Not to be cited without prior reference to the author'

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IGES 1997 G.H. 1997/ S:29Spatial Gradients in Estuarine Systems

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DIEL !ND LUNAR VARIATIONS IN AGOUSTIC MEASUREMENTS OF GLUPEOIDSIN THE BALTIC SEA

Andrzej OrlowskiSEA FISHERIES INSTITUTEP.O. BOX 3lJ:5'81-332.· GOYNIAPOLANO

ABSTRACT

Acoustic methods have become increasingly powerful and

effective tools for research into empirical studies of

ecosystems. However, for' fish stock assessment purposes more

informationis needed about fish behaviour to improve the level

of precision. This is particularly so in relation to the effects

of environmental factors and their consequences on the

properties of fish as acoustic targets.

This paper seeks to determine, and to analyse,

characteristics of the acoustic properties of certain species of

pelagic fish. It is based on measurements made over the period

from 1981 to 1996 in the southern Baltic at different seasons of

the year. Significant diel and lunar variations of the acoustic

properties of fish were observed. The correlation with fish

behaviour is discussed, emphasizing the potential effect on

acoustic methods used for fish biomass assessment.

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IIITRODUCTION

Acoustic methods for fish abundance estimation have been used for many years and continue to develop as a powerful fisheriesmanagement~ tool. Calibration of the acoustic systems can be very precise. It is the fish themselves that introduce a degree ofvariability into the assessment process. In the majority of publications, fish are treated as objects with time-stationarycharacteristics when observed as acoustic targets (Foote, 1987, 1991; Johannesson, Hitson 1983; Knudsen, 1990). Dependence of fishtarget strength on tilt angle was described in some papers [Foote. 1985; Foote, Ona, 1987; Johannesson, Ilitson. 1983} but wasnever expressed by time dependent target strength formula~.

An equation for target strength (TS) of individual clupeoid fish (Nakken, Olsen, 19ii) is:

TS =20 log I + b

where TS =fish target strength (dB)I =fork length of fish (cm)b=constant factor related to a given fish condition or species

( 1 )

e.To be strictly correct, the term (I) should take into account the effective acoustic length of a fish at any time e.g., if

fish are diving, or swimming upll'ards, their effective length is less than their true length. The term (b) expressescharacteristics such as morphological and physiological factors that influence the TS and which are often species specific.

Equation (1) is used in the process to convert acoustic back-scattering measurements to fish biomass. Weight-Iengthdependent functions are then applied to obtain the final result. For the family of clupeoids the value of factor (b) has beenrecommended as -T1.2 dB by the International Council for the Exploration of the Sea (ICES). This value has been used since 1983during the assessment of pelagic stocks in the Baltic Sea (Anon., 1990). It assumes that the acoustic reflection properties offish are independent of time. Ir either of the factors (l) or [bI are found to vary with time such an assumption could lead tosignificant errors in the assessment.

An analysis of physiology related to a live fish (Simmons, llacLennan. 1995; Foote, Ona, 1987; NikolskU, 1974; Ona, 1987;Thurman, 1982) predicts significant· relationships between its physical (body contents. end composition) and physiologicalcharacteristics. Different stages in the physiological development of fish INikolskij, 1974; Thurman. 1982) can be identified withlong-term (life period: maturation, feeding, spawning, hibernating) and short-term rhythms (diel) observed during the whole lifeprocess.

KATERIALS..,

The main subject of this paper is the analysis of echoes received from fish under the following selected conditions. Ouring .• fthe period from 1981 to 1995, ships of the Sea Fisheries Institute in Gdynia carried out aseries of research cruises col1ecting .... ,acoustic, biological, and environmental data in the southern Baltic. The research vessels used for this work were the "ProfesorSiedlecki" during 1981 to 1990 and "Baitica" for the last years.

