Diel vertical migrations of bathypelagic perch fry

M. CECH*, M. KRATOCHVIL, J. KUBECKA, V. DRAST IK AND

J. MATENA

Hydrobiological Institute, Academy of Sciences of the Czech Republic, Na sadkach 7,370 05 Ceske Budejovice, Czech Republic and Faculty of Biological Sciences, University

of South Bohemia, Branisovska 31, 370 05 Ceske Budejovice, Czech Republic

(Received 13 January 2004, Accepted 15 November 2004)

The behaviour of young-of-the-year (YOY) perch Perca fluviatilis as a dominant species in the

assemblage of fry in the pelagic of Slapy Reservoir (Czech Republic), was studied during late

May and mid-June 2002 using acoustic methods and complementary net catches. During the

day, perch fry were present simultaneously in littoral, epipelagic and bathypelagic habitats.

Bathypelagic perch fry, forming a scattering layer, migrated vertically each day between the

epilimnion and hypolimnion, with an amplitude of 11�0m in May and 12�5m in June. At dusk,

the migratory bathypelagic fry mixed in the epilimnion with non-migrating epipelagic fry and

spent the night close to the thermocline (abundance maximum at 3–4m in May, 0–2m in June).

In June, shoaling behaviour by some of the bathypelagic perch fry was also observed: the

shoaling fry remained higher in the water column than the non-shoaling fry. Both depths of the

scattering layer and the depths of the fry shoals were strongly controlled by the light intensity.

The contribution of the bathypelagic part of the population to the total numbers of pelagic

perch fry decreased from 28�1% in May to 4�7% in June, while the density of all pelagic perch

fry increased (c. 96 000 individuals ha�1 in May and 142 000 individuals ha�1 in June). In May,

the bathypelagic (average total length, LT, 11�9mm) and epipelagic (average LT 14�6mm) perch

fry differed in size while, in June, the epipelagic fry were divided into two distinct size groups.

The more abundant group, of small epipelagic perch fry (average LT 14�6mm), was similar in

size to the bathypelagic fry (average LT 14�6mm) while the less abundant group, of larger

epipelagic fry (average LT 34�4mm), was similar in size to littoral perch fry (average LT

35�0mm). The results suggest that in perch fry three different survival strategies with different

risks can be used in the same locality, time and year. # 2005 The Fisheries Society of the British Isles

Key words: echosounder; fry distribution; Gymnocephalus cernuus; ichthyoplankton; Sander

lucioperca; Slapy Reservoir.

INTRODUCTION

Perch Perca spp. spawn in April and May, in shallow littoral areas (depths 0–8m)at temperatures ranging predominantly from 7 to 11� C (Clady, 1976; Thorpe,1977; O’Gorman, 1983; Craig, 1987; Treasurer, 1988). Their eggs develop for 10to 20 days in temperatures ranging from 10 to 15� C and embryos hatch at sizesbetween 4 and 6mm (Thorpe, 1977; Whiteside et al., 1985). Smaller individualshatch first (Il’jina, 1973) and grow more slowly (Guma’a, 1978).

*Author to whom correspondence should be addressed. Tel.: þ42 03 87 77 58 70; fax: þ42 03 85 31 02 48;

email: carcharhinusleucas@yahoo.com

Journal of Fish Biology (2005) 66, 685–702

doi:10.1111/j.1095-8649.2005.00630.x,availableonlineathttp://www.blackwell-synergy.com

