ICES Journal of Marine Science, 59: 594–603. 2002
doi:10.1006/jmsc.2002.1193, available online at http://www.idealibrary.com on
Effect of deviations from target speed and of time of day on
catch rates of some abundant species under North Sea
International Bottom Trawl Survey protocol conditions
Sara Adlerstein and Siegfried Ehrich
Adlerstein, S. A., and Ehrich, S. 2002. Effect of deviations from target speed and of
time of day on catch rates of some abundant species under North Sea International
Bottom Trawl Survey protocol conditions. – ICES Journal of Marine Science, 59:
594–603.
Effort in trawl surveys is standardized by using a common gear and fixing haul
duration and vessel speed. Such standardization should result in a fairly constant
distance trawled and area or volume swept. Protocols for the North Sea International
Bottom Trawl Survey (IBTS) establish hauls of 30 min at a target speed of 4 knots over
ground with a standard GOV (Grande Ouverture Verticale) trawl. Primarily to
evaluate the effect of departures from the target speed and of trawl speed through
water, a fishing experiment was performed under IBTS protocol conditions. The
experiment consisted of 30 hauls made by RV ‘‘Walther Herwig III’’ in a small area in
the northern North Sea during 5 days in November 1997. Speed over ground was
calculated from the distance between GPS shooting and hauling positions. Current
speed and direction were continuously recorded by a current metre set a few metres
above the sea bottom in the centre of the area to allow trawl speed through water to
be calculated. We used generalized additive and linear models to analyse variation
in catch rates of Norway pout (Trisopterus esmarki), haddock (Melanogrammus
aeglefinus), whiting (Merlangius merlangus), dab (Limanda limanda) and grey gurnard
(Eutrigla gurnardus) with speed over ground and through water, and also with area
and volume swept by the gear, together with time of day to account for diel
fluctuations. Catch rates of small haddock and whiting, grey gurnard and dab
increased significantly with speed over ground while rates of Norway pout and large
whiting increased with speed through water. We propose that this difference is
indicative of the vertical distribution of the fish. Most affected by speed were small
haddock and whiting: catches in numbers doubled within the range of 3.9 to 5.2 knot
ground speed observed during the experiment. Catches of large haddock were stable.
Area swept affected small haddock and whiting and volume swept affected small
haddock only. With the exception of large whiting, catch rates of all species and sizes
varied with time of day typically within a factor of 2 between day and night.
2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.
All rights reserved.
Keywords: diel vertical migration, fishing experiment, speed over-ground, survey
protocols.
Received 23 July 2001; accepted 15 February 2002.
S. A. Adlerstein, and S. Ehrich: Bundesforschungsanstalt für Fischerei, Institut für
Seefischerei, Palmaille 9, D 22767 Hamburg, Germany. Correspondence to S. A.
Adlerstein: University of Michigan, School of Natural Resources, Museum Annex
138/Great Lakes Science Center, 1451 Green Road, Ann Arbor, MI 48105-2807,
USA. Tel: +1-734-214-9392; fax: 1-734-994-8780; e-mail: [email protected],
[email protected].
Introduction
The International Bottom Trawl Survey (IBTS) provides
annual recruitment estimates for a variety of commercially important North Sea fish species for stock assess1054–3139/02/060594+10 $35.00/0
ment (ICES, 2000a). Catches of older fish have been
used in fishery-independent assessment of haddock and
whiting stocks (Cook, 1997). Because several countries
and ships participate in the survey, much effort has been
put in standardizing survey methods. For instance, the
2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.
Catch rates of some abundant species under North Sea IBTS protocol conditions
IBTS manual (ICES, 1999) recommends a target ground
speed of 4 knots and a haul duration of 30 min. In
addition, information on actual shooting and hauling
positions and on gear geometry is requested for each
haul. In principle, such information might be used for
correcting for deviations in swept area, but its collection
is not mandatory and therefore corrections would only
be possible for a subset of all hauls made during any
particular survey. In practice, abundance estimates are
based on average catches per hour trawling (ICES,
2000b). In contrast, abundance indices of groundfish species from NAFO (North Atlantic Fisheries
Organisation) research vessel surveys are based on corrections for the deviation in the distance travelled during
a haul from the recommended distance based on a
ground speed of 3.5 knots (Halliday and Koeller, 1981).