The first cruises 11981-1985) were conducted during the summer and spring seasons and the last four cruises 11989-1996) wereorganized during the autumn; the latter as apart of an leES monitoring programme of pelagic fish stocks in the Baltic. Since 1918this programme has been carried out on a regular basis by Sweden, Denmark and Germany. Poland joined the programme in October1989. Each cruise of the Polish vessels lasted approximately two weeks and had the potential to collect data from more than onethousand nautical miles of acoustic transect.

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Samples were collected continuously, 24 hours a day. The time distribution of samples was homogeneous to give a good base onwhich to analyse the die I characteristics of fish echoes. Differences between the acoustic systems used were, Simrad EX38echo-sounder with QH echo-integrator ("Profesor Siedlecki" 1981-1988); EX400 with CD echo-integrator ["Profesor Siedlecki"1989-1990); ~d moo with a QD echo-integrator plus a pe ("Baltica"). This meant that the methods of survey and data collectionrequired unification of data files in addition to detai led analysis. All ten mentioned research cruise data fi les were used foranalysis of diel variability of the acoustic reflecting properties of fish.

During 1983 and 1985 two cruises in Hay represent the spring season. In 1981, 1983 and 1988, three cruises in July andAugust from these years characterize the summer months. Five cruises in October represent the autumn period for the years 1989,1990, 1994, 1995. 1996. Three fish species, herring, sprat and cod, are dominating in the southern Baltic. Fish observed duringthese surveys were mostly pelagic clupeoids (herring and sprat). Only in 1981 (Orlowski, 1996) the percentage of cod wassignificant (28.1%). The very 10'11 state of cod stocks between 13% and 2.4% of fish total biomass during the periods analysedallo.'ed the conclusions to be treated as valid for clupeoids.

HEIl10DS

The method of fish biomass measurements by acoustic means is described in (Johannesson, Hitson, 1983: Knudsen, 1990,Orlowski, 1989).

The first analysis refers to a question of the stability of fish acoustic reflecticn properties over a 24 hday. Assumingthat the sampling of an area during a 24 h per iod is homogeneous, the average value od SA, as an acoustic parameter, can beestimated over selected day and time intervals. The magnitude of SA was chosen as the main acoustic parameter to analyse for dielvariability. This parameter corresponds to the area backscattering coefficient per square nautical mile. It was introduced by theInstitute of Harine Research, Bergen, Norway [Knudsen, 1990) and is expressed as:

•

z2

51 d • .185t) 5v. '"

z1

where: 4n. 18522 is a constant to express column scattering strengthby area backscattering coefficient per n,mi2

Sv is the volume back-scattering strength.

[ 2 )

The SA values were collected over one mile intervals, but the average for each 4n.mi was taken as most representative tominimize autocorrelation effects (Orlowski, 1989). To ensure full comparability of results for the cruises being analysed,normalization was achieved on each cruise by dividing measured values of SA by the average figure obtained for SA. The finalparameter applied ror analysis is then equivalent to the values of SA/<SA>.

Next, the analysis addresses the matter of stability of fish acoustic reflection properties over a synodic month. It wasassumed that SA/<SA>, determined during the previous recalculation process, would be suitable ror estimation of the relativechanges of acoustic properties of fish, day by day.

Then, the task was to shift data files from a day, to a moon-orientated order. Aduration of a lunar month is not fullyfixed, lasting between 29.25 and 29.83 days with an average of 29.53 of the 24-hour astronomie days (Marks, 1970). Beeause eruisedurations were about half a lunar cycle, analysis to obtain results in relation to lunar cycles showed that a limited comparisonof eruise results would be possible. All eruise data had to be suitably shifted to a chosen reference point of the lunar period.

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Ihis is because the average value of SA/<SA} per day is taken as the parameter for analysis of lunar variations of the acousticproperties of fish.

A date of the full moon was chosen as the point of reference for such an operation and the date of each cruise day wasshifted. Ihis shift was in relation to the distance between the day of the year in the middle of the cruise, and the day of thepreceding ~nearest full moon. Anew number of a day called m-day was applied. Afirst quarter of the lunar period was correlatedwith the 22nd m-day, a full moon at the 30th m-day and a new moon at the 45th m-day.

