Sinusoidal swimming in fishes: the role of season, density of large zooplankton, fish length, time of the day, weather condition and solar radiation

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The sinusoidal swimming of fish, previously interpreted as foraging behaviour, was studied with respect to season, density of large zooplankton, fish length, time of the day, weather condition and solar radiation in Římov Reservoir, Czech Republic,
  PRIMARY RESEARCH PAPER Sinusoidal swimming in fishes: the role of season,density of large zooplankton, fish length, timeof the day, weather condition and solar radiation Oldr ˇich Jarolı´m  • Jan Kubec ˇka  • Martin C ˇech  • Mojmı´r Vas ˇek  • Jir ˇı´ Peterka  • Josef Mate ˇna Received: 18 February 2010/Revised: 7 July 2010/Accepted: 27 July 2010/Published online: 12 August 2010   Springer Science+Business Media B.V. 2010 Abstract  The sinusoidal swimming of fish, previ-ously interpreted as foraging behaviour, was studiedwith respect to season, density of large zooplankton,fish length, time of the day, weather condition andsolar radiation in Rˇı´mov Reservoir, Czech Republic,using a bottom-mounted, split-beam transducer (7  ,nominal angle; frequency 120 kHz). The proportionof sinusoidally swimming fish increased from Aprilto August while this behaviour was absent inOctober. The occurrence of sinusoidal swimmingshowed an apparent pattern throughout the day; itincreased sharply around sunrise, was highest within5–6 h around solar noon, and sharply decreasedaround sunset. Significantly less frequent occurrenceof sinusoidal swimming was recorded during cloudydays compared to sunny days. The vast majority of records came from fish of standard length rangingfrom 100 to 400 mm, which represents the typicalsize range of common bream  Abramis brama  androach  Rutilus rutilus  of age [ 1 ? , the main zoo-planktivores in the reservoir. The presence of theselarger fish in the open water of the reservoir, as wellas the presence of sinusoidal swimming, apparentlycorrelates with the presence of large zooplankton(  Daphnia ,  Leptodora  and  Cyclops vicinus ) in theepilimnion. The increase of sinusoidal swimmingbetween April, June and finally August resulted in anincrease of zooplankton component in fish guts. Itappears that high values of solar radiation, and stablecalm weather during high pressure periods, result inoptimal optical conditions for sinusoidal swimming,making this foraging behaviour more efficient andwidely used in fishes exploiting the zooplanktonproduction in the reservoir. Keywords  Common bream  Abramis brama    Daphnia    Echosounder    Leptodora    Roach  Rutilusrutilus    Rˇı´mov Reservoir    Sonar5 Introduction The mode of swimming of a fish has consequencesfor its ability to escape from predators or unfavour-able environmental conditions and its reproductivebehaviour, as well as its foraging (Wootton, 1998). With respect to swimming patterns and locomotionof fish, it is generally assumed that there are indi-vidual modes of straight swimming (Lindsey, 1978; Webb, 1984a, b; Videler, 1993), i.e. swimming in a Handling editor: Luiz Carlos GomesO. Jarolı´m    J. Kubecˇka    M. Cˇech ( & )   M. Vasˇek     J. Peterka    J. MateˇnaBiology Centre, Academy of Sciences of the CzechRepublic, Institute of Hydrobiology, Na Sa´dka´ch 7,370 05 Ceske Budejovice, Czech Republice-mail:  1 3 Hydrobiologia (2010) 654:253–265DOI 10.1007/s10750-010-0398-1  horizontal plane. Much less attention has been paid tofish swimming in a vertical plane represented by, e.g.gliding swimming patterns (Weihs, 1973, 1974) or sinusoidal swimming patterns (Cˇech & Kubecˇka,2002).Haberlehner (1988) was the first to notice roach  Rutilus rutilus  swimming up-and-down and consum-ing plankton in a backwater of the Danube (SCUBAdiving observation). Cˇech & Kubecˇka (2002) firstdescribed the term ‘sinusoidal swimming’ on thebasis of stationary uplooking acoustic