685# 2005TheFisheries Society of theBritish Isles

Soon after hatching the perch larvae migrate from the littoral into the pelagichabitat. Before they return to the littoral area after metamorphosis, perch frystay in the epilimnion for a month, or even longer (Ward & Robinson, 1974;Kelso & Ward, 1977; Coles, 1981; Whiteside et al., 1985; Treasurer, 1988),during which time they prefer higher temperatures (Ross et al., 1977) andseem to be positively phototactic (Disler & Smirnov, 1977; Craig, 1987). Atthis time, the transparency of perch larvae (Ward & Robinson, 1974; Coles,1981) is supposed to be an adaptation for life in the pelagic zone of lakes andreservoirs, making them less visible to predators (Faber, 1967).Many authors have noted maximum abundances of perch fry in the

upper 4m of a pelagic water column (Guma’a, 1978; Coles, 1981; Viljanen &Holopainen, 1982; Whiteside et al. 1985; Post & McQueen, 1988; Treasurer,1988; Wang & Eckmann, 1994; Matena, 1995a; Urho, 1996). There are alsorecords of perch fry from greater depths (Ward & Robinson, 1974; Cooperet al., 1981; Perrone et al., 1983; Kubecka & Slad, 1990), but their distribution isnot sufficiently understood and few studies give possible evidence for dielvertical migrations of perch fry in the depth range of 0–5m (Ward & Robinson,1974; Kelso & Ward, 1977). Older perch may undertake even bigger migrationsin various parts of water column (Hergenrader & Hasler, 1966; Goldspink,1990; Eckmann & Imbrock, 1996).The present study details diel vertical migration of a distinct portion of the

pelagic perch Perca fluviatilis L. fry stock in Slapy Reservoir. In addition, thepossible advantages of simultaneous acoustic and direct ichthyoplankton sam-pling for the estimation of real fry abundances are shown.

MATERIALS AND METHODS

STUDY AREA

The study was carried out in the canyon-shaped, meso- to eutrophic Slapy Reservoir,Czech Republic (49�4902800N; 14� 2505800 E, 40km south of Prague), which has an areaof 1392ha (length 42 km, mean width 313m), a volume of 269� 106m3, and maximumdepth of 58m. The littoral zone involves <5% of the total reservoir area but even less in thelacustrine study reach. The reservoir was built in 1954 by damming the River Vltava as thethird of nine recently built Vltava River cascade reservoirs. Because of a relatively highmean annual inflow of 85m3 s�1, the average theoretical retention time (reservoir volumedivided by the tributary discharge) is only 38�5 days (Hrbacek & Straskraba, 1966).

Slapy Reservoir is one of the few Czech reservoirs where cyprinids do not prevail. Since1961, cold profundal water has supplied the reservoir from the newly built, huge OrlıkReservoir upstream (Hrbacek, 1984) and has caused reversion of the fish stock develop-ment from cyprinid to perch dominance (Hanel & Cihar, 1983). The persistent dom-inance of perch was validated by Hanel (1988, 1990) in 1985–1987 using seines and a dropnet (perch contributed up to 88% by numbers of young-of-the-year, YOY, fish), and byCech & Cech (2001) in 1999–2001 using diet analysis of kingfishers Aleedo atthis (perchcontributed 71�9–86�5% by numbers of 0þ and 1þ year fishes eaten).

SAMPLING

Both acoustic and complementary net data (pelagic and littoral) were collected in two24 h sampling surveys during 29–30 May and 17–18 June 2002 in the study area fromZivohost’sky Bridge to Nova Zivohost’ [Fig. 1(a)]. The perch fry assemblage of Slapy

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Reservoir was divided into three parts (littoral, depth <2m; epipelagic, epilimnion of theopen water; bathypelagic, hypolimnion of the open water). Temperature and dissolvedoxygen were measured using a calibrated YSI 556 MPS probe. Light intensity on the watersurface was determined with a lux meter MDLX. In June, a LICOR LI-250 underwaterlight meter was also used to measure light penetration below the water surface.The acoustic investigation was carried out using a SIMRAD EY 500 split-beam

scientific echosounder, working with a frequency of 120KHz. The pulse repetition ratewas 10 pings s�1. The echosounder was controlled by a personal computer (Acer Extensa355). The transducer used (SIMRAD ES120–7G) had a circular beam pattern with a

N

TRIBUTARY

DAM

0 1 km

PragueSlapy Reservoir

RV-OO

T2

R

SB

CBW

FT1

IN

SEININGAREA

TRAWLING AREA

(a)