Dia VARUBILITY OF CLUPEOID ACOUsrlC PROPERTIES

Fish behaviour, as it .affects their acoustic properties remains an important subject as can be judged from the ICESSymposium on Fisheries and Plankton Acoustic in 1995 (Simmons, KacLennan, 1995). Complex research on behavioural-acousticcharacteristics of fish and scattering layers has been suggested (Orlowski, 1989) and environmental analysis of acoustic data wasbeen started in the Sea Fisheries Institute since many years (Orlowski, 1989). The variability of fish and plankton reflectingproperties and their vertical migrations during the 24-hour cycle has been observed and reported in numerous publications (Fetter,Davidiuk, 1986; Ona, 1987; Orlowski, 1989A, 1990; Protasov, 1978).

An analysis of the data bank from cruises between 1981 and 1995 in the southern Baltic may permit formulation of conclusionson the relative importance of this variability. The results of research on die I variability of the fish acoustic properties areseen in Figure 1. Distributions of values for SA/<SA} averaged over two-hour intervals. are sholo'O separately for each of theresearch cruises. For ease of identification in this paper cruises are designated by the two last digits of the year and thenumber of the month, e. g. 8808 means the cruise of August 1988.

Characteristics expressing the relationship between SA/<SA} and day-hour are shown in Figure 1 in a sequence correspondingto increasing differences (Euklides distance) between a given characteristics and the reference (ideal) of SA/<SA} : constant : 1for all time intervals. Kean characteristics for the period of research between 1981 and 1995 are located in the lowest part ofFigure 1. \

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Acoustic methods of fish abundance estimation assume that in theory the acoustic properties of the fish do not change, sothat all bars in Figure 1 should have a reference value of SA/<SA> : 1. Such near-ideal results are observed in one case only;when all data from the nine cruises were treated as one file (the lowest diagram of figure 1). It is surprising that the values ofSA/<SA} are not always greater than 1, for day or night-time. Values from the summer season in August 1983 show 101'1 levels duringday time while in the same season during August 1988 the greatest difference occurred when the ratio between the highest andlowest values was 3.52 but the opposite was true for the August 1988 cruise when the ratio was 2.36. No significant differences inweather between those surveys were observed. Kay 1985 produced the highest overall ratio of 4.27.

These results illustrate the time-varying nature of the acoustic properties of fish. Oue to physiological and behaviouralchanges fish form their 010'0 diel rhythm of life, divided into different periods, thereby changing their acoustic properties.Values of SA/<SA} are significantly hourly-dependent in some cases. Ihe diagrams in Figure 1 show that the biggest differencesoccurred between Hay and October, the extreme seasonal periods of the research.

Figure 2 shows the mean fish depth (HFD - depth of biomass gravity centre) and its standard deviation over the period from1983 to 1995 (data for 1981 were not available) for identical intervals of daytime to those in the previous figure. An importantand regular feature" of the fish diel-cycle is shown in Figure 2, where the fish behaviour pattern is fairly symmetric about thelevel reached at noon. Standard deviation of fish depth determined as the depth o~ "a gravity centre" of scattering strength inthe water column (Svc), was weil correlated to depth (linear correlation coefficient 0.948). Ouring daytime the fish were indeeper waters and distributed over a wider range of depths (standard deviation) 16m). During the night they were dispersed in the

surface layers but over a smaller range of depths [standard deviation (12 m). These relationships are typical of t~e fishdiel-cycle.

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The above does not in itself give proof of the variability found in the aeoustie properties of fish. For example take thesignifieant differenees between August cruises in 1983 and 1988, or between Oetober 1990 and 1994. In both cases the die Icharaeteristies of fish aeoustie properties for the same month of the year are opposite in respeet of the day and night values ofSA/<SA}. Historie knowledge on typical modulators of the rhythms of living ereatures may indieate the moon as a potential sourceof differen.tiation in behavioural activities of fish in the above examples.