To investigate the effects of the difference between
actual speed relative to target speed and of associated
variations in gear geometry on catch rates under normal
IBTS conditions, a small-scale experiment has been
carried out in a restricted area over a short time span.
Hauls were distributed evenly over the 24-h period
during five consecutive days and the direction of trawling was chosen at random. We report on the analysis of
variation of catch rates from the experiment with vessel
speed over ground (equivalent to distance travelled),
trawl speed through water, area and volume swept, and
time of day as variables. Time of day was included as a
covariate, because catches of several species are known
to fluctuate with time of day (Adlerstein and Trumble,
1993; Ehrich and Gröger, 1989; Pitt et al., 1981; Wieland
et al., 1998). The analysis was restricted to abundant
species that were thought to be associated with the sea
bottom in varying degree and therefore could react
differently to variation of the covariates. We used generalized additive models GAMs (Hastie and Tibshirani,
1990) following for example Swartzman et al. (1992).
This method allows for investigating variables that may
explain the observed variance without imposing restrictions of a pre-set functional form. This flexibility was
considered advantageous because the effects may be
non-linear.
Material and methods
Data
Catch data as well as information on vessel performance, gear geometry and environmental conditions were
collected for 30 hauls made by RV ‘‘Walther Herwig
III’’ between 22 and 27 November 1997. The experiment
was carried out within a restricted area in the northern
North Sea around 58N and 1W (Figure 1), one of the
‘‘boxes’’ of the annual German Small-Scale Bottom
Trawl Survey (GSBTS; Ehrich et al., 1998). The area
was selected because previous studies had indicated
595
62°N
60
58
56
54
52
50
− 4°W
−2
0
2
4
6
8
10°E
Figure 1. Distribution and towing direction of randomized
trawl hauls during the fishing experiment and location of the
study area.
optimum characteristics for the investigation of the
variables of interest, i.e. haddock and whiting were
generally abundant and fairly evenly distributed and
environmental parameters (such as sediment type,
depth, temperature and salinity) that might influence
catch rates are homogenous. By restricting the experiment to a 5-d period, variability caused by immigration
or emigration was minimized. Although the approach
may put limitations on the general validity of the
conclusions drawn, the advantage is that the covariates
of interest could be investigated in isolation from nonaccountable variables.
During the experiment, bottom temperature varied
from 10.6 to 10.8C and salinity from 35.18 to 35.22 psu.
Bottom currents were in a northerly direction with speed
varying from 3 to 32 cm s 1 with the tide. Weather
conditions remained fairly constant with mostly cloudy
skies and constant southeast winds of about 18 m s 1.
Day length was 8 h and 50 min (sunrise minus until
sunset plus 40 min).
Standard IBTS protocols were observed (ICES, 1999).
A standard GOV (Grande Ouverture Verticale) was
towed for exactly 30 min at a target speed of 4 knots.
Starting time was defined as the moment when the winch
had stopped at a predefined warp length. Shooting
positions and tow directions were chosen at random
(Figure 1). For each haul, speed and shooting and
hauling positions were determined by satellite navigator
(GPS). The catch was sorted and weighed, counted and
measured by species. If catches were large (e.g. haddock
and whiting), sub-samples of 200–400 individuals were
taken for length measurements.
Special effort was allocated to obtain information on
near-bottom water current characteristics and to
measure gear geometry. Current speed and direction
596
S. Adlerstein and S. Ehrich
600
(a)
500
400
300
catches represented bimodal length distributions with
peaks at 15 and 25 cm (Figure 2), while some hauls were
dominated by small or large fish. Accordingly, catch
rates of these two species <20 cm (0 group) and >20 cm
(mostly 1- and 2-yr-old fish) were analysed separately.
For other species, length compositions did not vary and
analysis was restricted to the total catch.
Analysis
Catch rates by species were modelled as a function of a
covariate representing either ground speed, speed
through the water, area or volume swept, plus time of
day. Ground speed in knots (GS) was calculated as twice
the geodesic distance (in nmi) covered in hauls of 30 min
duration. Trawl speed through water (TSW) in knots
was calculated as
200
100
0
TSW=GScos(VDCD)*(CS/51.4)
500
(1)
(b)
where VD and CD are vessel and current direction,
respectively, in radians, CS is near bottom current speed
in cm s 1 and 51.4 is the conversion factor to knots.