(b)

Zivohošt’ský Bridge

StaráZivohošt’

NováZivohošt’

FIG. 1. (a) A map of Slapy Reservoir and its location in the Czech Republic. The relative position of the

sampling sites is shown. *, the stratification variables (temperature and dissolved oxygen) were

measured at this point. (b) Diagram showing the sampling operation: T1, circular split beam

transducer (beam angle 7�1� ); RV-OO, research vessel Ota Oliva; R, 50m rope; T2, transducer of

beam angle 60� ; SB, support boat; W, 10 kg weight; IN, ichthyoplankton net; F, floater; CB,

collecting bucket.

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nominal angle of 7�1�. The transducer, beaming vertically, was held by a remotelycontrolled aluminium plate on the frame construction in front of the research vesselOta Oliva. Acoustic data were stored on the hard disk of the computer for later analysis.The whole sonar system was calibrated with a standard calibration copper sphere of23mm diameter (Foote et al., 1987). To detect all YOY fishes, including the smallest fishlarvae, the threshold for the primary noise filtering of the acoustic record during field-work was set to a minimal target strength (TS; MacLennan & Simmonds, 1992) of�80 dB.

At the same time, fish signals were validated using a conical ichthyoplankton net (2min diameter) with a rectangular mesh-size of 1� 1�35mm (Wanzenbock et al., 1997). Thenet had a 10 kg weight attached to the lower part of the frame and a styrofoam floater atthe surface. Sampling depth was adjusted by the length of the connecting line between theupper part of the net frame and the floater. The ichthyoplankton net was towed along aslightly curved trajectory (the net did not sample the area disturbed by the boat) c. 50mbehind the boat, usually for 5min, at a velocity of 0�8–1�1m s�1 (3–4 kmh�1). Thevolume of water filtered (comprising in total 27 200m3 in May and 35 150m3 in June; atotal of 85 hauls were made with the ichthyoplankton net) was calculated from the realsampling distance measured by Garmin eTrex Summit GPS and the opening area of thenet mouth. Several vertical tows of the net were made to verify the negligible contamina-tion of ichthyoplankton catches by fry from the upper layers when pulling the net fromthe deeper layers to the surface. The accuracy of the depth of the towed ichthyoplanktonnet was, in deeper layers (>4m), checked by a commercial echosounder (Eagle UltraClassic) equipped with a transducer of 60� beam width mounted on the supporting smallboat driven at the same speed during the tow, parallel to the floater [Fig. 1(b)].

Additional littoral samples of fry were taken during four diurnal periods: the day(0600–2000 hours), at dusk (2000–2200 hours), the night (2200–0400 hours) and at dawn(0400–0600 hours) with a beach seine of 10� 2m with rectangular mesh-size of1� 1�35mm (only three to six seinings per one diurnal period were made, 150m3 each,in order to avoid local overfishing of the poorly developed littoral area of the reservoir;seining places were randomly chosen along a shore 1 km in length).

Samples obtained from both littoral (nMay¼ 305, nJune¼ 947 YOY fishes) and ichthy-oplankton (nMay¼ 7422, nJune¼ 4313 YOY fishes) catches were immediately preserved in6–10% formaldehyde, in the field. All fish larvae and juveniles were determined to speciesaccording to the keys of Koblickaya (1981) and into developmental stages accordingto the keys of Pinder (2001), measured (total length, LT) and grouped into 1mm LT

classes.The acoustic data were analysed using the automatic tracking facilities of the new post-