LUNAR YARIABILITY OF FISH ACOUSfIC PROPERTIFS

There is a strong influence of the moon on oeeanographie phenomena, espeeially on oeean and sea tides [Harks," 1910; Thurman,1982). In respect of fish activity two basic physical factors could be taken into consideration, the change of gravity [the reason .of tides, water masses movement or internal waves in shallower areas) and the intensity of the moonlight. Both ones depend onposition of the moon in the sky and in relation to the sun. Oue to unequal periods of the moon and the sun apparent migrations inthe sky a moon position and its phase are continuously changing. Ouring the full moon the moon is in opposition to the sun and i~s

predominance is closely to the midnight. Ouring the new moon the moon is closely to the sun position and its predominanee momentis eloser to a noon. Apredominanee of the moon during the first quarter is elosely to the middle between the noon at the midnightand in the last quarter a predominance is eloser to the middle between the midnight and the noon. In a consequence of factorsmentioned above, a modulation of fish behaviour correlated with the lunar periods can be expected•

The task of analyzing of aeoustic properties of fish in relation to the lunar month is considerable and the observationscolleeted above are s_arized. Faetors for further eomparison in Table 1are eaeh defined as: rank corresponding to thesimilarity index between a particular cruise and the reference of:

[al SA/<SA} =constant =1 for all time intervals

[b) standard deviation of SA/<SA} =eonstant =0 for all intervals

(e) mean fish depth [mfd), as averaged for all cruises over the 1983 to 1995 period

[d) standard deviation of mean fish depth [mfd), averaged for cruises from 1983 to 1995

lOH = distance between the medial day of a given cruise and the date of the nearest full moon

H/m(SA) the maximum SA/<SA} to minimum SA/<SA} ratio for an analysed eruise

By analyzing faetors in Table 1 in relation to a time/distance to the nearest fuH moon (lOH) the following observations canbe made:

i) the diel acoustic properties of fish were the closest [rank 1, 2, 3, 4) to SA/<SA} =1 for the eases nearest to the full ornew moon per iods (lOH 1, 2, 1, 15)

ii) the biggest differenees appeared for cruises closest to the periods between the last quarter and the new moon [lOH 10) andthe first quarter and the full moon [lOK 26, 26, 24)

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iii) The ratio H/m[SA) was ordered in a similar way, showing the highest amplitude of modulation of the SA/<SA} faetor (>3) forcruises between the first quarter and the full moon (values 3.81, 4.21, 3.38) and the last quarter and the new moon [3.52).

iv) for the periods closest to the full and the new moon ratio H/m(SA) values were significantly lower (2.36, 2.44, 2.01, 2.52,2.61).

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Standard deviation of SA/<SA) values in two-hour intervals (b) were not visibly eorrelated to moon per iods. The lowestvalues of that faetor were typieal for the autumn, and the highest for the summer and spring. Fish depth, expressed by faetors (cl.and (d), was mostly eorrelated with the season. The latest relationship eould be eaused by similar day duration during the same.seasons.

Olle ~to differences among the seasons (shown above) further analysis of faetors, related to lunar influenee on iish aeousticproperties, was limited only to the autumn season, being most surveyed (5 research cruises 1989-1996). Analysed faetors wereexpressed by corresponding approximation curves. Values of factors were estimated for aeoustic data units (4 n.mi distaneeintervals). Heans of mentioned magnitudes for 2-m-days intervals were ealeulated for a11 autumn eruises together. Taking intoeonsideration periodieal form of a function (29-days period), destined to describe lunar fish behaviour charaeteristies,trigonometrie polynomial approximations for the models were applied (Polozy, 1966):

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Approximations for a11 basic factors were calculated UD to 6-th degree of polynomiaL Analysis of curves and approximation mors(eoefficients of random variation) allowed to limit approximation polynomials up to the fourth degree and such functions wereapplied for modeling the fish behaviour-cycle in relation to the lunar per iod and functions up to the third degree - in relationto the diel fish Iife-eyele.