Swept areas SA1 and SA2 (m2) were calculated as the
distance D in metres times wingspread or doorspread,
respectively. Volume swept SV (m3) was calculated as
the product of the distance D and an estimate of the area
of the net opening. This area was calculated assuming a
constant ellipse shape during tows.
400
300
200
VS=*(W/2)*(H/2)*D
(2)
100
0
10
20
30
40
Figure 2. Bimodal length compositions of haddock and whiting
catches (all hauls combined).
were constantly measured with a current metre set a few
metres above the sea bottom in the centre of the survey
area. Wing and door spread and headline height
were simultaneously monitored with wireless SIMRAD
sensors.
Data from 27 hauls were selected for analysis. Three
day-time hauls conducted at low ground speeds (<3.7
knots) were excluded because matching night tows
were lacking. Species evaluated are: Norway pout
(Trisopterus esmarki), haddock (Melanogrammus
aeglefinus), whiting (Merlangius merlangus), grey
gurnard (Eutrigla gurnardus), and dab (Limanda
limanda).
To guide appropriate levels of aggregation in relation
to fish size, length frequency distributions of all species
were compared among hauls. Whiting and haddock
where W is wingspread, H is headline height and D is
distance travelled (all in m).
The effect of the covariates on catch rates was analysed using routines to fit GAMs and generalized linear
models GLMs (McCullagh and Nelder, 1989) contained
in the S-Plus programming environment (Becker et al.,
1988), based on Hastie and Tibshirani (1990) and functions developed by Venables and Ripley (2000). Separate
models were run for speed over ground and through
water, and areas and volume swept. All covariates were
first introduced as continuous smooth variables and
were modelled non-parametrically using smoothing
splines described in Hastie and Tibshirani (1990),
Chambers and Hastie (1992) and Venables and Ripley
(2000). The logarithmic-link was used to relate expected
catch rates to the predictors according to
where ) is a fitted constant, is the mean catch rate, fj
are univariate smooth functions for Xi (time of day) and
Xi (either ground speed, speed through water, area or
volume swept), and the errors are assumed independent of the Xjs. The probability distribution of catch
Catch rates of some abundant species under North Sea IBTS protocol conditions
597
Table 1. Summary of haul performance parameters during the fishing experiment (dist: distance
travelled by the vessel in nmi; GS: ground speed in knots; TSW: speed through water in knots; wings
and doors: measures of the spread of the trawl in m; SA1 and SA2: area swept by the wings and doors,
respectively, in m2; SV the volume swept by the wings in m3).
Min
Max
Mean
s.d.
Dist
GS
TSW
Wing
Doors
Headline
SA1
SA2
SV
1.81
2.59
3.00
0.16
3.69
5.18
4.20
0.32
3.54
5.29
4.28
0.36
18.3
22.1
20.6
0.8
101
122
111
5
3.9
4.7
4.2
0.2
64 600
99 800
79 500
7 600
348 000
540 000
427 000
43 000
210 000
326 000
259 000
22 000
rates for all species was determined by regressing the
logarithm of the mean catch rates (by 4 h time and 0.2
knots speed intervals) against the logarithm of the
variance.
Explanatory variables were assessed using F-tests to
determine whether they explained a significant portion
of the corresponding model deviance (Hastie and
Tibshirani, 1990). The non-linearity and the appropriate
degrees of freedom of the smooth variables were
assessed (following Venables and Ripley, 2000) by
jointly increasing and decreasing the degrees of freedom
in the compared models (up to a linear term) until no
significant fit improvement was obtained at the 95%
confidence level. Additionally, time was modelled as a
day/night factor.
Results
The mean, range and standard deviation of haul performance parameters like distance travelled, speed over
ground and through water, wing and door spreads, areas
swept by the wings and doors and swept volume are
given in Table 1 and in Figure 3. Ground speed deviated
up to 30% from the target speed. The trawl geometry
measurements showed no unstable bottom contact
conditions.
