
of neighbouring stations and subjective judgment based
on experience. Thus for each acoustic sample within the
same sub-block all species have the same vertical sA
density profile (in relative terms). When the scrutinized
data are stored in the database single values are
averaged within blocks of 1 nmi (0.1 nmi for blue
whiting surveys) horizontal resolution and 10 m (25 m
for oceanic redfish) vertical resolution, relating to the sea
surface. In addition, the values from bottom up to 10 m
above bottom are stored with 1 m vertical resolution.
These profiles are used for investigating the vertical
distributions of fish in relation to environmental factors
and physiological limitations.
Cumulative relative frequency
To investigate the vertical distribution of a physostome
species or physoclists with pelagic distribution we
selected the acoustic samples with reasonably large total
Vertical density distributions of fish
683
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Summer 1996
Figure 2. 1996 winter survey in the Barents Sea. Trawl stations are marked with . Depth isolines are in metres.
aggregated sA-values over the water column, as in
Stensholt and Nakken (2001). To normalize the large
spatial variation in absolute values, for each sample s the
relative cumulative acoustic abundance C(s,v) of fish is
calculated in 10 m depth intervals from the surface down
to 10 · v metres, v=1, 2, . . ., D, with bottom depth in the
interval (10 · (D-1), 10 · D]. For each depth step (v=1, 2,
. . .) the calculations of C(s,v) need samples with bottom
depth at least 10 v metres, and thus the number of
samples is reduced with increasing v.
To correct for unequal bottom depth, the water
column is partitioned according to fractions of the
bottom depth, and the relative sA-values are accumulated from the surface down to the chosen fractions of
the bottom depth.
For each level of depth (Figure 16f) or relative depth
(Figure 17) the distribution over all samples of the
relative cumulative values are presented as box-plots.
The boxes indicate the inter-quartile ranges and a line
inside each box marks the median.
Relative pressure reduction
For physoclistous fish the extent of rapid vertical movement is restricted because of their closed swimbladder.
At any depth level, the change of swimbladder volume
and change of buoyancy status depend on the relative
change of pressure. For cod adapted to neutral buoyancy at depth d, the free vertical range (FVR) was
determined experimentally as the range between 25%
pressure reduction (at depth m) and 50% pressure
increase (at depth M) when compared to the pressure at
depth d (Harden Jones and Scholes, 1985). Thus the
extent of FVR increases with depth. With depths in
metres, the pressure ratios are
(m+10) · (d+10) 1 =0.75, (M+10) (d+10) 1 =1.5, and
(M+10) · (m+10) 1 =2
Thus m+10=0.5 · (M+10), i.e. the pressure is reduced
by 50% for fish ascending from M to m. A full free
vertical range for cod with the deeper limit at M (>10)
metres stretches up the water column to m=M/2–5 m. In
particular, when M is the bottom depth, the ‘‘bottom
restricted free vertical range’’ (FVRbot) stretches from
the seabed up to 5 m above the mid-water level. For
other physoclistous species the free vertical range may
have a different size, i.e. from m to M where
M+10=K · (m+10), and where K may be different from
2, but for the species considered here the results below
indicate that it is not much different.
The free vertical range of physoclistous fish adapted
to pelagic living, e.g. schooling fish such as blue whiting
in the deep sea, may be calculated using m equal to the
684
B. K. Stensholt et al.
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Summer 1996
Figure 3. 1996 summer survey in the Barents Sea. Trawl stations are marked with . Depth isolines are in metres. N shows the
release site of tagged cod.
minimum depth of the distribution layer or M equal to
the maximum depth of the distribution layer.
The acoustic density coefficient (sA-value) depends
on the fish buoyancy status. Moreover the acoustic
sampling unit represents a rather short time interval,
typically about 6 min per nautical mile along the
transect line. Therefore, a description of the vertical
distribution of acoustic density should be related to the
physiological limitation permitting rapid pressure
change and, consequently, rapid vertical migration for
individual physoclistous fish.
In the case of demersal physoclists, it is most useful to
investigate the relative vertical profiles of sA-values in
terms of relative pressure reduction with reference to
bottom pressure. Here the bottom-restricted, free vertical range (FVRbot) is defined as the FVR with its deepest
end (M) at the seabed, and its extension depends on the
fish physiology. It is used as a yardstick for measuring
the height above the seabed for investigation of the
acoustic vertical profiles. Since m+10=(M+10)K 1, a
fish that moves up one FVR experiences a relative
pressure reduction of 1K 1 (K=2 for cod).
The height above seabed is partitioned into steps
of the relative pressure reduction level (RPRL) with
reference to the bottom pressure. The samples with
various depth ranges due to unequal bottom depths are
transformed to have ranges from RPRL=0, at the
seabed with depth M, to M · (M+10) 1, at the sea
surface. In this study the relative pressure reduction at
the sea surface can be considered to be approximately 1
in all cases. Consequently, all samples with various
depth ranges have the bottom restricted FVR stretching
from RPRL=0 to RPRL=1K 1 (RPRL=0.5 in the
case of cod). This normalization allows the comparison
of relative profiles from locations with different bottom
depths. Then the factors affecting the vertical distribution can be investigated. The graphs show the extent of
the distribution range and the vertical distribution profile within each distribution range in terms of an FVR
and in relation to the bottom.
In the case of a pelagic distribution layer of a
physoclist species over great bottom depth, the reference
is to the deepest boundary of the distribution range
instead of bottom depth. The graphs show the extent of
the distribution range and the vertical distribution profile within each distribution range in terms of an FVR,
which is independent of the depth level.
Relative profiles
To normalize for the large variation in acoustic
abundance, the profile of each sample is converted into a
Vertical density distributions of fish
685
100
y75
80
y50
Box marks interquartile range,
25–75, of the distribution.
Line inside the box marks median
60
y25
40
20
cod
0
0.0
Seabed
x
0.2
0.4
0.6
0.8
1.0
RPRL
Figure 4. The horizontal axis shows the water column partitioned from seabed to surface according to the relative pressure
reduction level (RPRL) with reference to the pressure at the seabed. For each acoustic sample consisting of area backscattering
coefficients (sA-values), the relative acoustic density is accumulated in a stepwise manner from the seabed up to each RPRL. At
RPRL=x, the distribution over the entire sample set of the relative cumulative values up to RPRL=x is presented as a thin box,
showing the inter-quartile range and median. The relative cumulative distribution up to RPRL=x has median=y50, lower
quartile=y25, and upper quartile=y75, as shown on the vertical axis. Reference line indicates RPRL=0.5.
relative profile with the observed total sA-value in the
water column as 100%. Accumulating the relative
sA-value stepwise from the bottom up along the RPRL
axis, it grows from 0 to 100% as RPRL increases from 0
to 1.
In most of the cases studied here the fish are demersal,
and the increments of the relative cumulative vertical
profiles start from the seabed. The range of RPRL,
where the relative cumulative value grows from 0% until
it reaches 100% for most of the samples, may indicate
the presence of a physiological restriction in rapid
vertical movement due to the relative change of pressure
(Figure 4). This range of physiological restriction may
correspond to the free vertical range for the species, if
no other environmental factors influence the vertical
distribution.
In the case of pelagic distribution layers, the fish
vertical distribution is off the bottom entirely, as for
redfish and blue whiting shoals in the deep sea. Thus the
relative cumulative vertical profiles should be started
from the deepest end of each sample distribution range
(Figures 9e,f and 16a–e). When the profiles are described
in terms of the relative pressure reduction with respect to
the pressure at seabed the vertical extent of the school
stretches from RPRL=XM upward to RPRL=Xm, as
in the samples from 1995, 1996, 1998 of Figure 15f. In
order to compare the difference Xm XM to the FVR in
terms of relative pressure reduction, it must be converted
and expressed in terms of relative pressure reduction
from the deeper to the upper end of the distribution
range as a ratio
(Xm XM) · (1XM) 1 =
[(1XM)(1Xm)] · (1XM) 1 =
1[(1Xm) · (1XM) 1].
The last form corresponds to and may be compared to
1K 1 above.
Adaptation to pelagic life
For a physoclist, a rapid ascent too far above the upper
limit of its free vertical range is much more hazardous
686
B. K. Stensholt et al.
than a rapid descent below the lower limit (Tytler and
Blaxter, 1973). The latter results in temporary, excessive
under-buoyancy (Harden Jones, 1951, 1952; Harden
Jones and Scholes, 1981, 1985). When the observed
relative cumulative profile reaches 100% beyond one
bottom restricted FVR unit, there are samples with fish
detected more than one such unit above the sea bottom.
Based on the assumptions that the FVR restriction to
rapid vertical movement is effective, and that such a fish
has not ascended above its current FVR, we conclude
that the fish have adapted to pelagic living in the sense
that their current free vertical range does not include the
seabed.
Fish that are detected within the bottom-restricted
FVR may well have an individual, current FVR that
does not include the seabed. This means that they just
happen to be in the part of their current FVR that
partially overlaps with the bottom-restricted FVR, or
that they have accepted excessive negative buoyancy. It
is impossible to recognize from the acoustic signals if
there are fish adapted to pelagic living among those
detected within the bottom-restricted FVR. Therefore it
is necessary to use the very strict criterion that some fish
must have been detected above one bottom-restricted
FVR before concluding that part of the stock has
adapted to pelagic living.
If the interval [m, M] is the bottom-restricted FVR,
and there are fish with individual current free vertical
range [c, C], where c<m<C<M, then some of them may
happen to be detected in the depth interval [m, C], and
others, with excessive negative buoyancy, in [C, M].
Only fish that are detected in [c, m] satisfy the criterion.
If a fish detected in [m, M] actually has adapted to
pelagic living, this cannot be verified from the observed
samples.
Distribution of relative cumulative sA-values
In Figure 4 the water column is partitioned into RPRL
steps of 0.05 from ‘‘bottom’’ to surface, shown here
along the horizontal axis. The word ‘‘bottom’’ here
means the seabed in case of demersal distribution and
the deepest end of each sample distribution range for a
clear pelagic distribution. At each discrete RPRL, the
box-plot shows the distribution of the observed relative
cumulative sA-values. In each acoustic sample the relative sA-value is accumulated from bottom up to
RPRL=x. The distribution of these relative cumulative
values up to each discrete RPRL over the entire sample
set is presented as a box showing the inter-quartile range
with a line inside each box marking the median. The
accumulation levels, in percent, appear on the vertical
axis. The distribution of cumulative relative sA-values
up to RPRL=x has median=y50, lower quartile=y25,
and upper quartile=y75. For example, the lower
quartile=y25 means that 25% of the samples have the
values below, and 75% of the samples have the values
above y25.
The line joining the medians for all RPRL is used to
represent the relative cumulative acoustic density from
bottom up to each RPRL. The inter-quartile range
represents the variation of the distribution. If an RPRL
has large inter-quartile range, there is large variability
among the samples of the relative cumulative acoustic
density up to that level.
The FVR unit for each species is then estimated from
the median, i.e. an RPRL range where the median starts
to grow from 0% until it approaches 100%, and with the
quartiles indicating the uncertainty.
As explained above, a sample with fish above the
bottom-restricted FVR indicates that some of the fish
have adjusted to pelagic living. When the lower quartile
is clearly below 100% at the RPRL corresponding to the
upper limit of the bottom-restricted FVR (RPRL=0.5 in
the case of cod), at least 25% of the samples record fish
adapted to pelagic living. Of course this interpretation of
the figures builds on the assumption that the defined free
vertical range is an effective restriction for rapid vertical
movement.
