ICES Journal of Marine Science, 56: 561–570. 1999
Article No. jmsc.1999.0484, available online at http://www.idealibrary.com on
Acoustic studies of spatial gradients in the Baltic:
Implications for fish distribution
A. Orłowski
Orłowski, A. 1999. Acoustic studies of spatial gradients in the Baltic: Implications for
fish distribution. – ICES Journal of Marine Science, 56: 561–570.
Acoustic methods play an increasingly important role in studies of fish group
behaviour in relation to environmental factors. They are also becoming a promising
tool in the evolution of new standards in marine ecosystems research. This paper
presents two approaches to treating acoustic, biologic, and hydrographic data,
collected during surveys of significantly large spatial units of the marine ecosystem.
Both methods are designed for studying the spatial structure of abiotic and biotic
factors. Firstly, a method for estimation of vertical gradients in environmental factors
is defined. Corresponding to the main range of fish occurrence, it characterises fish
distributions by day and night in the seasons spring, summer, and autumn of the years
1983–1996. Secondly, the method of matrix macrosounding, which correlates acoustic
and hydrographic data, has been improved and employed to study the horizontal
gradients in fish distribution linked with environmental structures. A few applications
of the method for short-term and long-term studies are shown and discussed.
1999 International Council for the Exploration of the Sea
Key words: acoustics, spatial gradients, environment, fish distribution, Baltic.
Received 7 May 1998; accepted 12 April 1999.
A. Orłowski: Sea Fisheries Institute, PO Box 345, 81-332 Gdynia, Poland. Tel: +48 58
620 1728 ext. 215; fax: +48 58 620 2831; e-mail: [email protected]
Introduction
Marine ecosystems are characterized by well-defined
borders between sea water and the seabed. Seabed
configuration has a basic impact on the stratification of
physical and chemical properties of sea water. Such a
stratification which is variable in both vertical and
horizontal directions determines the abiotic conditions
in the aquatic habitat. The most significant elements of
stratification are formed by spatial gradients in environmental properties (e.g. temperature, salinity, oxygen,
nutrients). Those properties of sea water have a direct
influence on its gravity, viscosity, and energetic or
chemical condtions (Barnes and Mann, 1991). Spatial
gradients in the environmental structure influence the
distribution of organisms, conditioning primary and
secondary production, fish and plankton production and
reproduction, and nekton and benthos interactions.
The importance of the observations and analysis of
spatial gradients in abiotic and biotic factors at a scale
that matches the ecosystem structure is obvious. The
usefulness of acoustic methods. for this purpose was
reviewed by Holliday (1993). New examples, appear
year-by-year, e.g. Castillo et al. (1996), Corten (1997),
1054–3139/99/040561+10 $30.00/0
Orłowski (1997), Porazinski (1997), Roe (1996), and
Tameshi et al. (1996). The matrix macrosounding
method applied to data in this paper was first described
by Orłowski (1998).
When the localization of fish (schools or single fish) is
acoustically determined and the intensity of reflected
sound (target strength, volume, area, or column backscattering strength) is recorded it is possible to correlate
both with the magnitudes of selected abiotic parameters
simultaneously measured in the same area. Such a
multi-factor analysis is not simple and requires the
identification of a series of procedures to give adequate
and comparable information. A proposal is made
here of such procedures which apply acoustic, biologic,
and hydrographic data to estimate precisely defined
environmental fish distribution characteristics.
A method for the estimation of vertical gradients in
basic environmental factors, corresponding to the main
range of fish occurrence, is defined and applied to
characterize fish distributions for day and night during
spring, summer, and autumn and the years 1983–1996.
Furthermore the method of matrix macrosounding
has been improved and employed to study horizontal
gradients in fish distribution. A few examples of its
1999 International Council for the Exploration of the Sea
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A. Orłowski
application in the short- and long-terms, respectively,
are shown and discussed.