data from theopen water of Rˇı´mov Reservoir, Czech Republic(North Sea-drainage area). This term was usedbecause the trajectories of ‘sinusoidally swimming’pelagic fishes resembled a regular sinusoidal curvewhen displayed on the echogram (Fig. 1). Thepatterns of change in target strength (TS; Simmonds& MacLennan, 2005) revealed that the sinusoidalswimming is an active swimming mechanism, ratherthan a mechanism based on swim-bladder volumechanges, comprising tilting of the fish’s body duringthe ascending and descending phase of a sinusoidalcycle. The average amplitude of a sinusoidal cycle(distance between the uppermost and the lowermostpositions of the sinusoidal movement curve) was ca.1 m and the average frequency of cycling was nearly4 cycles min - 1 .Sinusoidal swimming was detected during allobservations from June to August and 83% of thefish [ 100 mm standard length (  L  S ) exhibited thisbehaviour. This movement pattern was not observedin November. A smaller data set from May showedless intense occurrence of sinusoidal swimming in theopen water, mainly because of coincidence withthe spawning of the dominant cyprinid species in thereservoir (Cˇech & Kubecˇka, 2002). Sinusoidal swim-ming was also closely dependent on time of day andweather conditions. It started after sunrise and wasreplaced by straight swimming before sunset. Sinu-soidal swimming was never observed during the night(Cˇech & Kubecˇka, 2002) or during extremely badweather (heavy rain, storms, strong winds; Cˇech,pers. observation).Following the work of  Janssen (1981, 1982) and Thetmayer & Kils (1995) dealing with visual fishfeeding, and considering the above findings, it hasbeen argued that sinusoidal swimming is an efficientmethod for fish visually searching for prey, mainlylarge zooplankton (  Daphnia ,  Leptodora ), whose spa-tial distribution in Rˇı´mov Reservoir is patchy insummer (foraging behaviour; Cˇech & Kubecˇka,2002).  Daphnia  and  Leptodora  are the most importantprey items of adult cyprinid species occupying theopen water of the reservoir in summer (Vasˇek et al.,2003; Vasˇek & Kubecˇka, 2004; Vasˇek et al., 2008).These two cladocerans are highly preferred in the dietof the larger common bream  Abramis brama  and of roach, the main zooplanktivores in Rˇı´mov Reservoir,having Ivlev electivity indices of 0.68–0.95 (Cˇech &Kubecˇka, 2002).The present study is focused on sinusoidal swim-ming with respect to the time of year, weatherconditions,solar radiation and the presence of juvenileand adult fish in the open water of Rˇı´mov Reservoir,from which this unique behaviour was first reported indetail. The main questions were: (1) Is the sinusoidalswimmingwidelyusedinfishesonlyduringsummerorcoulditbeobservedsimilarlythroughthespringandinearly fall? (2) Does the proportion of sinusoidallyswimming fish in the open water fish stock as well asthe total abundance and biomass of the open water fishstock increased during late spring and summer com-pared to early spring? (3) Is this increase to any extentcorrelated to the abundance of large zooplankton andits species composition? (4) Is the sinusoidal swim-ming reallyperformed only by a specific size cohortof fish (136–359 mm  L  S ; according to Cˇech & Kubecˇka,2002) or is it also widely used by both smaller andlarger fish? (5) Is the sinusoidal swimming influencedby weather conditions and intensity of solar radiation(i.e. by optical conditions), or is the number of fishperformingthisbehaviourindependentofthoseabioticfactors? 