The day-night time is calculated according to the sun’s
angle with the horizon, for winter surveys daytime starts
and ends when the sun is 5 below horizon. Each
acoustic sample is accordingly classified as a day- or
night-sample, and the vertical profiles may be presented
for day and night separately and compared.
The acoustic bottom dead zone and the day/night
cumulative relative frequencies
The observed total acoustic sA-value in the water
column may differ significantly between day and night.
Such differences could be caused by diurnal changes due
to fish behaviour and buoyancy status, combined with
technical limitations of the hydro-acoustic method to
detect fish (Harden Jones and Scholes, 1981, 1985;
Aglen, 1994). Thus the basis for calculating relative
frequencies changes correspondingly. Based on studies
of demersal species during winter in the Barents Sea
(Aglen et al., 1999), it is reasonable to assume that the
main reason for the difference in day and night loss
is migration of fish in and out of the near bottom
acoustic dead zone. An adjustment for the difference in
the day and night loss is based on this assumption and
that the total amount of fish is equal during day and
night.
Only when the daytime and night-time vertical profiles
are based on the same proportion of the fish present in
the water column, will the comparison of day/night
profiles give evidence of fish vertical movements. Thus
the observed relative frequencies have been adjusted
using the relative change in observed total acoustic
sA-value between day and night.
Vertical density distributions of fish
Starting with one acoustic sample, let A(h) be the
observed cumulative acoustic density from seabed up to
RPRL=h. With bottom depth b metre,
h=1[(d+10) · (b+10) 1]=(bd) · (b+10) 1
at depth d metres. At the surface, d=0, so
h=b · (b+10) 1, and N=A[b · (b+10) 1] is the
observed total acoustic sA-value.
The observed relative cumulative acoustic density is
F(h)=A(h) · N 1. Let X be the loss due to the acoustic
dead-zone. If the equipment had been able to detect fish
at the bottom, A(h) would have been replaced by
X+A(h), the total acoustic sA-value would have been
X+N, and the relative cumulative density at level h
would have been G(h)=[X+A(h)] · (X+N) 1. The relation between the observed F and the ‘‘correct’’ G is
therefore expressed by
G(h)=X · (X+N) 1 +[N · (X+N) 1] · F(h)
=X · (X+N) 1 +{1[X · (X+N) 1]} · F(h)
(1)
The intercept G(0)=X · (X+N) 1 is the relative loss
in the dead-zone, and N · (X+N) 1 is the reduction
factor for the slope. Increased loss X means higher
intercept and lower slope for G. In particular,
F(0)=0%, G(0)=X · (X+N) 1,
F(b · (b+10) 1)=G(b · (b+10) 1)=100%,
G(h)=F(h) · N · (X+N) 1,
1G(h)=[1F(h)] · N · (X+N) 1.
For each sample, s, one could draw a similar curve.
The observed total acoustic sA-value is N(s), and the
unknown relative loss is X(s) · [X(s)+N(s)] 1, which
could have been used to correct the curve. All sample
curves would have been shifted upward according to
Equation (1). This method can be applied to correct for
the unequal losses at the bottom due to different
reasons, e.g. day-night, or tidal current, or before and
after the passage of a vessel.
We use the median curve as a ‘‘virtual sample’’ to
represent the data. Let virtual dead-zone losses be Xd
and Xn, and observed total acoustic sA-value be Nd and
Nn for day and night, respectively. The total amount of
fish at any location does not change between day and
night, hence Xd +Nd =Xn +Nn. For Nd and Nn respectively, we may use the median of the observed day and
night total acoustic sA-value. For the purpose of day and
night comparison, we only need the difference
Xn Xd =Nd Nn.
Typically the daytime and night-time median curves
are concave, reach 100%, cross each other at RPRL=hc
(Figure 5), and in an interval around hc the daytime
curve is steeper than the night-time curve. There is then
a higher relative density of fish at day than at night in
this interval.
687
Before correction, the day and night cumulative frequencies are based on different total observed acoustic
densities. In order to draw conclusions about vertical
migration, one must consider the correction for difference in dead-zone loss, so that both curves are based on
the same total acoustic sA-value. Then one median curve
is shifted accordingly.
If Xn Xd =0, no correction is needed, and hc remains
the same.
If Xn Xd =Nd Nn >0, the correction shifts the
night-time
curve
upwards
to
intercept
at
(Nd Nn) · (Nd) 1. This intercept indicates the daynight difference in the relative acoustic density that is
lost due to fish in the dead-zone.
With X=Nd Nn and N=Nn, Equation (1) can be
rewritten in terms of the observed values as
G(h)=(Nd Nn) · (Nd) 1 +
{1[(Nd Nn) · (Nd) 1]} · F(h)
=F(h)+[(Nd Nn) · (Nd) 1] · [1F(h)].
Now, in the typical case, daytime accumulation
reaches 100% at a level RPRL=h* where the night value
is <100%. In other words, the median curves of day/
night relative cumulative acoustic sA-values indicate that
RPRL=h* is the upper limit of the daytime vertical
distribution, but that among night samples a significant
proportion of fish were still found above RPRL=h*. It
is then geometrically clear that the corrected night curve
still crosses the day curve, say at RPRL=hk, where
hc <hk <h* (Figure 5). Hence the median curves show
that in fact there must be net evening migrations both
upwards and downwards away from the level
RPRL=hk.
If Xn Xd =Nd Nn <0, there must be a similar
upward shift of the daytime curve (Figure 7). For small
shifts the intersections at 0 and hc are replaced by two
intersections between 0 and hc, and for large shifts there
is no intersection.
The normalizations for unequal bottom depths and
unequal acoustic densities together with corrections of
unequal bottom dead zone loss enable us to compare
vertical profiles and discuss vertical migration from the
overall line transect survey data. In particular the influence of some environmental factors on the vertical
migration can be discussed, e.g. daylight, bottom depth,
current and fish density.
Results
Vertical distribution of fish over different seasons
and geographical areas
The distribution of relative cumulative acoustic density
(sA-values) is calculated for all years combined as well as
annually for each physoclist and physostome of each
688
B. K. Stensholt et al.
h*
hk
100
80
night
60
40
20
day
haddock
50–600 m
0
0.0
hc
0.2
0.4
0.6
0.8
1.0
Figure 5. The median curves of day/night relative cumulative acoustic profiles indicate RPRL=h* as the upper limit of the daytime
vertical distribution, but among night time samples a significant proportion of fish are still found above RPRL=h*. The adjusted
night curve (broken line) crosses the day curve at RPRL=hk where hc <hk <h*. The higher slope of the daytime curve around
RPRL=hk shows higher density at daytime than night-time. Hence the median curves show that there must be net evening
migration both upwards and downwards away from the level RPRL=hk.
survey. The results are shown in Figures 6–17. For all
physoclists the relative cumulative vertical profiles are
started from the seabed, except for redfish when it is
pelagic (Figure 9e,f) and for blue whiting in the deep sea
(Figure 16a–e). In these cases the profiles are started
from the deepest boundary of each sample distribution
range. The figures are presented for each survey and
species, and within each species the acoustic samples are
grouped according to day-night or to density level and
bottom depth range, as indicated at the bottom right
corner of the figures. The species are cod and haddock
from winter surveys in the Barents Sea (Figures 6–8);
redfish from winter surveys in the Barents Sea and
Irminger Sea (Figure 9); cod, haddock, and redfish from
summer surveys in the Barents Sea (Figures 10–12);
saithe, haddock, and blue whiting from surveys along
the Norwegian coast (Figures 13, 14); saithe and blue
whiting from surveys in the North Sea (Figure 15); blue
whiting from surveys west of the British Isles (Figure 16)
and capelin and herring from winter surveys in the
Barents Sea (Figure 17).
The results for demersal fish surveys reflect the vertical
distribution of fish found in the shelf sea (Figures 1–3).
There are data sets where a few samples have very high
density and account for a high proportion of the
sampled fish. They have special profiles and this is
not reflected in the median of the entire collection
of samples. Therefore, such samples are investigated
separately.
The annual median curves of the relative cumulative
sA-values show the annual variation in vertical distribution (e.g. Figure 6b). The reference line at RPRL=0.5
indicates one bottom restricted FVR for cod. For other
species, the RPRL where the median gets close to 100%
and the inter-quartile range approaches 0 may indicate
the extent of their free vertical ranges. They seem to be
of roughly the same size as for cod (Figures 6a,
8a–13a,c,e, 9f, 15a,c, 16d,e,f). Below the reference line
Vertical density distributions of fish
100
100
(a)
80
80
60
60
40
40
20
20
689
(b)
sA>10
cod
cod
0
0
0.0
100
0.2
0.4
0.6
0.8
1.0
0.0
100
(c)
80
0.2
0.4
0.6
0.8
1.0
(d)
day
80
60
60
night
40
40
20
20
night
sA>1
cod
cod
50–600 m
0
0
0.0
100
0.2
0.4
0.6
0.8
1.0
0.0
100
(e)
80
80
60
60
40
40
sA>1
20
0.2
0.4
0.6
0.8
(f)
day
night
sA>10
20
cod
cod
50–200 m
0
0.0
0.2
0.4
0.6
0.8
1.0
1.0
200–600 m
0
0.0
0.2
0.4
0.6
0.8
1.0
Figure 6. Distribution of cod relative cumulative vertical profiles from the seabed to the surface in terms of RPRL, from winter
surveys in the Barents Sea, 1996–2001. Median and inter-quartile range (represented by thin boxes, left box for day, right box for
night) of the distribution, (a) all data; (b) annual median curves; (c) day- and night samples; (d) day–night median curves with
upward adjusted night median curve (broken line); (e) samples with bottom depth <200 m; (f) samples with bottom depth
200–600 m, more cod adapt to pelagic living.
690
B. K. Stensholt et al.
100
80
night
60
40
day
cod 1996
50–600 m
20
0
0.0
0.2
0.4
0.6
0.8
1.0
Figure 7. Day and night median curves of the distributions of
cod vertical profiles of relative cumulative acoustic sA-values in
relation to relative pressure reduction level, RPRL, with
upward adjusted day median curve (broken line). Samples are
from the winter survey in the Barents Sea, 1996. Reference line
indicates RPRL=0.5.
and close to the bottom the inter-quartile range is
relatively large, indicating a large variation of the relative cumulative values among different acoustic samples.
This means that within one FVR the fish are free to
distribute along the water column in a way suited to
local conditions.
Pelagic living for some fish in the samples is indicated
when the relative cumulative sA-values show there are
fish higher than RPRL=0.5, i.e. the lower quartile
reaches 100% beyond the reference line (Figures 6c,f, 8c,
9c, 10e, 11e, 13c, 15c,e). In such cases more than 25% of
the samples have some fish adapted to pelagic living.
This is observed especially in samples from areas of high
acoustic abundance and where the seabed is deeper than
200 m (Figures 6f, 8c, 9c, 10e, 11e, 14b,d). The most
notable species may be oceanic redfish and blue whiting
(Figures 15e,f, 16e,f), which are pelagic at great depth.