processing software, Sonar5, developed at the University of Oslo (Balk & Lindem, 2003).To obtain a detailed picture of diel vertical migration (DVM) of perch fry, the watercolumn was divided into 1m-thick layers down to a depth of 16m below the watersurface. Below this depth, no fry were observed. The uppermost 2m of the water columnwere not accessible to acoustic analysis due to the near field of the transducer (0�97m),possible avoidance, and the low sampling volume at closer ranges, and the fry abundancehad to be reconstructed using net catches. For each of the other 14 1m- thick layers, theabundance of fry was then calculated for each 10min of the acoustic record. This wasdone using echointegration, and by scaling the echointegrated energy with the averagebackscattering cross section (MacLennan & Simmonds, 1992). The backscattering crosssection came from the analysis of a single target population, and its quality was furtherimproved by tracking. Outside the shoals, the proportion of sizeable single targets in thetotal fry volume scattering strength (sv, MacLennan et al., 2002) ranged between 70–100%, of which >80% satisfied the tracking criteria. All sizing of acoustically detectedfry and setting of the size limits was done using the perch fry TSLT

�1 relationship fortheir dorsal aspect (Frouzova & Kubecka, 2004).

To examine whether the depth of bathypelagic perch fry was controlled by lightintensity, the depth of the main layer was defined as the 1m thick layer of the watercolumn with the highest abundance of migrating, non-shoaling perch fry. In addition, tocompare the selective advantage of shoaling behaviour (Magurran, 1990) to bathypelagic

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perch fry, the depth of the shoal (acoustic fish shoal; Freon et al., 1992) was also definedas the 1m thick layer of the water column where the centre of a perch fry shoal waspresent. For every shoal, its physical dimensions were measured: (1) the length of a fryshoal was calculated from the number of acoustic emissions produced per second (systemping rate) and the time spent by the shoal in between the borders of the emittedultrasound cone and the speed of the research vessel; (2) the height of the shoal wascalculated from the uppermost and lowermost shoal margins.To complete the data, in the most characteristic periods of the diurnal cycle (mid-day,

1000–1400 hours; mid-night, 2300–0300 hours), fry abundance in the upper two layers ofthe water column (0–1 and 1–2m below the water surface) was reconstructed from theresults of the towed ichthyoplankton net and the ratio between estimated ichthyoplank-ton net abundance of fry and acoustic abundance of fry in deeper layers sampledsimultaneously by the net and the acoustics.In May, the LT, of perch larvae and juveniles caught in the pelagic zone of Slapy

Reservoir ranged from 8�3 to 23�6mm. For the post-processing procedure, the TS thresh-old was set at �70 dB (6�2mm LT), to avoid acoustic under-estimation of perch fryabundance caused by inclination of fish larvae while maintaining constant depth(Frank, 1967; Ross et al., 1977) or tilting of the fish body during DVM and ascent anddescent in the water column (Cech & Kubecka, 2002). To exclude infrequently occurringlarger fish, targets > �57 dB (26�0mm LT) were manually erased from the analysis,using the erase function of Sonar5 (Balk & Lindem, 2003). In June, the LT of perchlarvae and juveniles caught in the pelagic zone of Slapy Reservoir ranged from 8�9 to40�0mm. Consequently, the TS threshold was set to �68 dB (7�7mm LT). Targets>�53 dB (40�5mm LT) were again erased from the analysis. The other configuration ofthe automatic tracking facility in May and June is given in Table I.The data were analysed using linear regression and t-tests. Where necessary, ANOVA

for unequal N was used instead of the t-test.

RESULTS

In May, for most of the day YOY perch were not dominant in the littoral(0–19�7% in abundance; the zero value occurred at dusk) except at dawn whenperch dominated this habitat [91�1%; Fig. 2(a)]. In contrast, in June perchstrongly dominated the fry assemblage of the littoral zone for most of the day(90�7–96�6% in abundance), except at dusk, when no perch fry were againrecorded in this habitat [Fig. 2(b)]. The distribution of fry in littoral zone wasrather variable due to local aggregations; absolute fry densities were from tensto hundreds of individuals 1000m�3 (Fig. 2).