Materials
During the period 1981–1997 ships of the Sea Fisheries
Institute in Gdynia carried out a series of research
cruises collecting acoustic, biological, and environmental materials in the area of the southern Baltic. The
first cruises were conducted during the summer and
spring seasons and the last six were organized during the
autumn as part of an ICES monitoring programme of
pelagic fish stocks in the Baltic. Each cruise lasted
approximately 2 weeks, and had the potential to collect
data from more than 1000 nmi of acoustic transect. An
analysis of the data bank from all cruises permitted
selection of eight surveys suitable for further environmental studies. The selected surveys represent the
springs of 1983 and 1985, the summers of 1983 and 1988,
and the autumns of 1989, 1990, 1994, 1995, and 1996.
For ease of identification in this paper cruises are
designated by the two last digits of the year and the
number of month, e.g. 8305 means the cruise of May
1983.
Acoustic samples (echo integrations, echograms) were
collected continuously, 24 hours a day, at an acoustic
frequency of 38 kHz. The time distribution of samples
was homogeneous and this gave a good basis for the
analysis of diel fish behaviour characteristics. The
acoustic magnitudes were collected over 1 nmi intervals
but the average for each five (1981–1989) or 4 nmi
(1990–1996) was taken as most representative to
minimize autocorrelation effects (Orłowski, 1989). The
results of echo integration were converted into values of
volume backscattering strength (Sv), column scattering
strength (Svc), and area backscattering coefficient (SA),
following Knudsen’s definition (1990). Acoustic magnitudes were related to the bottom depth, depth of the
lower and upper limits of fish layers, and depth of the
median of depth distribution of scatterers (depth centre
of gravity of biomass). The hydroacoustic system was
calibrated in situ by hydrophone, controlled from
SIMRAD hull monitor unit (1981–1993) or by standard
target (1994–1997). Precise calibration of the acoustic
system is not essential for this particular analysis. The
consistency of the system parameters was confirmed by
successive calibrations.
Biological samples were collected over the whole
period by the same type of pelagic trawl every 36 nmi of
acoustic transect on average. Hydrographic samples
(temperature, salinity, oxygen) were collected by Nansen
bottles (1981–1985) or a Neil-Brown CTD system
(1986–1997) approximately every 30 nmi. Oxygen
measurements were available for analysis from 1988.
The fish observed during the surveys were mostly pelagic
Figure 1. Basic elements applied in the estimation of the vertical
gradient (DF) of environmental factor (F) in relation to fish
distribution [P(F/S)]. DF range presentation below abscissa
corresponds to the presentation of gradients in Figures 2–4.
(94%), and comprised herring and sprat from the family
Clupeidae.
Vertical gradients
Method
Two significant rhythms have a basic influence on the
vertical structure of fish distribution: a short-term day
cycle and a long-term yearly cycle characterized by
different seasons. The first cycle is closely related to the
24-h day period and consequent variability of the zone
penetrated by light. In this case two configurations of
fish distribution may be distinguished: daytime and
night-time. The daytime is characterized by the presence
of shoal-like concentrations within a wide depth range.
During the night, most pelagic fish in the form of
scattering layers inhabit a reduced depth range in the
warmer near-surface waters. These fish distribution configurations are quasi-stable during the same season of
the year.
The long-term yearly cycle is dependent on the seasonal variability of total solar energy absorbed by sea
water and it is observed as a deep modulation of
environmental structure. In particular there are vertical
shifts of gradient in temperatures and changes of
average values of characteristic hydrographic factors.
If we take all of these into consideration we can
conclude that the day and the night ranges of selected
environmental factors, can be estimated for different
seasons of the year and used to characterize the influence
of abiotic vertical gradients on fish biomass distribution.
The characteristic range can be estimated (see Figure
1) on the basis of a probability density function P(F/S)
of selected factor values (F), empirically found, weighted
(normalized) by biomass density and expressed by SA
values. The range of selected factor DF, called ‘‘vertical
gradient’’ in this paper, is defined as an interval in which
Acoustic studies of spatial gradients
563
Figure 2. Vertical gradients (rectangles) in temperature (C), corresponding to main fish depths, for daytime (pattern 1) and
night-time (pattern 2), for different seasons and years (described on the right). During the summer, daytime gradients are estimated
for herring (pattern 4) and sprat (pattern 5) separately. Pattern 3 shows the effective range of temperature measured during the
same cruise. Circles inside rectangles represent values of medians.
the cumulative percentage of the P(F/S) distribution
comprises between 25 and 75% (DS). The gradient is
characterized by its range, and lower and upper limits.