Approximation functions expressing average values of SA/<SA} for m-days over the autumn period 1989-1995, separately for day(approximation error 11.11), night (11.91) and for 24-hours period (13.61) are ShOllll in Figure 3. The eurve representing 24-hoursper iod (medium dark) shows more stable and eonstant values, closer to expected value SA/<SA} : 1. Day and night eurves differ inaverage values of SA/<SA}. Ollring the day-time acoustic reflecting properties were approximately 201 lower then 24-hours average.The opposite phenomenon was observed during the night-time. The biggest differences between the curves occurred during the fullmoon and the last Quarter of the moon - periods of the most intensive light influenee of the moon on fish behaviour at night.

Approximation eurves expressing average va lues of fish minimum, main (as the average depth of the eentre of gravity ofbiomass distribution) and maximum depths during autumn season for m-days over the 1989 to 1996 period are shown in Figure 4,separately for day and night-time. Approximation mors of eurves are as follows:day-time - maximum depth: 2.15%, main depth: 4.05%, minimum depth: 8.01%; night-time - maximum depth: 6. MI, main depth: 10.91%,minimum depth: 8.80I.In order to avoid correlation of fish and sea bottom depths,.area of caleulations had to be limited for the bottom depths rangingfrom 50m up to 80m. It ean be easy to cbserve that variability of the charaeteristic depths in relation to the lunar eyele isQuite Iimited. One of the reasons could be assoeiated with additional influenee of environmental factors (mostly water temperaturestructure), "hieh were signifieantly different in seleeted cruises. Despite of environmental smoothing effect, eorrelation betll'eenthe fish main depth and the lunar cyele can be elearly observed, in particular during the night-time. Ollring the full moon and thelast Quarter (the strongest moonlight effect) the fish main depth was deereasing during the night and even during the day-time. Asimilar phenomenon was found for the minimum fish depth (mostly for the night). Depth of fish was greatest between the new moonand the first Quarter of the moon (the darkest period). Maximum fish depths were not visibly eorrelated with the lunar period.

All observations described above indueed to seareh another eorrelations and in a conseQuence to analyze all data eo11eetedover the per iod 1981-1996 and representing three different seasons. Sueh an analysis could be directly applied for modelingresults of hypothetieal average cruise, whieh per iod could be regularly shifted in relation to the lunar cyele, to identify themoon-dependent effeets. Neeessary ealeulations were made for 8 situations (A-H) representing two-weeks hypothetieal eruise period,shifted with the step of approximately 4 days one to the other to eover the whole lunar cyele. For eaeh situation the value ofSA/(SA) was estimated for two-hours intervals of the day (diel-eyele, eomparable with the bars at Figure 1) and eorrespondingFourier approximation curves were calculated (approximation errors ranging from 1.811 up to 11.4TI). Results are ShOllll at

Figure 5.

m

Tm (x): ~ (ak eo~ kx + bk sin kx)k : 0

where: ak, bk - Fourier' s coefficients,m- degree of approximation polynomial

( 3 )

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The upper diagram Aon the Figure 5 eorresponds to the situation when the middle of two-weeks-period is eorrelated to themoment of the first quarter of the moon. Eaeh step dOwil - means shifting the period 4 days later. The last diagram shows returningto the starting point and the eycle might start aga in. The loeal maximum of SA/<SA} values is marked by grey square lind the moonpredominance moment by white eirele at the'curves. An average day-time for the whole data eolleetion ean be fixed between 05:06and 17:52 (12 h 46' period). For easier identifieation of the lunar influenee on observed phenomenon the eurves are presented for36-hours time interval (1.5 of the 24h period).