TABLE I. Tracking variables: minimum track length, minimal number of detections totrack a fish (hits in beam); maximum ping gap, maximal number of missing pings per

track; gating range, maximal range between detections

29–30 May 17–18 June

Day Night All 24 h

Layer (m) 2–6 6–8 8–16 2–5 5–8 8–16 2–8 8–16Minimum track length (ping) 2 3 4 1 3 4 1 4Maximum ping gap (ping) 0 0 0 0 0 2Gating range (m) 0�07 0�07 0�07 0�07 0�07 0�07

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In both May and June, perch strongly dominated the fry stock of the pelagiczone in Slapy Reservoir (Fig. 3). In May, the bathypelagic fry community (10–16m below the water surface in the period 1000–1400 hours) was composed of95�5% perch, 3�2% zander Sander lucioperca (L.) and 1�3% ruffe Gymnocephaluscernuus (L.). In June, the daytime bathypelagic fry community was composed of92�1% perch, 5�7% ruffe and 2�2% zander.The large contribution of YOY perch to the fry community of the pelagic

zone of Slapy Reservoir led to the conclusion that the signals on the echogramwere related almost exclusively to perch. Consecutive acoustic analyses revealedthat, in both months, during the night most of the acoustic biomass of pelagicperch fry (>93% sv in May and >98% sv in June) was concentrated in the upper5m of the water column (Fig. 4). Night sv values were higher than day onessuggesting that some of the epipelagic fry descended deeper and could bedetected by the echosounder. In May, the night peak of acoustic biomass wasin the 3–4m layer, just above the thermocline [Figs 4(a) and 5(a)]. In June, the

0

20

40

60

80

100

DAY DUSK NIGHT DAWN

DAY DUSK NIGHT DAWN

Period

25±22

111±129

211

±226224

±189

0

20

40

60

80

100

Abu

nda

nce

(%

)

504(b)

(a)

±111467

±115528

±63598

±62

FIG. 2. Composition of the fry assemblage [cyprinids (&), ruffe ( ) and perch (&)] in the littoral zone of

Slapy Reservoir in (a) May and (b) June 2002 during day time (0600–2000 hours), dusk (2000–2200

hours), night (2200–0400 hours) and dawn (0400–0600 hours) estimated by seining. Numbers above

each column show littoral fry mean� S.D. individuals 1000m�3.

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# 2005TheFisheries Society of theBritish Isles, Journal of FishBiology 2005, 66, 685–702

peak of acoustic biomass was in the 2–3m depth layer. Cross referencing theacoustic results with the ichthyoplankton catches, however, revealed that, inJune, the maximum perch fry occurrence was even closer to the water surface(depth layer 0–2m, i.e. in the blind zone of the echosounder), which might beinduced by a less steep temperature stratification [Figs 4(b) and 5(b)]. Duringdawn, the acoustic biomass for the depth range 2–16m dropped suddenly andthe peak of acoustic biomass of perch fry started to descend into the deeperlayers. After reaching the depth maximum during the noon period (13–14m inMay, 11–12m in June), the peak of acoustic biomass started to ascend slowlytowards the surface during the afternoon and dusk. After dusk (in June already

0

20

40

60

80

100

268±45

156±53

63±8

953±212

345±71

0

20

40

60

80

100

BATHY-DAY

EPI-DAY EPI-DUSK

EPI-NIGHT

EPI-DAWN

Period

BATHY-DAY

EPI-DAY EPI-DUSK

EPI-NIGHT

EPI-DAWN

Abu

nda

nce

(%

)

21(b)

(a)

±4147±58

161±26

866±327

162±31

FIG. 3. Composition of the fry assemblage [cyprinids (&), ruffe ( ), zander ( ) and perch (&)] in the

pelagic zone of Slapy Reservoir in (a) May and (b) June 2002 during day time (0600–2000 hours),

dusk (2000–2200 hours), night (2200–0400 hours) and dawn (0400–0600 hours) estimated with the

ichthyoplankton net. Numbers above each column show pelagic fry mean� S.D. individuals

1000m�3. BATHY, bathypelagial (10–16m below the water surface, in the period 1000–1400

hours exclusively); EPI, epipelagial (0–4m below the water surface).