Thus the range can be considered as a gradient of a
selected factor in relation to the vertical biomass
distribution. The value Fm, corresponding to 50% of
cumulative percentage (median) is considered as its
characteristic value. In this way a defined range of the
environmental factor (DFi) and its median (Fmi) can
be used for the individual characterization of abiotic
vertical gradients in fish biomass distribution.
Data over the period 1983–1996 in the southern Baltic
were analysed and the values of temperature, salinity,
and oxygen at the depth of fish biomass gravity centres
were calculated. Values of corresponding environmental factors were estimated only for acoustic data
units, distance intervals, closest to STD/CTD stations.
Empirical distributions of separate factors [P(Fi/S)] were
found and the characteristic gradients (DFi) and median
values of factors (Fmi) calculated.
In summer the thermocline is usually very strong and
practically divides the Baltic waters into two non-mixing
layers (Piechura, 1984). Due to this, statistical distributions of values of abiotic factors at characteristic fish
depths, were found to be bimodal during daytime.
Therefore analysis of vertical limits was necessary for
both subdistributions separately. Analysis of associated
biological data has indicated that one group (higher
temperatures, over 9C) were mostly associated with
sprat and the other one (below 9C) with herring.
Consequently ‘‘summertime/daytime’’, characteristic
gradients of abiotic factors were calculated for sprat and
herring separately. During the night the fish were
randomly distributed in the water column and the
separation of data was not needed.
To improve comparisons the effective range of a
measured hydrographic parameter was defined and
shown below the fish distribution gradients as an
environmental background. The range was defined as an
interval in which the cumulative percentage of distribution of all values of selected parameter, measured at
standard oceanographic levels (10 m intervals), lay
between 25 and 75%.
Results and discussion
Results of the application of the method described above
are shown in Figures 2–4 in the form of charts of vertical
gradients in temperature, salinity, and oxygen. The
abiotic gradients of fish distribution were calculated
separately for fundamental time-dependent configurations (day and night, spring, summer, and autumn
seasons). Calculations for 1994 were not possible
because of the insufficient number of hydrographic measurements. Comparable effective range and
median value of each selected parameter are shown for
each survey below the gradients associated with fish
distribution.
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A. Orłowski
Figure 3. Vertical gradients (rectangles) in salinity, corresponding to main fish depths, for daytime (pattern 1) and night-time
(pattern 2), for different seasons and years (described on the right). During the summer day gradients are estimated for herring
(pattern 4) and sprat (pattern 5) separately. Pattern 3 shows the effective range of salinity measured during the same cruise. Circles
inside rectangles represent values of medians.
Figure 4. Vertical gradients (rectangles) in oxygen (ml l 1), corresponding to main fish depths, for daytime (pattern 1) and
night-time (pattern 2), for different seasons and years (described on the right). Pattern 3 shows the effective range of oxygen
measured during the same cruise. Circles inside rectangles represent values of medians.
Temperature gradients
Temperature gradients of fish distribution were very
characteristic for each analysed situation (Figure 2). In
the spring (8305, 8505) the fish occurred in waters
between 2.4 and 6.5C during the day (pattern 1), and
between 3.8 and 7.2C during the night (pattern 2).
Gradients were very comparable to the effective ranges
of temperatures (pattern 2) in each survey. Day and
night temperature gradients were similar but the fish
depths differed considerably. This is best seen in Figure
2. The reason is associated with specific thermal structure of sea water in the spring, when the minimum
Acoustic studies of spatial gradients
temperature appears between the sea bottom and the sea
surface with temperature increasing towards both the
surface and the bottom, respectively. Values of temperature medians for the day (4.4C–8305, 3.5C–8505) were
lower than the night (5.6C–8305, 5.8C–8505).
In the summer, temperatures usually attain maximum
values with a direct impact on the associated gradients in
water occupied by fish. The fish occurred (8308, 8808) in
water between 3.8 and 17.1C during the day, and
between 7.5 and 14.7C during the night. The extreme
temperatures were more comparable in the upper range.