135791113151719    D  e  p   t   h   (  m   ) sinusoidal'uncertain'straight Fig. 1  An example of sinusoidal, ‘uncertain’ and straighttrajectories of fish swimming in a vertical plane (raw 40 LogRTVG uplooking echogram)254 Hydrobiologia (2010) 654:253–265  1 3  Methods Study areaThe study was carried out in the meso- to eutrophicRˇı´mov Reservoir, Czech Republic (48  50 0 N, 14  29 0 E;170 km south of Prague), which has an area of 210 haand maximum surface elevation of 471 m a.s.l. Themaximum depth of the reservoir is 45 m and thevolume 33.6 millions m 3 (Sedˇa et al., 2000). Thisdimictic reservoir is inhabited by 30 species of fish,five of which, common bream, roach, bleak   Alburnusalburnus , perch  Perca fluviatilis  and ruff   Gymno-cephalus cernuus , have the highest ecologicallyrelevant abundances. The reservoir has had a stablefish composition, with a relatively low proportion of predatory fish, since 1988–1989 when a percid phasewas replaced by a cyprinid phase (Sedˇa & Kubecˇka,1997; Rˇı´ha et al., 2009).SamplingA scientific echosounder, Simrad EY 500, along withan ES 120-7G circular split-beam transducer (nom-inal angle 7.1  ) was used for obtaining the data.Signal frequency was 120 kHz, pulse length 0.1 ms,pulse interval 0.2 s and output power 63 W. As in theprevious work of Cˇech & Kubecˇka (2002), theuplooking transducer was placed at a depth of 38 mca. 1 m above the bottom, in the lacustrine part of thereservoir near the course of the old Malsˇe River(for location see Vasˇek et al., 2008). The transceiverand PC were placed 80 m away from the study areain the enclosed floating boat shed.The echosounder was continually in operationfrom April to October 2005. Four limnologicallydistinctive periods were chosen for further analysis:(1) the beginning of spring stratification (14–16April); (2) the clear-water phase (30 May–5 June); (3)the summer maximum of phytoplankton and zoo-plankton (8–12 August) from which there is acomparable data set from a previous study of Cˇech& Kubecˇka (2002); (4) the beginning of autumnmixing (12–14 October). During all these periods,there was stable, sunny weather, except in June andAugust when sunny days alternated with cloudy days.Simultaneously with the acoustic observations,fish were sampled using a series of Nordic multi-mesh gillnets (epi-, meso- and bathypelagic nets4.5 m high, 16 panels 2.5 m wide each, mesh sizefrom knot to knot 5.5, 6, 8, 10, 12.5, 16, 19.5, 24, 29,35, 43, 55, 70, 90, 110, 135 mm) and a purse seine(120 m long, 12 m deep, front/mid/rear mesh size6/8/10 mm; used in August only) (Vasˇek et al.,2008). Gillnets were installed 2 h before the sunsetand lifted 2–3 h after the sunset on each samplingoccasion. The exposition time throughout sunset waschosen on the basis of previous studies that foundincreased gillnet catchability at twilight period(Vasˇek et al., 2009) and gut fullness of planktivorousfish markedly higher during the day and evening thanin the morning and at night (Vasˇek & Kubecˇka,2004). Extensive purse seining was performed duringdaylight hours (noon–evening). In total, 850 fish werecaptured by gillnets and 182 fish were captured usingpurse seine. Fish  B 200 mm  L  S  were immediatelyanesthetised in MS 222 (500 mg l - 1 ; left in thesolution for 10 min), identified to the species, mea-sured to the nearest 5 mm  L  S  and transferred into10% formaldehyde solution. Fish [ 200 mm  L  S  werekilled by overdosing in MS 222 (1 g l - 1 ; left in thesolution for 10 min), dissected immediately and onlytheir guts were preserved in 5% formaldehyde forlater processing (Vasˇek et al., 2008). The death of allfish prior to immersion into the formaldehyde or priorto dissection was assured by decapitation.Zooplankton was collected by duplicate vertical nethauls (net diameter 20 cm, mesh size 200  l m) drawnthrough 5–0 m depth, which approximately corre-sponded to the extent of the epilimnion during theperiods studied (Vasˇek et al., 2008). These gillnet,purseseineanddietdata,alongwithinformationonthedensity and composition of the reservoir zooplankton,have been described elsewhere (Vasˇek et al., 2008).The acoustic records were processed by Sonar5 Propost-processing software. A total of 5,898 fish weremanually tracked, i.e. echoes were combined intotracks and counted, since automatic tracking was notable to distinguish individual