Catch rates were highest for haddock and whiting
>20 cm (up to 3000 fish per tow) and lowest for grey
gurnard (up to 200 fish per tow; Figure 3). Catches of
the selected species were never zero. Frequency distributions of catch rates varied from almost normal (large
haddock) to heavily skewed (small haddock; Figure 4)
and thus the slopes of regressions between the logarithms of the mean catch rates and the variance varied
from zero to around 2 (Table 2). Accordingly, catch
rates of large haddock were modelled with a normal
distribution (constant variance), rates of small haddock
with a Poisson distribution (variance proportional to the
mean) and that of large and small whiting, dab, grey
gurnard, and Norway pout with a gamma distribution
(variance proportional to the mean squared). Check of
the residuals indicated adequate model fits.
Analysis of deviance indicates that catch rates of all
species and size groups, with the exception of large
whiting, varied significantly with time of day (p<0.05;
Table 2). This variable explained up to 55% of the total
variation in the catches of large haddock. The variation
caused by time of day was significantly non-linear
(Figure 5), except for small whiting, and 3 degrees of
freedom were found adequate to model the effect on
catch rates for the remaining species. For small whiting,
a significant difference was found between day and night
catches. Catch rates for Norway pout, and haddock
(both size classes) tended to be higher during the day
while those of dab, grey gurnard and small whiting were
higher at night. The largest difference was observed in
dab, night catches being more than double the day
catches (Figure 5; Table 3).
The analysis of deviance also indicates that the effect
of ground speed was linear and significant in the model
for small haddock (p<0.01), small whiting (p=0.05) and
dab (p=0.05; Table 2). The effect of ground speed
explains up to 18% of the variation in small haddock.
The effect of speed through water was also linear in the
model and significant for small haddock and Norway
pout and at the 90% level for small and large whiting,
explaining up to 12% (small haddock) of the variation.
The variation with area swept by the wings was significant for small haddock (p=0.01) and small whiting
(p=0.05) and the area swept by the doors only for small
haddock (p=0.04). Volume swept significantly affected
catches of small haddock only (p=0.02). In the case of
large whiting, the first 8 consecutive hauls were characterized consistently by high catches (Figure 3). If this
was caused by a strong biological association such as a
feeding ‘‘hot spot’’ situation, the effect from the variables investigated might have been masked. Results of
an analysis excluding these stations still indicated nonsignificant effects of time of the day and vessel ground
speed, but a nearly significant effect of trawling speed
through water (p=0.06; Table 2).
Because catch rates varied linearly with speed, area
and volume covariates, the slopes of the effects could be
estimated (Table 3). Fitted values for ground speed and
water speed covariates are presented in Figures 6a and b,
respectively. Slopes for the effect of speed over ground
were highest (0.63–0.86) for small haddock, small
whiting, grey gurnard and dab. Slopes for Norway pout
598
S. Adlerstein and S. Ehrich
Numbers/30 min
3000
Norway pout
Large whiting
Large haddock
Norway pout
2500
8
6
4
2
0
2000
1500
200 400 600 800 1000 1200 1400
1000
Whiting > 20 cm
500
6
4
2
0
0
Numbers/30 min
3000
500
Small whiting
Small haddock
2500
1000
1500
1000
2500
3000
0
500
1000 1500 2000 2500 3000
500
Haddock < 20 cm
0
250
Numbers/30 min
2000
Haddock > 20 cm
6
4
2
0
2000
1500
6
4
0
Grey gurnard
Dab
500
200
1000
1500
2000
2500
3000
Whiting < 20 cm
150
8
4
0
100
50
0
0
500
1000 1500 2000 2500 3000
Grey gurnard
GS
WS
5.5
Speed (knots)
6
4
2
0
5.0
0
100
150
200
250
Dab
6
4
2
0
4.0
3.5
0
Swept area (m2)
and volume (m3)
50
4.5
SA1
SA2
SV
600
500
300
200
100
5
10
15
Haul
100
150 200
Numbers/hr
250
300
Figure 4. Frequency distribution of catch rates by species/sizes.