A more realistic picture appears when the daytime data
were separated into two main species, herring and sprat
as previously described. Extreme temperature limits for
herring (pattern 4) were between 3.4 and 5.8C but for
sprat were (pattern 5) between 12.9 and 18.3C. The
results were very similar for both (1983, 1988) cited
examples (Figure 2). Thus over the median values herring was associated with the lowest sub-range of effective
temperature range, while sprat was distributed within
the highest temperatures.
In the autumn, the mixing process is stronger and the
intensity of primary production is decreasing sharply.
Both factors influence the vertical gradients of temperature at the main depth of the fish. For the period
1989–1996 the fish occurred in waters between 4.7 and
12.7C during the day, and between 7.8 and 13.5C
during the night. Values of temperature medians for the
day (8.0C–8910, 10.6C–9010, 7.7C–9510, 6.3C–9610)
were considerably lower than for the night (12.2C–
8910, 12.1C–9010, 13.0C–9510, 10.0C–1996). Night
gradients were more closely dependent on the effective
range of temperature in each year. The differences
observed year-by-year among the autumn gradients may
result from both a diversity of hydrographic structure on
the one hand and the structure of the biological
resources, species and age composition for example, on
the other.
Salinity gradients
Salinity of the southern Baltic Sea does not show
seasonal changes (Piechura, 1984) and variability of its
gradients in relation to fish distribution is not related
easily to the seasons of the year. The charts in Figure 3,
first of all, show day and night differences in gradients
and values of medians of salinities, against a background of its effective range. During spring, daytime
gradients of salinities were very significant and shifted
towards high values, outside the estimated effective
range of the salinity. The reason was mostly associated
with the sprat-spawning concentration, localized in high
salinity water in the Bornholm Deep that comprises
most of the fish biomass.
In the summertime the daytime differences between
herring and sprat were easy to see. Differences observed
between 1983 and 1988 for herring resulted from the
565
influence of stronger in-flow of North Sea high salinity
water in 1983. It is interesting to observe that the in-flow
from 1983 had a relatively small influence on effective
range of salinity but the influence on herring was very
significant.
Median values of salinity gradients measured in the
autumn showed a monotonic decrease between 1989 and
1996 for the daytime (from 8.4 in 1989 up to 7.3 in
1996). The limits of the gradients were decreasing also.
The differences observed could result from a general
decrease of salinity of the southern Baltic waters over
the reported period stemming from a low North Sea
inflow. Night salinity gradients were more limited and
less variable from year-to-year than the daytime ones.
Oxygen gradients
Due to the limited availability of data on oxygen distributions, the comparison shown in Figure 4 is most
useful for the autumn. All charts correspond to the
average distribution of herring and sprat together.
In the summertime sprat occurred in well-oxygenized
and warm surface waters while herring occurred in
colder, high salinity, and less oxygenized deeper layers
during the daytime. In the autumn, daytime median
values of oxygen gradients increased from 4.2 ml l 1 in
1989 to 6.8 ml l 1 in 1996. It is important to notice, that
in 1989 the percentage of herring was 73%, while in 1996
it was only 33%. Thus the daytime oxygen gradients can
be partly dependent on species composition. During
the night fish were regularly migrating towards better
oxygenized surface layers.
It can be concluded that during the night fish occurred
in more highly oxygenated waters than in the day, but
in most cases an oxygen level (median value) was
lower than the median of effective range of this
parameter. This means that the oxygen level is influencing fish distribution in only a limited way in normal
circumstances.
Horizontal gradients
Method
Horizontal stratification of the environmental structure
of a marine ecosystem has implications on the geographical distribution of fish and is described usually
by charts of biomass surface density. The analysis of
horizontal gradients has to be done with reference to the
vertical cross-section of environmental structure to
enhance the knowledge on typical and unusual situations (very high or very low biomass densities).