For surveys in the Barents Sea, the distribution at
each RPRL has larger inter-quartile range in summer
than winter (Figures 6a, 8a–12a). The distribution in
summer 1999 shows more pelagic living for cod and
haddock than in most years. The depth distribution
100
100
(a)
(b)
80
80
60
60
40
40
20
20
haddock
haddock
0
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
100
100
(c)
80
80
60
60
40
40
0.2
0.4
0.6
day
1.0
(d)
night
20
20
0.8
haddock
50–600 m
haddock
0
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Figure 8. Distribution of haddock relative cumulative vertical profiles from the seabed to the surface in terms of RPRL, from
winter surveys in the Barents Sea, 1996–2001. Median and inter-quartile range (represented by thin boxes, left box for day, right
box for night) of the distribution (a) all data; (b) annual median curves; (c) day- and night samples; (d) day–night median curves
with upward adjusted night median curve (broken line).
Vertical density distributions of fish
100
691
100
(a)
(b)
80
80
60
60
40
40
20
20
sA>1
redfish
0
0.0
0.2
0.4
0.6
redfish
0.8
100
1.0
(c)
0
0.0
0.2
0.4
0.6
0.8
1.0
100
(d)
day
80
80
60
60
night
40
40
20
20
sA>1
redfish
0
0.0
0.2
0.4
0.6
redfish
50–1000 m
0.8
1.0
0
0.0
0.2
0.4
0.6
1.0
100
100
(e)
80
60
60
40
40
sA>1
20
(f)
day
80
night
20
sA>1
oceanic redfish
oceanic redfish
450–3000 m
450–3000 m
0
0.0
0.8
0.2
0.4
0.6
0.8
1.0
0
0.0
0.2
0.4
0.6
0.8
1.0
Figure 9. Distribution of redfish relative cumulative vertical profiles from the seabed to the surface in terms of RPRL, from winter
surveys in the Barents Sea, 1996–2001. Median and inter-quartile range (represented by thin boxes, left box for day, right box for
night) of the distribution (a) all data; (b) annual median curves; (c) day- and night samples; (d) day–night median curves with
upward adjusted night median curve (broken line); relative cumulative profiles from the deepest of the distribution layer to surface
of samples from Irminger Sea, 2001 (e) entire redfish distribution range; (f) only above and within deep scattering layer.
692
B. K. Stensholt et al.
100
100
(a)
(b)
80
80
60
60
40
40
20
20
sA>1
cod
cod
0
0.0
0.2
0.4
0.6
0.8
1.0
100
0
0.0
0.2
0.4
0.6
0.8
100
(c)
(d)
80
80
60
60
40
40
20
20
cod
sA>1
cod
50–200 m
0
0.0
0.2
0.4
0.6
50–200 m
0.8
1.0
100
0
0.0
0.2
0.4
0.6
0.8
1.0
100
(e)
(f)
80
80
60
60
40
40
20
20
cod
sA>10
cod
200–600 m
200–600 m
0
0.0
1.0
0.2
0.4
0.6
0.8
1.0
0
0.0
0.2
0.4
0.6
0.8
1.0
Figure 10. Distribution of cod relative cumulative vertical profiles from the seabed to the surface in terms of RPRL, from summer
surveys in the Barents Sea, 1995–2000. Median and inter-quartile range (represented by thin boxes) of the distribution (a) all data;
(b) annual median curves; (c) and (d) samples with bottom depth<200 m; (e) and (f) samples with bottom depth 200–600 m,
showing more cod adapt to pelagic living.
of individual cod from data storage tags has larger
variation in summer than in winter (Tables 1, 2), in
agreement with the above results.
Proportion of fish in the bottom channel
For the physoclist species, the ratio of the observed
sA-value from the bottom channel (c10m above seabed)
Vertical density distributions of fish
100
693
100
(a)
(b)
80
80
60
60
40
40
sA>1
haddock
20
0
0.0
0.2
0.4
0.6
haddock
20
0.8
1.0
100
0
0.0
0.2
0.4
0.6
0.8
100
(c)
(d)
80
80
60
60
40
40
haddock
20
0
0.0
1.0
0.4
0.6
haddock
20
50–200 m
0.2
sA>1
0.8
1.0
100
0
0.0
100
(e)
80
50–200 m
0.2
0.4
40
40
20
20
1.0
(f)
99
97
60
0.8
2000
80
60
0.6
96
98
sA>10
haddock
haddock
200–600 m
200–600 m
0
0.0
0.2
0.4
0.6
0.8
1.0
0
0.0
0.2
0.4
0.6
0.8
1.0
Figure 11. Distribution of haddock relative cumulative vertical profiles from the seabed to the surface in terms of RPRL, from
summer surveys in the Barents Sea, 1995–2000. Median and inter-quartile range (represented by thin boxes) of the distribution (a)
all data; (b) annual median curves; (c) and (d) samples with bottom depth<200 m; (e) and (f) samples with bottom depth
200–600 m, showing more haddock adapt to pelagic living.
to the observed total sA-value was calculated in each
sample and the distribution of the ratio investigated.
The median of the distribution is shown in Tables 3–4. A
general feature is that in deeper waters fish distributions spread relatively higher up into the water column.
Table 3 shows that cod and haddock have a higher
proportion in the bottom channel during night than
day but this is based on the observed acoustic values
without adjustment for unequal losses during day and
night.
694
B. K. Stensholt et al.
100
100
(a)
(b)
80
80
60
60
40
40
sA>1
20
sA>1
20
redfish
redfish
0
0.0
0.2
0.4
0.6
0.8
1.0
0
0.0
0.2
0.4
0.6
0.8
1.0
100
100
(c)
(d)
80
80
60
60
40
40
20
sA>1
sA>1
redfish
50–200 m
redfish
50–200 m
20
0
0.0
0.2
0.4
0.6
0.8
1.0
100
0
0.0
0.2
0.4
0.6
0.8
1.0
100
(f)
(e)
80
80
60
60
40
40
sA>10
sA>10
redfish
20
redfish
20
200–1000 m
200–1000 m
0
0.0
0.2
0.4
0.6
0.8
1.0
0
0.0
0.2
0.4
0.6
0.8
1.0
Figure 12. Distribution of redfish relative cumulative vertical profiles from the seabed to the surface in terms of RPRL, from
summer surveys in the Barents Sea, 1995–2000. Median and inter-quartile range (represented by thin boxes) of the distribution (a)
all data; (b) annual median curves; (c) and (d) samples with bottom depth <200 m; (e) and (f) samples with bottom depth
200–1000 m.
Table 2, based on depth records from data storage
tags, shows that individual cod spends more time in
the lower part of the daily depth range (within 10 m
above the daily maximum depth) during December–
January than during other periods. When cod is in the
‘‘lower part’’ it is likely that for some of the time it
would have been detected by acoustics and would have
contributed to the sA-value in the bottom channel,
and at other times it is in the acoustic bottom dead
zone.
Vertical density distributions of fish
100
695
100
(a)
(b)
80
80
60
60
40
40
sA>1
20
0
0.0
20
saithe
30–600 m
0.2
0.4
0.6
0.8
1.0
100
saithe
30–600 m
0
0.0
0.2
0.4
0.6
0.8
1.0
100
(c)
(d)
80
80
60
60
40
40
day
sA>1
20
20
haddock
30–600 m
0
0.0
0.2
0.4
0.6
0.8
1.0
100
haddock
30–600 m
0
0.0
0.2
0.4
0.6
0.8
1.0
100
(e)
(f)
day
80
80
night
60
night
60
40
40
sA>1
20
20
blue whiting
blue whiting
30–600 m
30–600 m
0
0.0
0.2
0.4
0.6
0.8
1.0
0
0.0
0.2
0.4
0.6
0.8
1.0
Figure 13. The distribution of relative cumulative vertical profiles from the seabed to the surface in terms of RPRL, from surveys
along the Norwegian coast 1992–2000. Day and night medians and inter-quartile ranges (represented by thin boxes, left box for
day, right box for night), from combined data for (a) saithe; (c) haddock; (e) blue whiting. Day–night median curves with upward
adjusted night median curve (broken line) for (b) saithe; (d) haddock; (f) blue whiting.
696
B. K. Stensholt et al.
100
100
(a)
(b)
80
80
60
60
40
40
sA>1
20
0
0.0
saithe
30–200 m
0.2
0.4
0.6
sA>5
20
0.8
1.0
100
0
0.0
saithe
30–600 m
0.2
0.4
0.6
0.8
100
(c)
(d)
80
80
60
60
40
40
sA>1
20
0
0.2
0.4
0.6
sA>1
20
haddock
30–200 m
0.0
0.8
1.0
100
0
0.0
haddock
30–600 m
0.2
0.4
0.6
0.8
1.0
100
(e)
(f)
80
80
60
60
40
40
sA>1
20
sA>10
20
blue whiting
blue whiting
300–600 m
200–300 m
0
0.0
1.0
0.2
0.4
0.6
0.8
1.0
0
0.0
0.2
0.4
0.6
0.8
1.0
Figure 14. The distribution of relative cumulative vertical profiles from the seabed to the surface in terms of RPRL, from surveys
along the Norwegian coast 1992–2000. Annual median curves for samples with bottom depth <200 m of (a) saithe; (c) haddock;
(e) blue whiting; and for samples with bottom depth 300–600 m of (b) saithe; (d) haddock; (f) blue whiting. The level of sA-value
indicated at bottom right corner.
Vertical density distributions of fish
697
100
100
(a)
(b)
80
80
60
60
40
40
sA>1
20
0
0.0
sA>1
20
saithe
30–600 m
saithe
30–600 m
0
0.2
0.4
0.6
0.8
1.0
100
0.0
0.2
0.4
0.6
0.8
1.0
100
(c)
(d)
80
80
60
60
1999
1998
40
40
sA>1
20
0
0.0
20
blue whiting
30–600 m
0.2
0.4
0.6
0.8
1.0
100
0
0.0
blue whiting
30–600 m
0.2
0.4
0.6
0.8
100
(e)
(f)
80
80
60
60
40
40
94
98
99
96
20
blue whiting
95
97
sA>1
20
300–600 m
0
0.0
1.0
0.2
0.4
0.6
0.8
1.0
0
0.0
blue whiting
300–600 m
0.2
0.4
0.6
0.8
1.0
Figure 15. The distribution of relative cumulative vertical profiles from the seabed to the surface in terms of RPRL, from surveys
in the North Sea 1992–2000. Median and inter-quartile range (represented by thin boxes) of (a) saithe; (c) and (e) blue whiting.
Annual median curves of (b) saithe; (d) and (f) blue whiting. Acoustic sA-values level, species and bottom depth ranges are shown
at the bottom right corner.
698
B. K. Stensholt et al.
100
100
(a)
(b)
80
80
60
60
night
40
40
1995
20
1995
sA>10
20
sA>10
300–600 m
0
0.0
0.2
0.4
0.6
0.8
600–1500 m
1.0
0
0.0
0.2
0.4
0.6
0.8
1.0
100
100
(c)
(d)
80
80
60
60
40
40
night
1996
20
1996
sA>10
night
20
sA>10
200–300 m
0
0.0
0.2
0.4
0.6
0.8
300–600 m
1.0
0
0.0
0.2
0.4
0.6
0.8
1.0
100
100
(e)
(f)
80
80
60
60
night
40
40
1996
20
sA>10
night
1996
sA>10
20
600–1500 m
600–1500 m
0
0
0.0
0.2
0.4
0.6
0.8
1.0
0
200
400
600
800
Figure 16. Median (full or broken line join day or night median) and inter-quartile range (represented by thin boxes, left box for
day, right box for night) of blue whiting from the distribution of relative cumulative vertical profiles from the deepest end of each
sample distribution layer to surface in terms of RPRL. Samples are from surveys west of British Isles. Bottom depth and year are
as specified at bottom right corner. The pelagic distribution layers are shown in (f), with accumulation from surface down in
metres.