As it was mentioned during the an~lysis of the Figure 1, diel ehanges of fish aeoustie properties (SA/<SA}) were founddifferent in respeet to the lunar eyele. The eurves in the Figure 5 shows how diel eharaeteristie of fish refleeting propertieseould vary due to the moon position and the phase. (mportant features of presented eurves eould be pointed out, as folIows:- loeal minimum values of SA/<SA) are elosely to the noon for all eurves (permanent periodieal dependenee on the sun eyele), .- loeal maximum values of SA/<SA} are varying between 23:00 and 5:00 (loeal time), with a direet eorrelation to the lunar eycle.Periodieal lunar modulation of presented eurves is easily seen. Stable situation was observed for A-D eurves, when the moon ispresent in the sky during the night-time (see white eirele positions). The modulation is the strongest after the full and afterthe new moon. The biggest differenees are observed between the eurves Cand G(the ful1 and the new moon) - eorrespondir:g toextreme moon influenee on a gravity (syzygial tides) and an effeetive light during the night-time: The amplitude of SA/<SA} dielehanges (scale on the left of the figure) does not exeeed 50% and it is signifieantly lower [averaglng effeet) then for singleeruises (Figure 1), Olle to a different number of samples (between 5000 and 2000 n.mi) used for ealeulation of shifted eurves,values of amplitude of diel ehanges are not direetly eomparable. Smaller dynamies of SA/<SA} die I ehanges was observed for theeases being represented by higher number of saffiples (B, C, D).

D1SCUSSIONThe purpose of this paper is to show the need for fish behaviour to be eonsidered in relation to the applieation of aeoustie

methods for fish biomass assessment. (n al1 aQuatie eeosystems there are many physieal and biologieal faetors modulated by shortand long-term rhythms and processes. Two such biologieal rhytrums, diel and lunar cyeles have been taken into account to show theirinfluence on the aeoustic properties of fish. The variability was greater than expected when referred to eaeh of the analysedaspeets..

Changes to the aeoustie properties of fish during a 24-hour eyele, examined in 2-hour intervals were over 100% of theextreme va lues (ratio of Hjm[SA)). The greatest ehanges found were for the springtime of Hay 1983 and 1985 whieh exeeeded 400%(Figure 1, Table 1).

The changes eould be eorrelated to seasonal and day Iife-eyeles, taking into eonsiceration feeding, metabolism, horizontaland vertieal migrations, reproduetion, ehanges in fish body eomposition (fat pereentage), reaetions to environmental eonditionsand movements to seek food. As an example, the volume of food inside fish stomaehs (Clupeidae family), was} 20% of the total fishweight in some measurements from the cruise in Oetober 1995.

Yery signifieant diel ehanges of fish aeoustie properties were found in all eruise data analysed. The smallest differeneesbetween the determined relationships, and the model normally assumed for fish stock assessment, was found for single eruiseselosest to the full and the new moon. This is when a phenomenon of syzygial tides oeeurs. (n those eases the differenee of averagefish baekscattering eross-seetion varied more than two times (Table 1); giving the potential for a similar range of error in the

. biomass estimation. Olle to the model assumed for eomparisons, the greatest differenees were observed between the first quarter ofthe moon and the full moon, and, the last quarter and the new moon. Estimation errors in such eases were } 400X. Fish mean depth(mfd), and its standard deviation in Figure 2 sholied an important and regular faetor of the diel fish eyele. No eorrelationbetween SA/<SA} and fish depth was found.

Hean aeoustie refleeting properties of fish analysed over the five autumn eruises were signifieantly higher (ap~roximately

TOX) during the night then during the day-time. The differenee was greater during the full moon and the last Quarter of the moon.Hodulation of fish charaeteristic depth by the moon phases showed timited range (approximately 25%) of dynamics, mostly

during the night-time.

The most important analysis was made for all data eolleeted over the period 1981-1996 and representing three different

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seasons. Hodeling of results of hypothetieal 2-weeks eruise. whieh period was regularly shifted in relation of the lunar eyele,enabled to identify very signifieant moon influenee on diel fish refleeting eharaeteristies. Observed variations were eloselydependent on both astronomie regulators [the sun and the moon) and the diel and lunar per iods were strongly marked in SA/<SA)patterns of empirieal approximation funetions (Figure 5). The observed phenomena shows a final effeet of astronomie influenee onfinal aeou,stie refleeting properties of the pelagie fish. Identification of the effect demands a wide research on its possiblesources. The reasons could be joint with a direct fish behavioural reactions, with fish feeding conditions or al1 factorstogether. Reaetion of the fish can be also considered as a secondary one, while the organisms, composing their feed could reactstronger on astronomie phenomena, causing strong fish behaviour reactions.