Interpretation of horizontal biomass density gradients
can be improved by the application of the modified
matrix macrosounding method, described by Orłowski
(1998). The method of macrosounding (Orłowski, 1990)
was based on computer transformation of acoustic data
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A. Orłowski
Figure 5. The application of the matrix macrosounding method to the study of the relationship between salinity gradients and
horizontal structure of fish distribution during the night (October 1996). Darker dots expose fish recordings exceeding
Sv> 61 dB. The chart of the cross-section is shown in the upper part.
collected over selected distance units, into a graphical
form showing, in macroscale, a vertical distribution of
fish targets in proportion to the corresponding value of
volume backscattering strength (Sv). In matrix macrosounding the whole area surveyed is divided (see Figure
5) into elementary units (rectangles) forming the matrix
of columns and rows. For each elementary rectangle
values of all factors describing fish distribution and
correlated environmental background can be estimated
from cruise results.
The method was modified recently by entering a
threshold for macrosounding visualization to enhance a
possibility of analysis of horizontal gradients in fish
distribution. In this the fish layers, for which Sv values
are exceeding assumed minimum values (Svmin) are
highlighted graphically. Hence the areas of higher or
lower biomass densities can be easily identified at the
macrosounding profile and compared to the available
associated characteristics of abiotic factors.
Results and discussion
Studies of fish distributions with reference to the
environmental structure of the ecosystem should be
limited to the most characteristic periods of fish behaviour. During the daytime fish mainly appear in ‘‘feeding’’ schools that are capable of significant vertical and
horizontal migrations within a wide range of values of
abiotic factors. During the night they are dispersed in a
form of scattering layers and are more passive and more
Acoustic studies of spatial gradients
567
Figure 6. Salinity gradients and horizontal structure of fish distribution during the night (October 1989). Darker dots indicate fish
recordings exceeding Sv> 65 dB. The chart of the cross section is shown within the bottom profile.
dependent on the environment. Indeed fish are not
capable of significant group migrations at that time.
Analysis of fish behaviour, described by Zusser (1971),
Barnes and Mann (1991), and surveyed by specialized
acoustic methods (Orłowski, 1998), shows that the night
distribution of fish is more stable and more closely
associated with the environment than the daytime one.
Consequently night-time was selected for further analysis of horizontal gradients in fish and abiotic factors
distributions.
Three examples were selected to demonstrate an application of the matrix macrosounding method for research
on the influence of spatial gradients of hydrologic
factors on fish effective biomass distribution in a
macroscale during night-time. Two of them show the
increase of fish biomass concentration caused by local
gradients in environmental structure. One example concerns the area of low fish biomass densities, repeatable
year-by-year.
The first example is taken from data collected in
October 1996 (Figure 5). The upper part of the figure
represents a selected cross-section from the standardized
chart of the matrix macrosounding method. The chart
also shows the division of the area into the elementary
rectangles, previously described. Corresponding
rectangle identification numbers are given below the
surface line in the lower part of the figure. The
selected cross-section (18 elementary units) starts at the
Bornholm Deep (rectangles 1–6), crosses Södra
Midsjöbanken (rectangles 8–10) and ends at South
Gotland Deep (15–18). In the area of shallow waters of
Södra Midsjöbanken dense sprat concentrations
(Sv> 61 dB; darker dots) were identified. The
lower part of Figure 5 shows a matrix macrosounding
visualization of the selected transect which is approximately 145 nmi in length. Gradients of salinity between
7.00–7.20, forming spatial limits of fish concentration,
can be seen in the figure. Localization of the sprat
concentration was closely associated with salinity gradients, which could be dependent primarily on the seabed
profile.
Figure 6, based on data from October 1989, is another
example of the influence of salinity on the night-time
distribution of fish. A chart of the transect is given in the
lower part of the top section of the figure. The transect
begins at Bornholm Deep (rectangles 1–3), crosses east
towards the Slupsk Sill (depth around 30 m), Slupsk
Furrow (80 m) and Slant Sill. In the area to the east of
the Slupsk Sill a concentration of fish (Sv> 65 dB) was
observed and the thickness of fish layer was decreasing
sharply. Scanning the transect by isolines has shown the
existence of local upwelling in that area. The phenomenon was exposed by vertical deformation of the 7.86
salinity isoline. During the night the lower depth limit
of fish occurrence is very precisely matched to water
density (Orłowski, 1998). It can be seen clearly that the
variability of its magnitude is a strong influence on the
spatial distribution of fish. This phenomenon could be
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A. Orłowski
Figure 7. Application of the matrix macrosounding method for long-term studies on horizontal gradients in the Gdansk Deep area.