Vertical density distributions of fish
100
100
(a)
80
80
60
60
40
40
20
699
(d)
20
capelin
0
0.0
herring
0
0.2
0.4
0.6
0.8
1.0
100
0.0
100
(b)
80
80
60
60
40
40
0.2
0.4
0.6
(e)
capelin
herring
0
0
100
100
(c)
(f)
80
80
60
60
day
night
40
20
night
20
capelin
0
0.0
1.0
20
20
40
0.8
0.2
0.4
0.6
0.8
herring
1.0
0
0.0
0.2
0.4
0.6
0.8
1.0
Figure 17. The distribution of relative cumulative vertical profiles from the surface to the bottom in terms of relative depth (0.0 at
surface, 1.0 at bottom), from winter surveys in the Barents Sea, 1996–2001. Median and inter-quartile range (represented by thin
boxes, left box for day, right box for night) of the distribution for (a) capelin; (d) herring; annual median curves (b) capelin;
(e) herring; day and night for (c) capelin; (f) herring.
Dead-zone loss during day and night
The typical case in the Barents Sea winter surveys
1996–2002 was that the observed acoustic abundance
during the daytime was higher than at night, Nd >Nn, i.e.
that night-time loss in the acoustic dead-zone exceeds
daytime loss. The night-time median curve is adjusted by
an upward shift. The values for Nd and Nn are the
medians of the total acoustic sA-value in the water
column over the day and night samples, respectively.
Only the samples with total sA-value greater than 1 are
used here.
700
B. K. Stensholt et al.
Table 1. Temperature and depth distribution, in winter (January–March) and summer (July–August). From 1996–1997 time series
of 6 data-storage tags attached to North East Arctic cod released in the Barents Sea.
Tag no.
Temperature
117
131
191
204
206
246
Depth
117
131
191
204
206
246
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
Mean
Std
Min
Max
3.1
3.9
2.2
2.4
3.9
4.0
2.6
4.0
3.1
2.4
2.4
4.2
2.0
0.8
1.5
1.4
1.2
0.3
1.5
0.4
1.2
0.9
1.5
0.3
1
1.5
1.3
0.1
1.3
3.4
0.7
3.0
0.3
0.1
1.0
3.5
6.4
5.2
5.0
5.0
9.2
4.7
4.9
4.9
6.3
3.6
5.3
5.3
237
286
208
261
209
318
236
309
130
201
235
311
78
75
69
43
96
40
84
31
92
94
81
11
0
0
60
81
1
174
2.4
193
0
49
3
241
For the six winter surveys and the three species
cod, haddock, and redfish, the dead zone corrections,
i.e. the relative difference in losses at the bottom
(Nd Nn) · (Nd) 1 if Nd >Nn (night correction), or
(Nn Nd) · (Nn) 1 if Nd <Nn (day correction), are given
in Table 5. The material may be insufficient to allow the
conclusion that 1996, with its large day corrections, is an
exceptional year (Figure 7). One possibility is that there
is a separation of both the cod and the haddock stocks
into groups with very different diurnal vertical migration
patterns and that switches between Nd <Nn and Nd >Nn
from one year to another is explained by relative
changes in the abundance of such groups. The relative
475
475
406
371
410
404
389
376
353
353
370
331
P5
0.6
2.5
0.1
0.2
2.1
3.6
0.4
3.4
1.0
0.6
0.2
3.5
113
176
92
198
52
235
103
256
20
72
92
297
Q1
1.3
3.4
0.9
1.0
3.4
3.8
1.3
3.8
2.1
1.7
0.9
3.9
174
271
151
237
136
303
150
288
55
86
166
303
Median
3.9
4.2
2.1
2.9
3.9
4.0
3
4.0
3.3
2.6
2.7
4.1
243
296
218
255
224
321
272
308
99
207
264
312
Q3
4.7
4.5
3.6
3.5
4.3
4.2
3.8
4.4
3.9
3.1
3.7
4.4
294
322
257
293
281
344
305
334
207
293
300
320
P95
5.4
4.8
4.1
4.3
6.3
4.6
4.4
4.6
5.3
3.3
4.4
4.6
353
379
309
335
351
377
344
352
300
327
329
328
cumulative acoustic frequency figures for day and night
(Figures 6d, 8d, 9d) indicate that such separation exists,
although the acoustic data give no information on which
parts of the stock ascend or descend. The pattern of
vertical migration in 1996 corresponds to the high
proportion of the one-year-old fish (Table 6), in agreement with Aglen et al. (1999). Unless the species and age
allocation of acoustic samples can be made according to
age and species composition in smaller depth intervals,
such profiles may also occur in areas where different
species and age groups have opposite vertical migration.
Diel pattern in vertical distribution
Table 2. Percentage of time spent within 10 m above the daily
maximal depth from depth records during April–June (4–6),
July–August (7–8), September–November (9–11), December–
January (12–1), and February–March (2–3). Depth time series
from April 1996 to March 1997 are from 6 data-storage tags
attached to North East Arctic cod released in the Barents Sea.
Tag number
4–6
7–8
9–11
12–1
2–3
117
131
191
204
206
246
43
46
45
46
35
67
41
33
45
45
48
37
71
37
33
57
75
42
76
65
50
70
73
74
29
51
57
31
44
56
In most cases, when the samples from winter surveys
(cod, haddock, and redfish 1996–2001) are classified as
day or night, the relative cumulative distribution of
sA-values near the sea bottom shows lower median
values at daytime than at night-time. With accumulation
further up the water column, the day median increases
faster than the night median, and the order is reversed
around mid-range in the lower half of the water column
(Figures 6d, 7d, 8d). This reversal appears as the day
median curve’s crossing and passing over the night
median curve, with a steeper slope of the day median
curve than the night median curve. When the crossing
remains after the correction for unequal dead zone loss,
it shows that during night-time a higher proportion of
Vertical density distributions of fish
701
Table 3. Cod and haddock: medians of the distributions of the relative acoustic sA-values from bottom channel (10 m above
seabed) in percent of total sA-values. Acoustic samples are from summer and winter surveys in the Barents Sea 1995–2002 and are
classified according to bottom depth interval, day and night.
Summer survey
Winter survey
50–600 m
Cod
Cod
Year
50–200 m
200–600 m
1995
1996
1997
1998
1999
2000
2001
2002
97.3
82.7
93.3
99.9
74.7
100.0
26.3
23.8
9.7
15.7
11.7
52.3
Haddock
Haddock
50–200 m
200–600 m
50–200 m
200–600 m
Day
Night
Day
Night
55.6
67.0
55.5
63.6
59.6
52.9
72.9
31.5
36.0
24.5
24.7
19.1
39.3
37.2
72.7
58.2
69.1
57.6
49.5
42.8
39.9
25.5
22.4
23.2
18.0
13.1
37.5
16.0
29.4
38.4
20.5
22.5
13.7
22.5
26.2
37.0
39.3
30.2
31.5
29.0
52.6
46.9
21.9
18.8
18.2
10.4
13.6
25.6
12.7
37.5
28.7
30.5
29.2
19.9
49.5
21.4
fish is distributed near the bottom while another higher
proportion also distributes further up into the water
column than during daytime. During daytime there is a
higher proportion of fish around mid-range of the
bottom half of the water column. This vertical movement and replacement may cause unequal amounts of
acoustic information to be lost in the dead zone at
night-time and daytime. Moreover, the slope of the
median curve indicates the density level, and thus
doubling the slope, e.g. from 2 to 4 (i.e. a small angle at
the crossing), means a doubling of the density.
Table 5 shows that for cod and haddock there is a
higher loss during night than day in five out of seven
years, and for redfish night loss is the higher in six out of
seven years. After correction, the median profiles for
‘‘day’’ and ‘‘night’’ are based on equal dead zone losses.
Table 4. Median and lower quartile (P25) of the relative
acoustic sA-values from bottom channel (10 m above seabed) in
percent of total sA-values. Blue whiting from saithe survey
along the Norwegian coast north of 62N 1992–1997. Saithe
from North Sea survey south of 62N 1992–2000. Classification
by bottom depth.
Saithe survey
Blue whiting
50–200 m
300–600 m
Cod
North Sea survey
Saithe
30–600 m
Year
Median
Median
P25
Median
1992
1993
1994
1995
1996
1997
1998
1999
2000
100.0
68.3
68.7
100.0
15.7
100.0
2.5
13.2
35.1
14.9
11.2
9.5
41.3
100.0
100.0
100.0
46.9
100.0
63.9
100.0
97.2
81.2
100.0
100.0
100.0
86.3
100.0
100.0
100.0
100.0
The acoustic data show the net effect if there are
migrations in opposite directions but do not show to
what extent migrations with opposite direction cancel
out.
Table 5. The relative difference between day and night average
acoustic sA-values in percent of the larger of these two. The day
column shows the correction percentage when day loss>night
loss, and similarly for the night column.
Cod
Year
Day
1996
1997
1998
1999
2000
2001
2002
24.3
Haddock
Night
24.7
2.3
Day
Night
Redfish
Day
21.8
4.8
28.8
31.7
21.1
38.1
30.8
4.7
11.4
63.4
38.6
Night
20.8
31.6
54.7
7.0
29.3
39.9
9.7
Table 6. NE-Arctic cod: age composition based on
acoustic abundance indices in percent (Aglen, 2002) from the
Norwegian winter survey in the Barents Sea.
Year
1995
19961
19971*
19981*
1999
2000
2001
2002
1
Age 1
Age 2
Age 3
Age 4
Age 5
Age 6+
74.0
77.7
67.4
71.4
36.7
15.6
55.5
3.2
8.3
11.0
17.9
13.3
31.2
22.5
5.6
39.1
4.7
3.0
7.8
9.0
15.4
24.9
12.2
10.0
4.5
2.3
2.1
3.8
9.9
16.1
15.1
19.3
5.9
2.8
2.0
0.9
4.6
14.4
6.8
17.0
2.5
3.2
2.7
1.6
2.2
6.4
4.7
11.4
Survey covered a larger area.
*Adjusted indices, survey covered only Norwegian zone.
702
B. K. Stensholt et al.
The eastern part of the Barents Sea, which is an
important feeding area for young cod, was not surveyed
in 1997–1998. In 1996, 2000, and 2001 there are sufficient samples east of 3530E for analysis, and separate
investigation shows a relative day correction of 36% for
1996 and relative night corrections of 60% in 2000 and
57% in 2001. The results from the areas west of 3530E
are similar to the overall results in Table 5.
Day and night samples along the Norwegian coast
and from west of the British Isles are situated far apart
in terms of geographic locations, so only cautious conclusions can be drawn. The diel vertical migration of
saithe and haddock is not immediately clear from the
combined data 1992–2000 (Figure 13a,c). Their diel
vertical migration patterns appear more clearly when the
relative cumulative vertical profiles for night-time have
been adjusted upward by correcting for the estimated
losses in the acoustic dead-zone (Figure 13b). Yearly
data show some evidence of diel vertical migration but
the direction of migration changes from year to year.