In the light of data, described above. an assumption of quasi-stationary fish aeoustic reflection properties, common in mostliterature [Foote, 1981; Johannessan, Hitson, 1983; Knudsen, 1990; Orloll'ski, 1989) should be reconsidered. The main problem .appears to be in determining factars which express the aeoustical properties of the fish being surveyed. Calibration measurementsof fish target strength should be carried out with respect to all presented conclusions. Both the fish length term (I) and the"speeies" term [b) in eqn. (1) need to be addressed before the length-weight functions can be employed to determine biomass.

In order to predict circumstances when irregularities may occur the conditions under which TS measurements are made shouldbe recognized. The conditions include the season of year and the lunar-per iod, which will determine the physiological state of thefish and its feeding base. Also, the time of a day, which may indicate that the effeetive length of the fish is likely to benormal, i.e., swimming horizontally, or less than normal when the tilt angle inereases. Consequently, eqn. (1) has to bedetermined by reference to a few known options, or given as an average value, taking into account the circumstances. In order toobtain a higher level of preeision for fish stock assessment purposes, all the behavioural aspects of fish acoustic propertiesshould be taken into consideration.

ACXNllliLEDGlIENT

The work was supported by the Polish Committee of Science as PB/16 Grant.

REFERENCES

!non., 1990: Report of the Planning Group for Hydroacoustic Surveys in the Baltic, Copenhagen, 11-18 April, 1990, C.H. 1990/J:34.

ICES, 1995: Fisheries and Plankton Acoustics, Eds. Simmonds, E. J. HacLennan, D. N. ICES Journal of Harine Science. 53(2).

Fetter, H. H., A. P. Davidiuk, 1986: Sezonnoje raspriedielenije sieldiej iuznoi Baltiki i faktorov sredy. Fisch. Forsch., 24, 2 •16-19 [in Russian).

Foote, K. G., 1985: Rather-high-frequency sound scattering by swimbladdered fish. J.Acoust.Soc.Am. 18(2), 688-100.

Foote, K. G., E. Ona, 1981: Tilt angles of schooling penned saithe. J. Cons. Int. Explor. Her, 43, 118-121.

Foote, K. G., 1981: Fish target strength for use in eeho integrator surveys, J. Acoust. Soc. Am. 82(3). 981-981.

Foote, K. G., 1991: Summary of methods for determining fish target strength at ultrasonic frequeneies, ICES J. mar. Sei., 211-211.

Johannnesson, K. A., R. B. Hitson. 1983: Fisheries Acoustics. Apractical manual for aquatic biomass estimation. FAD Fish. Techn.Pap.• (240).

Knudsen, H. P., 1990: The Bergen Echo Integrator: An introduction. J. Cons. Int. Explor. Her 41, 167-174.

Harks. A. I 1910: Ksi~iyc, PWN, Warszawa.

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Nakken, 0., K. Olsen, 1917: Target strength measurements of fish. Rapp. P. -V. Reun. CIEM, 110, 52-69.

Nikolskij, G. B., 1914: Ekologia Ryb, Izd. Vysshaia Shkola, Moskva (in Russian).

Ona, E., 1981: Physiological factors causing natural variations in target strength, International Symposium on FisheriesAcoustics, June 22-26, Seattle, Washington, USA.

Orlowski, A., 1989: Zastosowanie akustycznych metod do badania rozmieszczenia ryb i warstw rozpraszaj~cych na tle srodowiskamorskiego (Appl ication of acoustic methods for study of distribution of fish and scattering layers vs the marine environment). St.Hat. Mor. Inst. Ryb., Gdynia, sero B, 51, 134 pp (in Polish).

Orlowski, A., 1989: Seasonal fluctuations of biomass distribution based on results of hydroacoustic surveys of the Polish fisheryzone, Fish. Res., 8, 25-34.