400
0
50
20
25
30
Figure 3. Variation in catch rates of species/sizes and in
covariates (GS: vessel speed over ground; WS: trawl speed
through water (WS); SA1, SA2: swept areas between wings
and doors, respectively; SV: swept volume) during the consecutive 30 hauls (day hauls: 6–10, 13–18 and 26–29; hauls 16, 17
and 29 were excluded from the analysis).
and large whiting and haddock were negligible. In
contrast, Norway pout and large whiting exhibited steep
slopes for the effect of speed through water (0.46–0.47),
although slopes for small haddock and small whiting
were still relatively high (0.5–0.6). Steepest slopes in both
cases are for small haddock and whiting. Slopes for large
haddock, dab and grey gurnard were insignificant. For
the area swept by the wings, highest values were
obtained for small haddock and whiting (0.00003–
0.00004) and for the area swept by the doors for grey
gurnard (0.000009), small haddock and whiting
(0.000005), and dab (0.000004). Finally, the highest
slopes for the effect of the volume swept were also for
Catch rates of some abundant species under North Sea IBTS protocol conditions
599
Table 2. Mean catch rates (C in #/30 min; standard deviation in brackets) and results of analysis of
deviance from GAM by species: catch rates are modelled as smooth function of time of day (tod) and
linear functions of other covariates (see Table 1); probabilities for each term are given and the
percentage of the deviance explained in brackets; b: slope of the regression of the logarithms of mean
and variance.
Species
Norway pout
Large whiting
Large whiting†
Large haddock
Small haddock
Small whiting
Grey gurnard
Dab
C
GS
TSW
SA1
SA2
SV
tod
b
610
(320)
1153
(623)
894
(348)
1251
(649)
782
(568)
644
(371)
71
(57)
84
(66)
0.77
<1%
0.95
<1%
0.86
2%
0.44
1%
<0.01
18%
0.05
13%
0.24
2%
0.05
7%
0.05
8%
0.16
5%
0.06
16%
0.26
2%
0.04
12%
0.10
8%
0.97
<1%
0.57
<1%
0.77
<1%
0.99
<1%
0.50
3%
0.81
>1%
0.01
15%
0.05
12%
0.63
<1%
0.15
3%
0.74
<1%
0.98
<1%
0.45
3%
0.65
>1%
0.04
13%
0.09
9%
0.96
<1%
0.37
1%
0.17
4%
0.99
<1%
0.75
<1%
0.61
<1%
0.02*
14%
0.18
5%
0.70
<1%
0.16
3%
<0.001
41%
0.23
19%
0.31
20%
<0.001
55%
<0.001
34%
0.05
14%
<0.001
30%
<0.001
32%
1.86
p<0.001
1.91
p<0.001
1.83
p<0.001
0.34
p>0.1
0.96
p<0.001
1.94
p<0.001
1.74
p<0.001
1.78
p<0.001
†Selected data excluding eight hauls with relatively high catches.
Table 3. Slopes in the logarithmic link scale of the effect of various covariates (for abbreviations see
Table 1) in a GAM for each species/size class and coefficients of the time of day effect (tod) introduced
as factor in a GLM (*: significant at 95%; ne: not estimated because effect not significant).
Species
Norway pout
Large whiting
Large whiting†
Large haddock
Small haddock
Small whiting
Grey gurnard
Dab
GS
TSW
AS1
AS2
SV
tod
0.09
0.08
0.05
0.12
0.86*
0.78*
0.63
0.67*
0.46*
0.39
0.47*
0.19
0.60*
0.46
0.05
0.19
4e-6
1e-7
8e-6
2e-7
4e-5*
3e-5*
1e-5
2e-5
7e-7
4e-8
1e-7
4e-7
5e-6*
5e-6
9e-6
4e-6
5e-7
3e-9
1e-6
1e-6
1e-5*
1e-5
4e-6
9e-6
0.52*
ne
ne
0.76*
0.70*
+0.20*
+0.70*
+0.97*
†Selected data excluding eight hauls with relatively high catches.
small haddock and whiting (0.00001). Thus, the catch of
small whiting and haddock, grey gurnard and dab
roughly doubles with an increase of the speed covariate
from about 4 to 5 knots, equivalent to a difference in
distance trawled of around 1000 m in 30 min, in the area
swept by the doors of about 20 000 m2, in the area swept
by the wings of about 100 000 m2 and in the volume
swept of about 50 000 m3.