fish when their trajec-tories overlapped (Balk & Lindem, 2000). The TS thresholdwas set to - 56 dB. Timeof day, depth in thewater column, TS, trajectory shape and change of vertical range were recorded for individual fish. Threebasic types of fish swimming trajectories in thevertical planewere distinguished—sinusoidal,straightand ‘uncertain’. Sinusoidal swimming was defined asswimming along the trajectory that resembled asinusoid curve with at least one full sinusoidal cycle Hydrobiologia (2010) 654:253–265 255  1 3  on the echogram (Cˇech & Kubecˇka, 2002). A straighttrajectory (straight swimming) resembled a straightline with no apparent change of vertical range.Sometimes the fish trajectory was classified as‘uncertain’ in cases when it was not swimmingsinusoidally (no regular up-and-down movement)but did seem to change its vertical range (‘uncertain’swimming; Fig. 1). Fish trajectories shorter than 15 s(ca.the average durationofonesingle sinusoidalcycleaccording to Cˇech & Kubecˇka, 2002; 586 trajectoriesin total) were excluded from further analysis tominimise misinterpretation of sinusoidal and ‘uncer-tain’ trajectories. In total (day and night, April toOctober records), 2,496 straight trajectories, 941‘uncertain’ trajectories and 1,875 sinusoidal trajecto-ries were included into individual analyses.Vertical profiles of temperature (  C) and dissolvedoxygen (mg l - 1 ) were measured with a calibratedOXI 196 probe (WTW, Germany). Meteorologicaldata were recorded by an HBI meteorological probeMS16 (Fiedler-Ma´gr, Czech Republic), which com-prises a global radiation probe GR01, temperatureprobe HST005 and rain gauge SR02. Global radiation(GR, in W m - 2 ), comprised of direct radiation,reflected radiation from the ground and diffusedradiation of the visible spectrum (Gjelland et al.,2004), was used for defining day and night. The timeinterval when GR  =  0 W m - 2 was defined as sunrise/ sunset, GR [ 0 W m - 2 corresponds to day (daylighthours).Calculations and statisticsThe  L  S  of fish was calculated from the acoustic TS byusing the formula:  L  S  ¼ 10  TS  m ð Þ = n ;  ð 1 Þ where  n  =  19.65,  m  = - 92.8; ventral aspect (Fro-uzova´ et al., 2005). For sinusoidally swimming fishonly TS readings in the uppermost and lowermostpositions of the sinusoidal cycle, when the fish are nottilted, were considered (Cˇech & Kubecˇka, 2002).Weights of fish were calculated using the length–weight relationship: W   ¼ a   L  b S ;  ð 2 Þ where  a  =  1.0943  9  10 - 5 ,  b  =  3.1387. Parameters a  and  b  were obtained from length–weightrelationships for common bream and roach caughtin Rˇı´mov Reservoir in 2005. Those two cyprinidsmade up the majority of the pelagic fish stock thatyear (Peterka et al., 2007). Biomass of fish wascalculated for each hour using the formula:  B ¼ W    N   t S    10 ;  ð 3 Þ where  B  is total biomass (kg ha - 1 ),  W   is hourlyaveraged fish weight (g),  N   is the number of observations per h,  S   is water surface area coveredby the acoustic beam (m 2 ),  t   is the average time spentin the beam (s) and 10 is the coefficient for correctionto kg ha - 1 .Abundance of fish (ind. ha - 1 ) was computed as:  A ¼  BW  :  ð 4 Þ The role of season and density of large zooplankton The data were tested using regression analysis[course of (1) fish abundance, (2) fish biomass and(3) proportion of sinusoidal fish in the open water of Rˇı´mov Reservoir from April to August; course of themass of zooplankton in the contents of fish guts fromApril to August] and  t   test (depth distribution of sinusoidally compared to straight swimming fish). The role of time of the day and fish length The data were tested using ANOVA (proportion of  juvenile fish in the open water fish stock during daycompared to night; comparison of   L  S  of sinusoidalfish during individual periods of the year) andWilcoxon rank sum test (size composition of thestock of sinusoidally compared