Diel vertical migration of blue whiting is seen all years
both in samples from the Norwegian coast (Figure 13e,f)
and from the North Sea survey, when the RPRL is
calculated based on the bottom restricted FVR.
West of the British Isles the RPRL is calculated based
on the deepest boundary of each distribution range. The
1995 samples show diel behaviour in areas of depth
<600 m (Figure 16a). For depth >600 m, the day
samples are mainly from deeper areas than the
night samples; however, there is no clear diel pattern
(Figure 16b). The 1996 samples show clear diel pattern,
with higher daytime concentration at the deeper end of
the vertical distribution range (Figure 16c,d,e). Samples
from areas deeper than 600 m have the night-time distribution about 50–100 m higher up than the daytime
distribution (Figure 16f).
For oceanic redfish, the RPRL is calculated from the
deep end of the distribution range. In the relative
cumulative vertical profiles of total sA-values, i.e. above,
within, and below the deep scattering layer (Figure 9e),
the median mark is higher during the day than at night
from just above the deepest end of the distribution
range. Further up, but within RPRL=0.5, the night
median is higher, but beyond RPRL=0.5 the day
median is again higher, and the night distribution
stretches higher in the water column. This may be an
effect of how the samples are gathered, as biologists
suspect that there are two separate stocks of redfish
(Magnússon et al., 1996; Dalen and Nedreaas, 2001).
When only fish from above and within the deep scattering layer are used, then each daytime sample is mainly
distributed within one (cod) FVR. However, night
samples stretch higher up (Figure 9f).
Diel vertical migration is also found for capelin in all
winter surveys. The median indicates that capelin are
distributed throughout the entire water column during
night-time and are concentrated closer to the bottom
during the day (Figure 17c). The large inter-quartile
range may indicate that the capelin’s vertical profile
varies according to local conditions. Similar diel behaviour is found for herring in the years for which we have
a significant amount of data (1999–2000). In 1998 and
2000 most herring samples were distributed in the
bottom half of the water column (Figure 17d,e,f).
Some results species by species
Cod
Data from both winter and summer surveys
(Figures 6a,b, 10a,b) show that the vertical distribution
is in agreement with a theoretical free vertical range
(FVR) as determined by Harden Jones and Scholes
(1985). The samples with bottom depth less than 200 m
have relative vertical profiles within the bottomrestricted FVR (Figures 6e, 10c,d). With bottom depth
more than 200 m combined with high acoustic abundance, more cod adapt to pelagic living in both winter
and summer survey samples (Figures 6f, 10e,f and Table
3). Both acoustic samples and data-storage tag records
show larger variation in depth distribution during summer than in winter (Figures 6a, 10a and Tables 1, 2).
Haddock
Data from winter and summer surveys show a similar
vertical profile to that of cod. In data from all four
surveys haddock mainly stay within one ‘‘cod’’
bottom restricted FVR, but more often adapt to pelagic
living (Figures 8a,b, 11a,b, 13c,d, and 14c,d). Summer
samples have larger inter-quartile range than winter
samples (Figures 8a and 11a). This suggests a greater
variation in the relative vertical profiles in summer.
The vertical distribution in terms of pressure change
in comparison to the bottom restricted FVR seems
to depend on bottom depth and the size of acoustic
abundance (Figures 11c,d,e,f, 14c,d). Haddock samples
from areas deeper then 200 m or with high acoustic
abundance show clearly that some of the fish adapt to
pelagic living (Figures 11e,f, 14c,d, and Table 3). In the
North Sea surveys and summer surveys, however, they
spread more upward into the water column than in
winter-survey samples. The summer 1998 median curve
in Figure 11f shows that most haddock samples had
more than half of the relative acoustic density well above
the bottom restricted FVR (RPRL=0.5). The two selection criteria together were satisfied by samples in one
stretch of line transects along the northern coast of
Norway. Most individual samples were similar to the
median curve, with two separate high concentrations of
acoustic abundance, one close to the bottom and the
other in the upper half of the water column.
Vertical density distributions of fish
Redfish
In general redfish are found in regions deeper than
100 m. The vertical profile shows that redfish mainly
stay well within the 50% RPRL with respect to the
bottom pressure in both summer and winter (Figures 9
and 12). Even with depths greater than 300 m only small
parts of the samples go into the upper half of the water
column. Redfish is not a target species and none of the
surveys cover its distribution areas completely.
Saithe
Along the Norwegian coast, saithe mainly stay in the
bottom half of the water column for all bottom depths
(Figures 13a,b, 14a,b), but as depth increases offshore
they spread relatively higher into the water column
(Figure 14b). For depths less than 200 m the saithe stay
close to seabed (Figure 14a), but for depth greater than
300 m (Figure 14b) the median of relative cumulative
frequency in some years is well below 100%, at
RPRL=0.5.
In the North Sea survey, saithe are found mainly in
the north and northwest part of the survey area, west of
the Norwegian Trench. Saithe in the North Sea mainly
stay within the bottom channel (10 m above seabed)
(Figure 15a,b) and Table 4.
Blue Whiting
Along the Norwegian coast in areas deeper than 300 m
blue whiting usually have a relatively high acoustic
abundance and the distribution stretches over more than
one cod bottom-restricted FVR in some years (Figure
14f). In the total samples (Figure 13e,f) and samples in
areas shallower than 300 m (Figure 14e and Table 4),
however, the relative cumulative sA-values mainly
reaches 100% well below the RPRL=0.5, with reference
to the bottom pressure.
Blue whiting samples in the North Sea are few in
number and there are only night samples in 1998. The
median curves of most years, except 1999, have relative
cumulative acoustic density profiles within or stretching
just above RPRL=0.5, with reference to the bottom
pressure (Figure 15c,d). For 1999 (August–September)
the relative cumulative frequency indicates a mixture of
pelagic and demersal living. Samples from deep areas
(>300 m) are mainly from the area north of Shetland.
The annual median curves show that effective accumulation starts higher in the water column and reaches
100% at a level higher than RPRL=0.5 (Figure 15e,f),
indicating pelagic living in mid-water. The reason for the
S-shape of the median curve may be that a large part of
the echo abundance comes from a distribution range
stretching over roughly one FVR in mid-water. In
samples from 1995, 1996, 1998, the vertical extent of a
distribution range has a relative pressure reduction at
the upper end, with reference to the pressure at the
deeper end at approximately 0.5.
703
For samples of spawning blue whiting from surveys
west of the British Isles, the relative cumulative vertical
profiles of sA-values show that most blue whiting adapt
to pelagic living. The main distribution range gradually
leaves the seabed for samples with depths greater than
300 m and is all well above seabed at depths greater than
600 m (Figure 16f). Thus the relative vertical profiles are
accumulated from, and the RPRL=0 refers to, the
deepest end of each sample distribution range. The
distribution of both years’ profiles shows that the
main concentration of the stock is within RPRL=0.5
(Figure 16).
Physostome species
Capelin and herring have open swimbladders and thus
the relative vertical profiles of acoustic density are
accumulated from surface down to bottom. The depth
axis is partitioned as a relative proportion of depth from
sea surface to bottom.
Capelin
Data from winter surveys show that the capelin’s
relative vertical profile varies greatly over the survey
area (Figure 17a,b,c). High acoustic abundance is found
along the Polar Front in mid-water and mainly in areas
with a bottom depth more than 200 m. There are only a
few samples during summer and all of the fish are then
found close to the surface.
Herring
Samples from the North Sea show that herring are
mainly found in the upper half of the water column. The
vertical profiles of samples along the Norwegian coast
vary greatly. The influx of herring into the Barents Sea is
variable. In most years the data are insufficient for
conclusions to be made about diel vertical migration. In
winter, herring in the Barents Sea are mainly distributed
in the bottom half of the water column (Figure 17d,e,f),
and samples from year 2000 have diel vertical migration
with night-time ascents.
Temperature and salinity
Temperature, salinity, and sA-values from the winter
and summer surveys in the Barents Sea have been
interpolated onto the same locations. The distribution of
temperature and salinity, weighted by sA-values for each
survey and species, are presented in Tables 7–8.
Summer and winter the vertical profiles for cod,
haddock, and redfish show that in most cases the higher
concentration is below 100 m where the vertical temperature gradient is low. The survey coverage of the
eastern parts varies, and in 1997–1998 the survey vessels
did not enter the Russian zone. This may affect the range
704
B. K. Stensholt et al.
Table 7. Temperature and salinity weighted by acoustic sA-values for cod, haddock, and redfish.
Summer surveys in the Barents Sea, 1996–2000. All years and annual average. s.d. is standard
deviation; P1, P5, . . . are the percentiles.
Species
Mean
s.d.
P75
Median
P25
P5
3.30
5.77
4.39
4.32
9.07
3.06
3.40
3.11
2.10
8.56
1.99
3.18
2.22
1.26
7.38
0.21
1.93
1.54
0.11
3.80
3.6
6.5
4.8
7.0
5.6
3.3
3.9
3.0
4.0
3.6
3.16
2.40
2.27
3.32
2.59
2.7
1.0
1.1
2.5
1.1
1.0
0.3
0.5
1.0
0.2
0.4
0.5
1.1
0.4
0.8
Haddock summer temperature, Barents Sea
1996
3.30
0.97
6.6
6.2
1997
5.69
0.78
8.1
7.6
1998
5.93
0.61
8.8
8.1
1999
5.26
0.92
8.7
8.3
2000
5.19
0.60
8.3
7.2
3.3
6.6
7.0
7.2
6.1
3.2
5.8
6.1
4.9
5.4
3.2
5.0
5.1
3.6
4.5
1.8
3.0
2.8
2.3
2.3
1.0
2.1
1.8
0.9
0.8
Redfish summer temperature, Barents Sea
1996
3.81
0.88
6.5
1997
2.99
0.36
6.2
1998
3.58
0.53
6.2
1999
3.54
0.70
6.6
2000
3.10
0.69
6.2
6.4
5.2
6.1
6.5
5.4
5.5
3.7
5.2
4.1
4.0
3.9
2.8
3.1
3.1
3.0
1.9
2.4
2.5
2.6
1.9
1.2
1.0
1.2
1.6
1.6
1.0
0.1
0.6
0.8
1.6
35.02
35.01
35.11
35.05
35.04
34.94
34.89
35.05
35.01
34.92
34.87
34.83
35.02
34.97
34.66
34.76
34.55
35.00
34.90
34.38
34.39
34.22
34.92
34.71
34.11
34.22
34.13
34.71
34.35
33.90
Summer temperature 1996–2000,
Cod
2.76
1.45
Haddock
4.33
1.03
Redfish
3.41
0.68
Capelin
2.80
2.57
Herring
7.98
2.01
P99
P95
Barents Sea
7.13
5.20
8.34
7.58
6.49
6.23
8.09
6.74
10.39
9.73
Cod summer temperature, Barents Sea
1996
2.86
1.51
5.4
1997
2.62
1.76
7.8
1998
2.10
1.32
6.2
1999
3.36
1.52
8.2
2000
2.45
0.87
6.4
Summer salinity 1996–2000, Barents Sea
Cod
34.82
0.20
35.06
Haddock
34.70
0.15
35.05
Redfish
35.02
0.03
35.12
Capelin
34.93
0.17
35.07
Herring
34.64
0.36
35.07
of temperature distribution and also the relationship
between temperature and fish distribution. Most of the
cod, haddock and redfish are found in the range 2–6C
but a significant proportion of cod is in subzero temperatures where no haddock and very little redfish are
found. The subzero records for cod stem from its high
concentration along the Polar Front. For all species the
temperature variance is smaller in winter than in
summer, which may simply reflect the fact that fish
spread more out in summer. For cod this is in agreement
with the distribution of temperature and depth in
storage-tag data (Table 1).