Discussion
The use of a common factor across species to correct
survey data from tows of fixed duration for speed/
distance travelled may easily lead to biased estimates of
abundance indices, because our results indicate that
vessel performance affects species and size groups differ-
ently. Catch rates of small haddock, small whiting, grey
gurnard and dab increased by a factor of two with a
variation of target speed from 4 to 5 knots while catches
of Norway pout, large whiting and large haddock were
not significantly affected. Effects may differ because the
reaction to the fishing gear is determined partially by the
fish distribution in the water column, their size or shape,
and behaviour (Engås and Godø, 1986; Aglen, 1996;
Godø, 1990; Fréon et al., 1993). We propose that the
differences found reflect the degree of association of the
fish to the seabed, i.e. catch rates of fish closely associated with the seabed are more affected than more pelagic
fish. Since our results are restricted to particular conditions, studies are needed to investigate how the
effect of towing speed/distance varies by species in time
and space, preferably in combination with studies of
600
S. Adlerstein and S. Ehrich
s(time, 3)
0.5
0.0
–0.5
Norway pout
–1.0
s(time, 3)
0.5
0.0
–0.5
Haddock > 20 cm
0.5
s(time, 3)
0.0
–0.5
–1.0
Haddock < 20 cm
–1.5
1.5
Grey gurnard
s(time, 3)
1.0
0.5
0.0
–0.5
–1.0
Dab
s(time, 3)
1.0
0.5
0.0
–0.5
–1.0
0
5
10
15
Time
20
24
Figure 5. Fitted time-of-day (incorporated as smooth variable)
effects (scaled so that 0 corresponds to the mean; solid lines)
and approximate 95% confidence bands (dotted lines) from
GAM for those species/sizes exhibiting significant non-linear
variation in catch rates, while accounting for the linear effect of
speed over ground or speed through water (whichever explained
more of the variation). Marks on the x-axis represent individual
observations.
their distribution over the water column. Meanwhile,
emphasis should be put on compliance with prescribed
target speed and on actually measuring water speed.
If cpue is representative of local abundance, an
increase in speed or area swept should result in
higher catches. However, this was not observed in large
haddock. Main and Sangster (1981) observed that large
haddock may escape over the headline in substantial
numbers. Such behaviour in combination with a lower
vertical net aperture caused by increased trawling speed
(as observed by Koeller, 1991, and also in this experiment) could make escapement over the headline easier.
In addition, increased speed intensifies the noise produced by survey vessels which shortens the fish
reaction time to the gear (Neproshin, 1979; Ona and
Chruickshank, 1986; Olsen et al., 1982; Olsen, 1990;
Ona and Godø, 1990). Thus, large haddock may have
better chances to escape at higher speed than at lower
speed given the effects on net geometry as well as
their behavioural response and swimming capabilities.
Overall, decreased catchability might compensate for the
longer distance trawled. Another possible explanation
might be that the number of hauls in this experiment has
been insufficient to detect the effect. This could be due to
the aggregation of large haddock in schools if catchability depends on tow direction relative to shape and
direction of the schools.
Maintaining a constant ground speed does not guarantee that catches are not biased by variation in trawling speed through water. Norway pout catches were
unaffected by ground speed but increased significantly
with speed through water. Catches of small haddock and
all sizes of whiting were also significantly affected.
Unfortunately, maintaining constant speeds both
through water and over ground is impractical. Thoughts
must be given to defining tow direction with respect to
bottom currents.
Our analysis of swept-area and volume effects is
deceptive and shows – for a 50% increase in these parameters – only a significant increase for small haddock
and marginally so for whiting. The conclusion may be
that our estimates of area and volume swept are imprecise. The geometry of the net changes during trawling,
for instance by filling of the net (Koeller, 1991), and our
assumption of a rigid ellipse form may be too restrictive.
Although the gear was always in contact with the
bottom, also changes in the contact area between net and
seabed may make estimates of swept area suspect. Furthermore, the effective width of the trawl may be more
than the wingspread because of herding effects of the
sweeps but less than the doorspread because of escapement over the sweeplines. We share the view of Koeller
(1991) who states that the effective area swept by a gear
is virtually always unknown.