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during dusk), the value of acoustic biomass analysed for the whole watercolumn again increased dramatically. The sequence of echograms [Fig. 6(a),(c)] revealed that around dawn, the bathypelagic perch fry left the upper layersof the water column (epilimnion and metalimnion) and, as a scattering layer,migrated into the deeper layers (hypolimnion). During the afternoon, the layer

2·5

4·5

6·5

8·5

10·5

12·5

14·5

0000

–020

0

0400

–060

0

1000

–120

014

00–1

600

2000

–220

0

0

0·0000002

0·0000004

0·0000006

0·0000008

0·0000010

0·0000012

0·0000014

0·0000016

0·0000018

s v (

m2 m

–3)

s v (

m2 m

–3)

Depth (m)

Depth (m)

Period

2·5

4·5

6·5

8·5

10·5

12·5

14·5

0000

–020

004

00–0

600

0800

–100

012

00–1

400

1800

–200

022

00–2

400

0

0·0000001

0·0000002

0·0000003

0·0000004

0·0000005

0·0000006

0·0000007

0·0000008

Period

(a)

(b)

FIG. 4. Diurnal fluctuation of acoustic biomass (sv) of pelagic fry in 1m layers of the acoustically

sampled water column of Slapy Reservoir in (a) May and (b) June 2002 time period (hours): ,

0000–0200; , 0200–0400; , 0400–0600; , 0600–0800; , 0800–1000; , 1000–1200; &,

1200–1400; , 1400–1600; , 1600–1800; , 1800–2000; , 2000–2200; , 2200–2400].

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of bathypelagic fry migrated in the opposite direction and around dusk enteredthe epilimnion. It was evident that changes in the light intensity were followedby changes in the depth of the main layer of bathypelagic perch fry. Theincrease in light intensity during dawn was followed by the descent ofbathypelagic perch fry into the deeper layers while decreasing light intensityduring late afternoon and dusk was followed by the ascent of these fry towardsthe surface [Fig. 6(b), (d)]. The maximum amplitude of the DVM of bathypelagicperch fry was 11m in May and 12�5m in June and the depth of the main layerof bathypelagic fry was therefore strongly controlled by the light intensity(regression analysis May; F1,27, P< 0�001, r2¼ 0�84; regression analysis June;F1,36, P< 0�001, r2¼ 0�93; Fig. 7).In contrast to May, in June, shoaling behaviour was observed in some of the

bathypelagic perch fry and the shoals (n¼ 58) were significantly higher in the

Light (µmol m–2 s–1)

0 1000 20000

2·5

5·0

7·5

10·0

12·5

15·0

17·5

20·0

Dep

th (

m)

O2 (mg l–1)

0

2·5

5·0

7·5

10·0

12·5

15·0

17·5

20·0

5 15 25 0 5 10

Temperature (° C)(a)

(b)

FIG. 5. Comparison of the vertical distribution of temperature, dissolved oxygen and photosynthetically

active light (radiation) measured during the noon period in (a) May and (b) June 2002 in Slapy

Reservoir. Photosynthetically active light was measured only in June.

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1 3 5 7 9 11 13 15 17 19 2116

2000

1600

3004

5405

1708

2812

14

(a)

1 3 5 7 9 11 13 15 17 19 21

Depth (m)

1615

1910

2044

2140

2217

0059

0410

0535

0930

1204

(c)

(b)

(d)

Tim

e (h

ours

)

0 2 4 6 8 10 12 14 16

1212125613571559190220102105005701590258045408420935115702040608010

0

120

140

020406080100

120

140

Light intensity (103 lx)

0 2 4 6 8 10 12 14 16

12131634174018402056215100430142040904590557094310471144

Tim

e (h

ours

)

Depth of main layer (m)

1904

2046

2107

FIG.6.Ilustrationofthedielverticalmigrationofperch

fryin

SlapyReservoir

in(a),(b)Mayand(c),(d)June2002.(a),(c)Sequence

ofraw

20LogR

TVG

(for

definitionseeMacL

ennan&

Sim

monds,1992)echogramsrepresentingday,dusk,night,dawnanddayagain

and(b),(d)dependence

ofthedepth

ofthemain

layer

(––)onthelightintensity

(--

--)measuredatthewatersurface.Tracesbelow

thefrylayer

(May,time0016,0030,0454hours)correspondto

chironomid

pupaeenteringupper

layersofthehypolimnionduringnight-time.