to straight swimmingfish). The role of time of the day, weather conditionand solar radiation The difference in percentage of sinusoidal fish duringsunny compared to cloudy days in June and Augustwas tested using  t   test.Time series analyses were performed in R (a freesoftware environment for statistical computing; pack-age stats version 2.7.0). A seasonal-trend decompo-sition procedure, based on the loess model (STL,nonparametric analogy of Fourier analysis; see 256 Hydrobiologia (2010) 654:253–265  1 3  Becker et al., 1988; Cleveland et al., 1990), was applied. The seasonal component (originally  S  v ;Cleveland et al., 1990) was substituted by the dailytrend component (  D v ). The trend component (srci-nally  T  v ; Cleveland et al., 1990) was substituted bythe general trend component (GT v ). Remainders(srcinally  R v ; Cleveland et al., 1990) were substi-tuted by residuals (  R v ). Raw data ( Y  v ) are equal to thesum of all components (the original equation inCleveland et al., 1990, was adapted with regard tochanges in names of individual components): Y  v  ¼  GT v  þ  D v  þ  R v :  ð 5 Þ The model worked on the basis of iterations (twoinner iterations were used), when the significances of all fitted elements were compared with each other andwith the variability of the residuals. R functions,  ts (with argument  freq  equal to 24) and  stl  (withargument  s.window  equal to ‘periodic’ and argument t.window  equal to 6), were used to create and plottime series.All regression analyses, ANOVAs, Wilcoxon rank sum tests and  t   tests were performed in Statistica(StatSoft). Results The role of season and density of largezooplanktonThe acoustic abundance of fish as well as theiracoustic biomass in the open water of Rˇı´mov Reser-voir steadily increased from April to August (regres-sion analysis day abundance ;  F  1,11  =  5.12,  r  2 =  0.32, P \ 0.05; regression analysis day biomass ;  F  1,11  = 26.68,  r  2 =  0.71,  P \ 0.001; Fig. 2a, b). The onlyexceptions were the nights in June when very lowabundance, and biomass of fish was recorded (coin-cidence with intensive bream spawning events in thelittoral zone). There were also very few fish inOctober in the open water, especially the daytimeabundance and biomass was very low (over 8 timeslower abundance and 16 times lower biomass com-pared to August).The temperature profiles showed sharp stratifica-tion in April, June and August with an apparentthermocline shallower than 10 m and relativelyweak temperature stratification in October with athermocline below 25 m. A lack of dissolved oxygeninthedeepeststratawasrecordedinallmonthsstudiedexcept April, but in all periods the oxygen values werelower in the thermocline compared to the epilimneticlayers. The water transparency was highest during theclear-water phase at the beginning of June (Fig. 3).  Daphnia  dominated the reservoir zooplankton inlate May/early June (28.8 ind. l - 1 ) and in August(31.7 ind. l - 1 ). The density of   Leptodora  peaked inAugust (0.6 ind. l - 1 ). In April, large cladoceranspecies were almost absent from the reservoir(0.4 ind. l - 1 ), but they were partly replaced by larger,early-spring cyclopoid copepods (14.6 ind. l - 1 )mostly  Cyclops vicinus . In October, both largecladocerans and cyclopoid copepods had almostvanished from the open water of the reservoir(2.5 ind. l - 1 ) (for more details see Vasˇek et al., 2008). 050100150200250300350400    A   b  u  n   d  a  n  c  e   [   i  n   d .   h  a   -   1     h   -   1    ] 050100150200250 April June August October    B   i  o  m  a  s  s   [   k  g   h  a   -   1     h   -   1    ] (a)(b) Fig. 2 a  Abundance (mean  ?  SD) and  b  biomass(mean  ?  SD) of all fish during ( square ) day and (  filled square )night in April, June, August and October 2005 in the openwater of Rˇı´mov ReservoirHydrobiologia (2010) 654:253–265 257  1 3
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