Acoustic studies of spatial gradients in the Baltic: Implications for fish distribution

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1 ICES Journal of Marine Science, 56: Article No. jmsc , available online at on Acoustic studies of spatial gradients in the Baltic: Implications for fish distribution A. Orłowski Orłowski, A Acoustic studies of spatial gradients in the Baltic: Implications for fish distribution. ICES Journal of Marine Science, 56: 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 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 International Council for the Exploration of the Sea Key words: acoustics, spatial gradients, environment, fish distribution, Baltic. Received 7 May 1998; accepted 12 April A. Orłowski: Sea Fisheries Institute, PO Box 345, Gdynia, Poland. Tel: ext. 215; fax: ; orlov@miryb.mir.gdynia.pl 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), 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 Furthermore the method of matrix macrosounding has been improved and employed to study horizontal gradients in fish distribution. A few examples of its /99/ $30.00/ International Council for the Exploration of the Sea

2 562 A. Orłowski application in the short- and long-terms, respectively, are shown and discussed. Materials During the period 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 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 means the cruise of May 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 ( ) or 4 nmi ( ) 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 ( ) or by standard target ( ). 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 ( ) or a Neil-Brown CTD system ( ) approximately every 30 nmi. Oxygen measurements were available for analysis from 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

3 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 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 9 C) were mostly associated with sprat and the other one (below 9 C) 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.

4 564 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 6.5 C during the day (pattern 1), and between 3.8 and 7.2 C 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

5 Acoustic studies of spatial gradients 565 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 (4.4 C 8305, 3.5 C 8505) were lower than the night (5.6 C 8305, 5.8 C 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 17.1 C during the day, and between 7.5 and 14.7 C 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 5.8 C but for sprat were (pattern 5) between 12.9 and 18.3 C. 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 the fish occurred in waters between 4.7 and 12.7 C during the day, and between 7.8 and 13.5 C during the night. Values of temperature medians for the day (8.0 C 8910, 10.6 C 9010, 7.7 C 9510, 6.3 C 9610) were considerably lower than for the night (12.2 C 8910, 12.1 C 9010, 13.0 C 9510, 10.0 C 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 influence of stronger in-flow of North Sea high salinity water in 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 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

6 566 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

7 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 , 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

8 568 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

9 Acoustic studies of spatial gradients 569 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 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 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 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. References Barnes, R. S. K., and Mann, K. 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