The variations in salinity are, to our knowledge, too
small to make a significant physiological difference
(Tables 7, 8). The particularly low salinity variation for
redfish may reflect that it does not migrate as much
between different water masses as the other species. The
average temperature where the fish were found can vary
from year to year, the clearest difference is seen between
the median winter temperatures of the water masses for
redfish between 1996 and 2000.
P1
0.57
0.98
0.86
0.78
2.71
Discussion
Vertical distribution and relative pressure
reduction
The rapid vertical movements of a physoclist are limited
by the relative change in the ambient pressure because of
swimbladder physiology. Therefore, in the case of
demersal physoclists, it is meaningful to investigate the
relative vertical profiles of sA-values in terms of relative
pressure reduction with reference to bottom pressure
and to measure their extent in FVR units. This allows
the comparison of relative profiles from locations with
different bottom depth. Then the factors affecting the
vertical distribution can be investigated.
Each acoustic sample is a ‘‘snapshot’’ of the fish
distribution in the water column expressed as an integral
over 1 nmi, each fish staying in its own individually
adaptable range. The relative cumulative frequency of
the observed sA-values in each water column represents
the profile of the sample. If all individual FVR include
Vertical density distributions of fish
705
Table 8. Temperature and salinity weighted by acoustic sA-values for cod, haddock, and redfish.
Winter surveys in the Barents Sea, 1996–2000. All years and annual average. s.d. is the standard
deviation; P1, P5, . . . are the percentiles.
Species
P95
P75
Median
P25
P5
P1
Winter temperature 1996–2000, Barents Sea
Cod
3.10
1.28
6.8
5.8
Haddock
4.12
0.63
6.8
5.9
Redfish
3.66
0.62
6.8
6.1
Capelin
1.25
1.51
5.6
4.0
Herring
3.23
0.75
6.8
5.7
4.3
4.6
4.7
2.3
4.1
3.17
4.09
3.70
1.46
3.32
1.9
3.6
2.4
0.02
2.2
0.1
2.5
1.8
1.2
1.2
0.5
1.8
1.3
1.3
0.4
Cod winter temperature, Barents
1996
2.33
1.11
1997
2.97
1.00
1998
2.97
1.37
1999
3.13
1.18
2000
4.10
1.33
5.3
4.5
5.3
6.1
6.6
3.3
4.0
4.3
4.5
5.2
2.3
3.4
3.1
3.2
4.2
1.3
2.0
1.8
1.8
3.0
0.3
0.4
0.2
0.3
1.5
0.5
0.4
0.4
1.1
0.03
Haddock winter temperature, Barents Sea
1996
3.81
0.64
6.2
1997
4.00
0.49
5.7
1998
4.21
0.43
6.4
1999
4.17
0.60
6.9
2000
4.54
0.78
7.0
5.9
4.6
5.8
6.4
6.6
4.4
4.4
4.7
5.0
5.2
3.6
4.0
4.2
4.2
4.5
2.9
3.7
3.7
3.3
3.7
2.3
3.0
2.6
2.2
2.8
1.6
2.1
1.7
1.5
1.9
Redfish winter temperature, Barents Sea
1996
3.23
0.60
6.2
1997
3.28
0.59
5.5
1998
3.85
0.73
6.5
1999
4.19
0.37
6.9
2000
4.87
0.47
7.0
5.6
5.0
6.3
6.4
6.8
4.2
4.0
4.7
5.1
5.9
2.72
3.58
4.04
4.32
5.08
2.1
2.3
2.7
3.3
3.8
1.7
1.6
1.8
1.6
2.4
1.0
1.3
1.4
1.3
2.3
35.06
35.06
35.08
35.04
35.07
35.00
35.00
35.03
35.01
34.93
34.96
34.96
35.00
34.94
34.73
34.89
34.83
34.96
34.87
34.63
34.64
34.56
34.76
34.56
34.38
34.41
34.24
34.34
34.50
34.21
Winter salinity
Cod
Haddock
Redfish
Capelin
Herring
Mean
s.d.
P99
Sea
6.0
5.4
6.3
6.9
7.0
1996–2000, Barents Sea
34.92
0.10
35.10
34.90
0.11
35.10
34.97
0.06
35.11
34.92
0.12
35.05
34.76
0.11
35.15
the seabed, the distribution of the relative cumulative
sA-values, with its lower quartile, will reach 100% within
one bottom-restricted FVR, i.e. at the RPRL where the
median and quartile curves approach 100%. When the
distribution of the relative cumulative frequency reaches
100% well beyond one bottom-restricted FVR, e.g. when
the inter-quartile range remains large at an RPRL where
the median curve approaches 100%, it indicates that
some fraction of the stock has adapted to pelagic living.
The free vertical range and the extent of the
vertical distribution
The cod’s vertical distribution is related to the physiological restrictions of the individual cod’s vertical movements. This was recognized by Ellis (1956), who
reported from acoustic observations at the Skolpen
Bank in the Barents Sea, September 1955, that cod
shoals were found to be well within the theoretical limits
imposed by the swimbladder. The cod’s FVR was determined experimentally by Harden Jones and Scholes
(1981, 1985) as a depth interval with a pressure ratio of
0.5 between the upper and the lower end. It is used here
as a unit for comparison although other physoclists may
have a somewhat different FVR. The restriction to an
FVR is not absolute, as cod, saithe and haddock tolerate
60–70% pressure reduction from the pressure at neutral
buoyancy level before the swimbladder bursts (Tytler
and Blaxter, 1973).
The relative cumulative sA-values often reach 100% at
RPRL=0.5, which indicates that the experimentally
determined FVR is in fact a reasonable description of
the cod’s natural behaviour. The restriction to change of
pressure during rapid vertical movement, imposed by
swimbladder physiology, is also evident in the analysis
of acoustic samples of other physoclists. Their free
vertical ranges seem to be of roughly the same size as for
cod. This restriction is shown both in physoclists living
as demersals (Figures 4–14) and in pelagic distribution
layers (Figures 9e,f, 15f, 16).
Consider the deepest end of the vertical distribution
range of a sample, at depth M, to be the deepest end of
an FVR. Then the neutral buoyancy level, at depth d, is
the level where the relative pressure reduction is 1/3, and
706
B. K. Stensholt et al.
the upper end of FVR, at depth m, is the level where the
relative pressure reduction is 1/2. In most cases the
relative cumulative values are approximately 80% at
RPRL=1/3 (Figures 4–16). If the bottom-restricted
FVR is their natural FVR these results indicate that
demersal physoclistous fish spend most of their time
being under-buoyant. This supports the hypothesis of
Harden Jones and Scholes (1981), that if the overall
strategy is to conserve energy, such behaviour would be
adopted by demersal physoclists, while pelagic physoclists might maintain neutral buoyancy throughout
the day. In the case of blue whiting shoals, they appear
to be mainly within one FVR whether they distribute above seabed or as pelagic shoals in mid-water
(Figures 15d,f, 16).
which was mainly along the Polar Front and around
Svalbard, including a steep shelf edge. Acoustic signals
show demersal fish shoals that are distributed at the
seabed on a steep shelf and then extending outwards in
mid-water over the edge.
There is great diversity of prey species for cod,
haddock and redfish during the summer surveys, while
capelin is the main prey during the winter. The variation
in spatial distribution and behaviour among different
prey species may result in a large variation of the vertical
distribution of the predators among the different
samples collected from different locations (Neilson and
Perry, 1990). The depth distribution of individual cod
from data-storage tags has larger variation in summer
than in winter (Table 1, Stensholt, 2001) in agreement
with these results.
The distribution of vertical profiles
The FVR unit expresses the capacity of the physoclists
to perform rapid vertical moves but the fish may adapt
to make use of this capacity to a variable degree. This
degree is reflected in the vertical profile of the relative
cumulative acoustic density. Investigating such profiles
may reveal factors that influence the vertical migration
behaviour.
The median profile represents most of the samples.
Small samples of fish may have vertical profiles that are
modified due to special factors, e.g. the prey species’
vertical distribution, depth, density, and current
(Neilson and Perry, 1990). Thus the median of the entire
set of samples may not be a good representative of these
special cases. In these cases the analysis of such samples
should be carried out separately. Our results indicate
that great bottom depth, high density, and strong
currents affect the vertical distribution in all studied
physoclist species.
Because of the survey design only one sample is
available in each geographic location. Classification of
the locations for uniform environmental conditions,
which may affect the vertical profiles, could improve the
analysis. On the other hand such grouping of the data
means a reduced number of samples within each group
and so over-refinement could also reduce reliability.
Strong local environmental gradients in the saithe
surveys along the Norwegian coast lead to some
uncertainty with respect to the geographical separation
of day-night effects from environmental effects. The
larger variation in the extent of vertical distribution
observed during summer surveys than winter surveys
could be caused partly by the variation over different
years in the summer survey coverage, partly by fish
migration due to climate change (Midttun, 1965),
and partly by the vertical distribution being location
dependent. The distribution in summer 1999 shows more
pelagic living for cod and haddock than in most years.
This may be due to the survey line-transect coverage,
Evidence for pelagic living
The relative sA-values from the bottom channel (10 m
above seabed) generally decrease with increasing bottom
depth (Tables 3–4). The relative cumulative profiles
(Figures 6, 8–15) show more detail. In particular, the
lower quartile sometimes reaches 100% beyond one
bottom-restricted FVR, which is evidence that there are
fish above the bottom-restricted FVR in 25% of the
samples.
Neilson and Perry (1990) summarized from several
reports evidence that predators’ migration behaviour is
modified or governed according to food availability and
behaviour of their main prey species. Physoclists must be
adapted to pelagic living in order to forage more than
one bottom-restricted FVR above sea bottom. Such
adaptation means that the fish have an individual free
vertical range that does not include the seabed. After a
descent to the bottom these fish will be excessively
negatively buoyant staying below the lower free vertical
range limit. They compensate by gas secretion but the
process consumes time and energy (Harden Jones, 1951;
Blaxter and Tytler, 1972; Harden Jones and Scholes,
1985). Generally, with higher fish density and deeper
water, a larger fraction of the fish is found to adapt to
pelagic living. With increased density there is increased
competition for food, and this may cause more individuals to search for prey relatively higher in the water
column.
With increasing bottom depth there is an increasing
number of FVR-units from the seabed to the surface.
For physoclist fish adapted to stay near the bottom this
means that the upper layers become less accessible via
rapid ascents. With increased bottom depth fish adapted
to pelagic living can more easily reach the upper layers.