Dots indicate only the fish recordings exceeding Sv> 65 dB. The chart of the cross-section is shown in the left corner of the
temperature transect. Patterns of temperature, salinity, and oxygen against the fish distribution at night are plotted separately. The
situation shown is reconstructed on the basis of data collected over 1989–1995.
Acoustic studies of spatial gradients
caused by the natural easterly movement of saline water
assisted by the wind.
Figure 7 demonstrates another practical application
of the method of matrix macrosounding. This time the
main purpose was to identify hydrographic reasons for
the permanent decrease of the fish concentration in the
middle of the Gdansk Deep at night. A cross-section
(1–10) of 72 nmi profile, starting to the North from the
Vistula estuary (rectangles 1–2) and crossing the Gdansk
Deep (4–9) in direction to the North, was chosen
(Figure 7). The area is well known for higher fish
densities during the autumn acoustic surveys and characteristic abiotic horizontal and vertical gradients are
associated with specific sources (river estuary, coastline
pattern, bottom contour). The importance of the area
was clearly seen in the satellite images shown and
described by Horstmann (1986). The matrix macrosounding cross-section was calculated as the average for
autumn surveys over the period 1989–1995. In 1996 a
strong thermal anomaly was observed and fish densities
were very low, insufficient for analysis. In Figure 7 there
is a quasi-symmetrical distribution of fish along the
profile. Lowest fish densities appeared in the centre of
the Gdansk Deep and close to the Vistula estuary. These
minima were characteristic for 1989 and 1990. In 1994
and 1995 night time data from the central rectangle of
the profile were not available. Comparison of fish horizontal distribution with associated temperature, salinity,
and oxygen patterns shows a great coincidence of
observed gradients. During the night fish concentrations
were directly associated with warmer waters. The lower
limit of fish depth was correlated with the water density
gradient, resulting from the vertical distribution of salinity. The oxygen gradient was very close to the salinity
one. All abiotic gradients showed quasi-symmetrical
patterns similar to those of the fish layers in relation to
the Gdansk Deep contour. The decrease of fish biomass
density in the central part of the area is clear. It might be
caused by the presence of low salinity or oxygen in the
same place. The phenomenon might also be caused by
other abiotic factors not measured during the surveys
(i.e. biogens or suspended matter), that have been modified by river run-off and concentrated in the centre of the
Gdansk Deep. A similar result was obtained for the
cross-section perpendicular to that described.
Conclusions
The main aim of this paper is to use data from acoustic
surveys to enhance marine ecosystem characteristics by
evaluating spatial gradients in abiotic and biotic factors
in selected situations. Two different methods of
approach have been described and discussed.
First a method for estimating the vertical gradients in
environmental factors, corresponding to the main range
569
of fish occurrence, was used for short- and long-term
cyclical studies of fish behaviour in relation to abiotic
factors. The examples and reported results could be
treated as initial data for further comparisons of fish
distributions for day, night, and various seasons. The
results extend significantly knowledge of the interactions
between fish and factors which control overall community structure. The method seems to be more efficient,
and accurate than that used earlier and described in
(Orłowski, 1989).
Secondly the method of matrix macrosounding,
improved and employed for research on horizontal
gradients in fish distribution seems to be useful for
large-scale analysis of horizontal gradients in biotic and
abiotic factors in marine ecosystems. The examples
described give more detailed information on average and
extreme situations influencing fish distribution in the
Baltic ecosystem and demonstrate the importance of
such analyses in particularly interesting areas. The effectiveness of both methods is dependent on having an
adequate number of well sited STD/CTD stations and of
their being repeated from survey to survey. Application
of these methods in different areas could considerably
enhance our understanding of fish behaviour in relation
to marine ecosystem abiotic characteristics.
Both methods are particularly appropriate in regions
like the Baltic Sea, which are characterized by a very
specific, time variable (in-flows) structure of spatial
gradients in the abiotic factors.
Acknowledgements
The work was supported by the Polish Committee of
Science as PB/16 Grant. Computer programs were
prepared in cooperation with Mr S. Kurzyk from Sea
Fisheries Institute.
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