Orlowski, A., 1990: Hydroacoustic characteristics of scattering layers in the Northeast Atlantic Ocean, J.Acoust. Soc. Am., 88(1),July, 1990, 298-309.

Oriowski, A.. 1996: Increase of cod biomass in the southern Baltic observed during 1994/5 autumn acoustic surveys, in ScientificPapers Presented at the Pol ish-Swedish Symposium on Baltic Coo, Gdynia, Po land, 21-22 March, 1995, IHR Report No. 321, Lyseki I,49-58.

Poloiy, N., 1966: Hetody przybliionych obIiczen. Plm, Warszawa (in Polish).

Protasov, W. R., 1918: Poviedienie Ryb., Pishchevaia ProlllYsIennost, Moskva (in Russian).

Thurman, H. V., 1982: Zarys Oceanologii. Wyd. Horskie, Gdansk (in Polish).

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Table 1. Main factors characterizing variability of fish

acoustic properties determined by analysis of data from each,,~

cruise over the period 1981 to 1995 (in the same order as

Figure 1) . it"'

Cruise date SA/<SA> Fish depth Moon Ratio

Year Month (a) (b) (c) (d) rOM M/m(SA)

1988 August 1 6 6 3 1 2.36 •1995 October 2 1 4 7 2 2.44

1989 October 3 3 1 5 1 2.07

1990 October 4 4 2 4 15 2.52

1981 July 5 9 * * 2 2.67

1994 October 6 2 5 8 24 3.38

1985 May 7 5 7 6 26 4.27

1983 May 8 8 8 2 26 3.87

1983 August 9 7 3 1 10 3.52

* data not available •\.

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Fig. 1. Characteristics of die! variability of the acoustic reflecting properties of fish expressed by the factor SA/,(SA> shown in2-hours intervals of a day for cruises over 1981-1995 period in the southern Baltic.

- 12 -

600

~ 45I

::r:: .~

30B::!:QJ

::r: 1U")I-I

15w....~I

~:

0.--.0

15~.........b

30 •0 6 12 18 HOUR

Fig. 2. Kean fish depth (KFD) and its standard deviation in 2-hours intervals of a day for cruises over the 1983-1995 period inthe southern BaIt ic.

SA / <SA>

1.4

1.3

1.2

1 .1

1.0

9

a

'/ .:::.:::::••::.:••••::==::::::::::••

G

Day

•

5

.4

3

'ThZJ

19 21 23 25 27 29 31 33 35 37 39 41

M-daynumher

43 45 47 49

Fig. 3. Yariability of day, night and 24-hour period of acoustic reflecting properties of fish expressed by approximation curvesof SA/<SA) for days oriented in aeeordanee to a lunar eycle (m-days), observed during the autUlIll over the 1989-1996 per iod. Moonperiods are marked over the x-axis.

- 13 -

DAY-TIME

Fish depth [m]

70 Maximum

I

.""

60

50

40

30

20

Minimum

M-daynumber

19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Main

M-daynumber

Minimum

NIGHT-TIME70 Fish depth [m]

60

50

40

- 30

20

10

,~ ®

19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Fig. 4. Day and night-time variability of minimum. main and maximum fish depths expressed by approximation curves for daysoriented in accordance to a lunar cycle (m-daysl, observed during the autumn over the 1989-1996 period. Koon periods are markedover the x-axis.

- 14 -

•24201612OB0424201612

SA<SA>1 0

9 ) \.

8

®7

.6

.5 ~4

3

2

. 1 •I0 ~~

Fig. 5. The approximation curves (A-H) showing a lunar dependent variability of SA/<SA) for 24-hours day period, calculated on thebasis of all data collected (1981-t996). Each curve represents two weeks period las the eQuivalent of a typical cruise), shiftedsuccessively (4 days steps] in relation to the lunar cycle. Hoon per iods are marked on the right side of the figure. Local maximumof SA/<SA) va lues is marked by SQuare and the moon predominance moment by white circle at the curves. Approximation curves areshown for 16 hours time segment to make a comparison more clear.

.,