There are clear differences between the species. Among
the physoclists studied here blue whiting is most often
seen to be pelagic, especially in areas deeper than 300 m,
while redfish almost always stay within the bottom
Vertical density distributions of fish
half of the water column. The observations that the
physoclists in the demersal fish surveys stay within one
bottom-restricted free vertical range, may come from the
fact that these surveys are mainly in shelf areas of depths
<500 m. Moreover, the allocation of acoustic density to
species is based mainly on bottom trawl catches. Therefore the species composition found is not considered
reliable in the pelagic channel. In the surveys west of the
British Isles, blue whiting is the target species and the
allocation of acoustic values in the pelagic zone is much
better supported by pelagic trawl catches (Monstad et al.
1995, 1996). The pelagic distribution layers of blue
whiting are mostly within the limit of RPRL=0.5 (one
FVR for cod), when the RPRL is calculated based on
the deepest of the distribution layers. Similar results are
found for oceanic redfish. Observations from other
surveys report that occasionally cod, redfish and blue
whiting are found to distribute in or above mid-water
over great depths (Harden Jones and Scholes, 1985;
Magnússon et al., 1996; Monstad et al. 1995, 1996).
Records from data storage tags attached on cod
released in the Barents Sea (Stensholt, 2001) and the
North Sea (Righton et al., 2001) show that occasionally
individual cod have their daily range larger than the free
vertical range. In these exceptional cases the cod adapts
to pelagic living and seems to accept excessive negative
buoyancy at the bottom part of the daily range. Righton
et al. (2001) found that this occurs more often at shallow
depths. This is natural as in shallower waters the fish has
a shorter interval of excessive negative buoyancy to pass.
Even though the tags from the Barents Sea show that
this occasionally happens also during the periods of
summer and winter surveys, the acoustic profiles
demonstrate that the cod are mainly distributed well
within the bottom-restricted FVR in shallow depths.
Such a large daily range may be too rare to change the
general picture of the distribution of relative cumulative
acoustic profiles. However, in shallow areas (<200 m),
the profile of relative acoustic density may be distorted
or altered by avoidance reactions among fish in the
upper layers due to the approaching survey vessel (Ona
and Godø, 1990). Studies of the cod, haddock, and
saithe show that avoidance reactions are not always
evident and it is still unclear whether the avoidance
reaction is predominantly vertical or horizontal (Aglen,
1994; Godø et al., 1999; Totland, pers. comm.). Recent
studies have documented cases where no avoidance
could be detected (Fernandes et al., 2000a). These
observations were made with RV ‘‘Scotia’’ which is far
less noisy than the vessel used in our study (Fernandes
et al., 2000b).
Tidal currents
Most saithe found in the North Sea stay, for the most
part, within 10 m from the bottom (Figure 15a,b) and
707
Table 4. They are found mainly in the northern and
northeastern part of the survey area, along the west side
of the Norwegian Trench. Saithe along the Norwegian
coast north of 62N, are clearly distributed higher in the
water column (Figures 13a,b, 14a,b). The differing
behaviour in the two survey areas is most likely due to a
difference in the environmental conditions with the
strong tidal currents in the North Sea as a possible
cause. A similar observation was made in the cod’s
acoustic abundance on the Svalbard bank. Analysis of
storage-tag data shows examples of cod with reduced
vertical activity in connection with tidal currents
(Stensholt, 2001), and the saithe may stay close to the
bottom for the same reason.
Age composition, vertical movement, and
dead-zone loss
In most winter surveys the vertical profiles show that
during night-time a higher proportion of fish is distributed both near the bottom and further up into the
water column respectively than during daytime (Table 3,
Figures 5, 6d, 8d, 9d). During daytime there is a higher
proportion of fish around mid-range of the bottom half
of the water column. The degree of separation and
replacement of one part with another part, which results
in the observed day-night vertical profiles, may depend
on the size composition of the stock.
Several studies indicate that large fish ascend during
daytime and descend to the bottom during night-time,
while the opposite holds for small fish (Beamish, 1966;
Aglen et al., 1999; Korsbrekke and Nakken, 1999;
Einarsson, 2001; Stensholt and Nakken, 2001). The
storage-tag data also confirm a daytime ascent of
50–100 m for large cod during winter (Stensholt, 2001).
In 1996 there is higher relative cumulative acoustic
density near the bottom during day than night and
higher relative cumulative acoustic density at night
compared with day from the upper part of the vertical
distribution range (Figure 7). This may be due to the
very high proportion of one- and two-year-old cod in the
stock composition (Table 6). Smaller fish may migrate to
a depth level with a lower concentration of larger fish to
avoid predation. In 1996 the survey transect has the
most extensive coverage in the eastern part of the
Barents Sea, the main living area for young cod. In
1997–1999 there are no surveys east of 40E. In 1992–
1997 the 0-group index was relatively high for cod and
intermediate for haddock (Anon., 1998).
Table 5 shows very high night-time corrections in
2001. A possible cause is that too much of the acoustic
abundance from cod, haddock, and redfish was ascribed
to blue whiting, which were unusually abundant in the
survey area due, it is thought, to relatively high temperatures. There is a high proportion of one-year-old cod in
2001 and, if the proportion that ascends at night has
708
B. K. Stensholt et al.
been recorded as blue whiting, the acoustic loss at night
may have been correspondingly over-estimated, resulting in too high correction of the night-time acoustic
dead zone loss.
Logarithm of pressure ratio
With bottom depth D metres, a depth of x metres is
converted to RPRL=(Dx) · (D+10) 1. One may
regard the FVR as a logarithmic yardstick and convert a
depth of x metres to the logarithm of the pressure
ratio, i.e. log[(10+D) · (10+x) 1]. For small Dx,
which is usual for demersal fish, the RPRL is roughly
proportional to the difference of the logarithms:
ln[10+D]ln[10+x]= ln[1(Dx) · (D+10) 1]]
(Dx) · (D+10) 1.
Thus it makes no practical difference whether depths
are converted to RPRL or to logarithms of pressure
ratio. However, one FVR unit for cod corresponds
to x=0.5 · D5 and the pressure ratio is
(10+D) · (10+x) 1 =2, and the logarithm base 2 of the
pressure ratio is 1.
Conclusions
In order to compare the vertical profiles from the
overall acoustic line-transect survey data, large spatial
variations in absolute values among samples are
normalized using the relative acoustic sA-values. The
samples with unequal bottom depths are normalized by
expressing the relative vertical profiles of each acoustic
sample in terms of the relative pressure reduction level
(RPRL) from the seabed, or from the deepest end of the
vertical distribution range, up to the surface. The
normalized profile is suitable for a discussion of the
physiological limitation to change of pressure during
rapid vertical movement, the free vertical range (FVR),
among demersal physoclists. The observed acoustic vertical distribution of cod is generally within one FVR,
RPRL ranging from 0 to 0.5. Other physoclists show
similar distributions, which indicate that they have
roughly the same FVR as cod. For blue whiting, the
extent of observed pelagic distribution layer is mainly
within one cod FVR.
Taking into account the physiological limitation, the
fish vertical distribution is shown to change with
changes in factors such as age, fish density, prey species’
vertical migration patterns, day–night, bottom depth,
pressure change, strong current, and vessel noise. In
deep water or areas of high fish density, cod, haddock,
saithe, and blue whiting adapt to pelagic living in the
sense that their FVR does not include the seabed. In
areas with strong current or in shallow waters the fish
distribution is closer to the seabed and the vertical
distribution range is mainly less than the FVR.
Due to unequal day and night losses of acoustic
sA-values the profiles have been adjusted, and a method
for adjustment is suggested. The adjusted relative
acoustic vertical profiles for day–night suggest a net
vertical movement, while different parts of the stock may
have different diel migration. Discussion in terms of
stock composition, in the case of cod, indicates that this
is linked to changes in the age composition. When
vertical profiles are expressed in terms of FVR, then
trawl, acoustic and storage-tag information can be combined. A structure of spatial density distribution of fish
in relation to the environment and physiological limitation can be established. This information can be used
to improve the accuracy of stock estimates.
Acknowledgements
Special thanks to colleagues at the Institute of Marine
Research, Bergen, Norway, who gave excellent help on
different aspects: Kaare Hansen for organizing the
acoustic data, Jari Jacobsen and Bjørn Helge Bjørnestad
at the IT department, Odd Nakken for comment and
suggestion, Egil Ona for information on swimbladder
physiology, Adnan Ajiad, Tore Jacobsen, Terje
Monstad, Kjell H. Nedreaas, Odd Smestad, Atle
Totland. Thanks are also offered to editor John
Ramster and referees for their interest and comments.
References
Aglen, A. 1994. Sources of error in acoustic estimation of fish
abundance. In Marine Fish behaviour in Capture and
Abundance Estimation, pp. 107–133. Ed. by A. Fernö, and
S. Olsen. Fishing News Books, Oxford, UK.
Aglen, A. 1999. Report on demersal fish surveys in the Barents
Sea and Svalbard area during summer/autumn 1996 and
1997. Fisken og Havet, nr.7, 46 pp.
Aglen, A. 2001a. Report on demersal fish surveys in the Barents
Sea and Svalbard area during summer/autumn 1998 and
1999.
http://ressurs.imr.no/bunnfisk/raporter/sommertokt98.htm. 41 pp.
Aglen, A. 2001b. Results from the joint Russian-Norwegian
demersal fish survey in the Barents Sea, Winter 2001. Working Document AFWG, Bergen, 24 April–3 May 2001, 29 pp.
Aglen, A. 2002. Results from the joint Norwegian-Russian
demersal fish survey in the Barents Sea, February 2002.
Working Document AFWG, Copenhagen, 16–26 April 2002,
29 pp.
Aglen, A., Drevetnyak, K., Jakobsen, T., Korsbrekke, K.,
Lepesevich, Y., Mehl, S., Nakken, O., and Nedreaas, K. H.
2001. Investigations on demersal fish in the Barents Sea
winter 2000. Detailed report. IMR-PINRO Joint Report
Series no. 5, Bergen, Murmansk. 74 pp.
Aglen, A., Engås, A., Huse, I., Michalsen, K., and Stensholt, B.
1999. How vertical fish distribution may affect survey results.
ICES Journal of marine Science, 56: 345–360.
Vertical density distributions of fish
Aglen, A., and Nakken, O. 1996. Bunnfiskundersøkelser i
Barentshavet og ved Svalbard August 1995. In Norwegian.
Fisken og Havet, nr. 30, 33 pp.
Alexander, R. McN. 1966. Physical aspects of swimbladder
function. Biological reviews of the Cambridge Philosophical
Society, 41: 141–176.
Alexander, R. McN. 1970. Swimbladder gas secretion and
energy expenditure in vertically migrating fishes. In Proceedings of an International Symposium on Biological Sound
Scattering in the Ocean, pp. 75–86. Ed. by G. B. Farquhar.
US. Gov. Print. Off., Washington, D.C.
Alexander, R. McN. 1972. The energetics of vertical migration
by fishes. Symposia of the Society for Experimental Biology,
26: 273–294.
Anon. ICES Preliminary Report of the International Saithe
Survey in October–November 1992–1996.
Anon. Preliminary report of the international 0-group
fish survey in the Barents Sea and adjacent waters in
August–September 1998. 26 pp.
Anon. 1999. Internal cruise report ‘‘R/V Michael Sars’’ survey
No. 1999110. In Norwegian.