Significant variation (up to a factor of 2) between day
and night catch rates of all species analysed (with the
Catch rates of some abundant species under North Sea IBTS protocol conditions
Norway pout
Haddock < 20 cm
1.0
Partial for TSW
Partial for GS
601
0.5
0.0
0.5
0.0
–0.5
–0.5
1.0
Whiting < 20 cm
Partial for TSW
Partial for GS
1.5
1.0
0.5
0.0
Haddock < 20 cm
0.5
0.0
–0.5
–0.5
1.0
Grey gurnard
Partial for TSW
Partial for GS
1.0
0.5
0.0
–0.5
0.5
0.0
–0.5
3.5
Partial for GS
1.5
Whiting < 20 cm
Dab
4.0
4.5
TSW
5.0
1.0
0.5
0.0
–0.5
4.0
4.2
4.4
4.6
GS
4.8
5.0
5.2
Figure 6. Fitted time-of-day (incorporated as a covariate) effect from GAM for species/sizes exhibiting significant linear variation
in catch rates with (a) vessel speed over ground (GS); (b) trawling speed through water (TSW). See also Figure 5 for explanation.
exception of large whiting) are in line with most previous
findings for the North Sea (Wieland et al., 1998; Ehrich
and Gröger, 1989) and the Barents Sea (Engås and
Soldal, 1992; Michalsen et al., 1996; Aglen et al., 1999).
In contrast, Wieland et al. (1998) report that catches of
1- and 2-yr-old whiting were higher during the day than
at night and Aglen et al. (1999) report a pelagic distribution of large haddock during the day. Such variations
are not surprising because diel fluctuations may be
related to light in different ways. We suggest that species
characterized by higher night catches are more closely
associated with the seabed while higher day catches are
indicative of more pelagic fish. If all species were staying
deeper in the water column during day time than at
night, more benthic species escaping under the ground
rope during the day might become more vulnerable as
they move slightly off bottom at night, while less benthic
species that are available to the gear during the day
might escape over the headline at night. Walsh (1989)
found that bottom trawls were more efficient at night in
catching fish with demersal habits. Dahm and Wienbeck
(1996) demonstrated that losses of grey gurnard of
602
S. Adlerstein and S. Ehrich
around 40% occur beneath the GOV trawl, while losses
of haddock, whiting and Norway pout (over all sizes)
were under 10%. Irrespective of the mechanisms
involved, our results endorse the recommendation in the
IBTS manual to limit tows to daylight. In practice, over
20% of records in the IBTS database collected from
1990–1999 were from night hauls. It may be better to
exclude these when calculating abundance indices, as
long as there are no definitive models by species to
correct these data for bias. This necessarily requires
further investigation. An alternative for the future is to
randomize survey design with respect to time.
Effects of speed over ground and distance travelled on
catch rates cannot be differentiated in studies from hauls
of fixed duration. Nevertheless, one aspect to notice here
is that the highest effect for covariates related to speed
was found for small whiting and haddock. This is in
line with general relationships found between fish size,
swimming speed and endurance (He, 1993) that predict
that small fish should be less able to escape faster
moving gear. After all, selectivity occurs mainly during
herding, swimming with the trawl in the mouth area and
in the cod end (He, 1993) and is always related to
swimming behaviour and endurance.
In summary, diel variation in catch rates and effects of
trawling speed are bound to be species and size specific
and could vary with season and site depending on fish
behaviour. Further research, including fishing experiments at fine resolution, is needed to evaluate the
general applicability of our findings. Nevertheless, our
results might be used to establish data selection criteria
for calculating abundance indices, to stress the importance of complying with survey protocols, and to
raise the point of randomizing hauls with respect to
extraneous variables.
Acknowledgements
We thank Holger Klein (Bundesamt für Seeschifffahrt
und Hydrographie) and Manfred Stein (Bundesforschungsanstalt für Fischerei, Institut für Seefischerei)
for providing the hydrographic data, and N. Mergardt
and C. Hammer (Institut für Seefischerei) for providing the catch data. Funding support was provided by
the European Commission; Directorate General Fisheries (DG XIV) under project contract 98/029 (‘‘Surveybased abundance indices that account for fine spatial
scale information for North Sea stocks’’ – FINE).
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