Arnold, G. P., and Greer Walker, M. 1992. Vertical movements
of cod (Gadus morhua L.) in the open sea and the hydrostatic
function of the swimbladder. ICES Journal of marine
Science, 49: 357–372.
Beamish, F. W. H. 1966. Vertical migration by demersal fish in
the Northwest Atlantic. Journal of Fisheries Research Board,
Canada, 23: 109–139.
Björnsson, B., Steinarsson, A., and Oddgeirsson, M. 2001.
Optimal temperature for growth and feed conversion of
immature cod (Gadus morhua L.). ICES Journal of Marine
Science, 8: 29–38.
Blaxter, J. H. S., and Tytler, P. 1972. Pressure discrimination in
Teleost fish: The effects of Pressure on organisms. Symposia
of the society for experimental biology, 26. The University
Press, Cambridge. 417–443.
Blaxter, J. H. S., and Tytler, P. 1978. Physiology and function
of the swimbladder. Advances in Comparative Physiology
and Biochemistry, 7: 311–367.
Brawn, V. M. 1962. Physical properties and hydrostatic function of the swimbladder of herring (Clupea harengus L.).
Journal of Fisheries Research Board, Canada, 19(4):
635–656.
Cressie, N. A. C. 1991. Statistics for Spatial Data. John Wiley
& Sons. 900 pp.
Dalen, J., and Nedreaas, K. H. 2001. Acoustic abundance
estimation and target strength observations of redfish
(Sebastes Mentella) in the Irminger Sea. Institute of Marine
Research, Bergen, Norway. 13 pp.
Einarsson, H. A. 2001. Length related diurnal vertical migration of cod (Gadus morhua L.), haddock (Melanogrammus
aeglefinus L.) and redfish (Sebastes spp.) in the Barents Sea.
Dept. of Fisheries and Marine Biology, University of Bergen.
Ellis, G. H. 1956. Observations on the shoaling behaviour of
cod (Gadus Gallarias) in deep water relative to daylight.
Journal of the Marine Biological Association of the UK, 35:
415–417.
Fernandes, P. G., Brierley, A. S., Simmonds, E. J., Millard,
N. W., McPhail, S. D., Armstrong, F., Stevemson, P., and
Sqires, M. 2000a. Oceanography: Fish do not avoid survey
vessels. Nature, 404(6773): 35–36.
Fernandes, P. G., Brierley, A. S., Simmonds, E. J, Millard,
N. W., McPhail, S. D., Armstrong, F., Stevemson, P., and
Sqires, M. 2000b. Oceanography: Fish do not avoid survey
vessels. Nature, 404(6801): 152.
Foote, K. G., Knudsen, H. P., Korneliussen, R. I., Nordbö,
P. E., and Röang, K. 1991. Postprocessing system for echo
709
sounder data. The Journal of the Acoustical Society of
America, 90: 37–47.
Godø, O. R., Somerton, D., and Totland, A. 1999. Fish
behaviour during sampling as observed from free floating
buoys – Application for bottom trawl survey assessment.
ICES CM 1999/J: 10.
Harden Jones, F. R. 1949. The teleostean swimbladder and the
vertical migration. Nature, 164: 847.
Harden Jones, F. R. 1951. The swimbladder and the vertical
movements of Teleostean fishes, I. Physical factors. Journal
of Experimental Biology, 28: 553–566.
Harden Jones, F. R. 1952. The swimbladder and the vertical
movements of Teleostean fishes, II. The restriction to rapid
and slow movements. Journal of Experimental Biology, 29:
94–109.
Harden Jones, F. R., and Scholes, P. 1974. The effect of
low temperature on cod, Gadus morhua. Journal du
Conseil International pour l’Exploration de la Mer, 35(3):
258–271.
Harden Jones, F., and Scholes, P. 1981. The swimbladder,
vertical movements, and the target strength of fish. In
Meeting on hydroacoustical methods for the estimation of
marine fish populations, 25–29 June 1979. II: Contributed
papers, discussion and comments. The Charles Stark
Draper Laboratory, Inc., Cambridge, Massachusetts, USA.
964 pp.
Harden Jones, F. R., and Scholes, P. 1985. Gas secretion and
resorption in the swimbladder of cod Gadus morhua. Journal
of Comparative Physiology, 155b: 319–331.
ISATIS. 1997. Transvalor and Geovariance Avon France.
Knudsen, H. P. 1990. The Bergen Echo Integrator: An introduction. Journal du Conseil International pour l’Exploration
de la Mer, 47: 167–174.
Korsbrekke, K., and Mehl, S. 2000. Abundance of saithe
Finnmiark-Møre autumn 2000. Int. rep. Institute of Marine
Research, Bergen, Norway, 20pp. In Norwegian with English
legends.
Korsbrekke, K., and Nakken, O. 1999. Length and species
dependent diurnal variation of catch rates in the Norwegian
Barents Sea bottom-trawl surveys. ICES Journal of Marine
Science, 56: 284–291.
Kristiansen, T. S., Michalsen, K., Jacobsen, J. A., and Huse, I.
2001. Optimal selection of temperature areas by juvenile cod
(Gadus morhua L.) in the Barents Sea modeled by dynamic
optimization. ICES Journal of Marine Science, 58: 172–182.
Kutchai, H., and Steen, J. B. 1971. The permeability of the
swimbladder. Comparative Biochemistry and Physiology,
39A: 119–123.
MacLennan, D. N., Fernandes, P. G., and Dalen, J. 2001. A
consistent approach to definitions and symbols in fisheries
acoustics. ICES Journal of Marine Science (in press).
Magnússon, J., Magnússon, J. V., Sigurðsson, Þ., Reynisson,
P., Hammer, C., Bethke, E., Pedchenko, A., Gavrilov, E.,
Melnikov, S., Antsilerov, M., and Kiseleva, V. 1996. Report
on the Joint Icelandic/German/Russian Survey on Oceanic
Redfish in the Irminger Sea and Adjacent Waters in June/
July 1996. ICES CM 1996/G: 8 Ref. H.
Marshall, N. B. 1972. Swimbladder organization and depth
ranges of deep-sea teleosts. Symposia of the Society for
Experimental Biology, 26: 261–272.
Mehl, S. 1997. Botnfiskundersøkingar I Barentshavet (Norsk
sone) vinteren 1997. Fisken og Havet, nr. 11. 72 pp.
Mehl, S. 1998. Botnfiskundersøkingar I Barentshavet (redusert
område) vinteren 1998. Fisken og Havet, nr. 7. 69 pp.
Mehl, S. 1999. Botnfiskundersøkingar I Barentshavet vinteren
1999. Fisken og Havet, nr. 13, 70 pp.
Mehl, S., and Nakken, O. 1996. Botnfiskundersøkingar I
Barentshavet vinteren 1996. Fisken og Havet, nr. 11. 68 pp.
710
B. K. Stensholt et al.
Michalsen, K., Aglen, A., and Pennington, M. 2000. Report on
demersal fish surveys in the Barents Sea and Svalbard area
during summer/autumn 2000. Internal report Institute of
Marine Research, Bergen, Norway. 41 pp.
Midttun, L. S. 1965. The relation between temperature
conditions and fish distribution in the Southeastern Barents
Sea. ICNAF Special Publication, 6: 213–220.
Monstad, T., Belikov, S. V., Shamrai, E. A., and McFadzen,
I. R. B. 1995. Investigations on Blue Whiting in the area west
of the British Isles, spring 1995. ICES CM 1995/H: 7 Pelagic
fish committee, Ref. B, 21 pp.
Monstad, T., Belikov, S. V., and Shamrai, E. A. 1996. Report
of the joint Norwegian-Russian acoustic survey on Blue
Whiting during spring 1996. ICES CM 1996/H: 12 Pelagic
fish committee, 23 pp.
Nedreaas, K. H. 1995. Annual acoustic surveys on 2-5 year old
North-East Arctic saithe in October. ICES Saithe study
group, Aberdeen 30 May–2 June 1995, 8 pp.
Neilson, J. D., and Perry, R.I. 1990. Diel vertical migrations of
marine fishes: an obligate or facultative process? Advances in
Marine Biology, 26: 115–168.
Ona, E., and Godø, O. R. 1990. Fish reaction to trawling
noise: the significance for trawl sampling. Rapports et
Procès-Verbaux des Réunions du Conseil International pour
l’Exploration de la Mer, 189: 159–166.
Ona, E., and Mitson, R. B. 1996. Acoustic sampling and signal
processing near the seabed: the deadzone revisited. ICES
Journal of Marine Science, 53: 677–690.
Righton, D., Metcalfe, J. D., and Arnold, G. P. 2001. Vertical
reality: utilizing knowledge of cod behaviour to interpret
survey results. ICES Annual Science Conference 2001, CM
2001/Q: 20, 15 pp.
Rose, G. A., and Porter, D. R. 1996. Target-strength studies on
Atlantic cod (Gadus morhua) in Newfoundland waters.
Simmond, E. J., and MacLennan, D. N. (eds.) Proceedings of
an ICES International Symposium on Fisheries and Plankton Acoustics, Aberdeen, Scotland, 12–16 June 1995. ICES
Journal of Marine Science, 53(2): 259–265.
Ross, L. G. 1976. The permeability to oxygen of the swimbladder of the meso-pelagic fish Ceratoscopelus maderensis.
Marine Biology (Berl.), 37: 83–87.
Ross, L. G. 1979. The permeability to oxygen and the guanin
content of the swimbladder of a physoclist fish, Pollachius
viren. Journal of the Marine Biological Association of the
UK, 59: 437–441.
Stensholt, B. K. 2001. Cod migration patterns in relation to
temperature: analysis of storage tag data. ICES Journal of
Marine Science, 58: 770–793.
Stensholt, B., and Nakken, O. 2001. Environmental factors,
spatial density and size distributions of 0-group fish. In
Proceeding of the symposium on Spatial Processes and
Management of Marine Populations, Oct 27–30, 1999,
pp. 395–413. Ed. by G. H. Kruse et al. Anchorage Alaska,
USA.
Stensholt, B. K., and Sunnanå, K. 1996. Spatial distributions of
variables in marine environ-mental and fisheries research:
Geostatistics and autocorrelated environmental and fisheries
data. ICES Annual Science Conference, Reykjavik, Iceland
27 Sept–1 Oct 1996. 27 pp.
Sundnes, G., and Bratland, P. 1972. Notes on the gas content
and neutral buoyancy in physostome fish. Fiskeridirektoratets skrifter, Serie Havundersøkelser, 16: 89–97.
Tytler, P., and Blaxter, J. H. S. 1973. Adaptation by cod and
saithe to pressure changes. Netherlands Journal of Sea
Research, 7: 31–45.
Tytler, P., and Blaxter, J. H. S. 1977. The effect of swimbladder
deflation on pressure sensitivity in the saithe Pollachius
Virens. Journal of the Marine Biological Association of the
UK, 57: 1057–1064.
Woodhead, P. M. J., and Woodhead, A. D. 1959. The effects of
low temperatures on the physiology and distribution of the
cod, Gadus morhua L., in the Barents sea. Proceeding of the
Zoological Society of London: 181–199.
Woodhead, P. M. J., and Woodhead, A. D. 1965. Seasonal
changes in the physiology of the Barents Sea cod, Gadus
morhua L., in relation to its environmient. II. Physiological
reactions to low temperature. ICNAF Special Publications,
6: 717–734.