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water column than the depth of the main layer (t-test, d.f.¼ 114, P< 0.001;Fig. 8). Although occurring nearer the water surface, the depth of the shoals wasalso strongly controlled by light intensity (regression analysis, y¼ 2�58x� 2�00;r2¼ 0�72, F1,56, P< 0.001). The length of bathypelagic perch fry shoals rangedfrom 1 to 4�7m; the average length of a shoal was 2�6m; the height of the shoalsranged from 0�37 to 1�31m with an average height of 0�67m. The distributionwas highly asymmetric, with only a few shoals >1m in height.Average fry (all species) densities in every depth layer (layers 0–2m corrected

by the netting results) provided an estimate of total fry numbers and theproportions of epi- and bathypelagic fry (Fig. 9). Day and night abundancesin whole water column differed significantly, especially in the June data [1980and 14 210 individuals 1000m�2, Fig. 9(c), (d)] which probably indicated day-time net avoidance in the surface layers. This is why the night results integrated

0

2

4

6

8

10

12

14

160 1 2 3 4 5 6

Light intensity (log10 lx)

Dep

th o

f m

ain

laye

r (m

)

FIG. 7. The relationship between light intensity and the depth of the main layer of bathypelagic perch fry

in May ( , – –) (y¼ 2�38xþ 1�42; r2¼ 0�84, P< 0�001) and June ( , —) (y¼ 3�48x–4�86, r2¼ 0�93,P< 0�001) in Slapy Reservoir.

0

2

4

6

8

10

12

14

16

0537

1004

1052

1114

1130

1136

1151

1202

1205

1212

1215

1615

1707

1714

1758

1841

1846

1911

1922

2042

Time (hours)

Dep

th (

m)

FIG. 8. Comparison of the depth of main layer of bathypelagic perch fry (created by non-shoaling perch

fry individuals exclusively, ) and the depth of bathypelagic perch fry shoal (created by shoaling

perch fry individuals exclusively, n) in Slapy Reservoir in June 2002. (t-test, d.f.¼ 114, P< 0.001).

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for all depths [Fig. 9(b), (d)] were more likely to provide the total estimate of thepelagic fry cohort and the share of bathypelagic fry on the total cohort shouldhave been related to night rather than day total abundance. During May 28�1%

(c) (d)

0 1000 2000 3000 4000 5000

thermocline

epilimnion

hypolimnion

0 2000 4000 6000 8000

thermocline

epilimnion

hypolimnion

0 500 1000 1500 2000

0·5

2·5

4·5

6·5

8·5

10·5

12·5

14·5

16·5

Abundance (individuals 1000 m–3)

thermocline

epilimnion

hypolimnion

0 500 1000

0·5

2·5

4·5

6·5

8·5

10·5

12·5

14·5

16·5

Dep

th (

m)

thermocline

epilimnion

hypolimnion

(a) (b)

**

**

5380

1980

9600

14210

FIG. 9. Comparison of meanþ S.D. pelagic fry abundance (all species) in the upper 16m of the water

column during (a), (c) mid-day (1000–1400 hours) and (b), (d) mid-night (2300–0300 hours) in Slapy

Reservoir in (a), (b) May and (c), (d) June 2002 estimated acoustically. Numbers above each graph

show pelagic fry abundance (individuals 1000m�2; sum of all depths). The distribution of the

epilimnion, hypolimnion and thermocline are given. Note different scales of the x-axis for each

plot. *, Fry abundance in the upper two layers of water column (0–1, 1–2m below the water surface)

was calculated from the results of a towed ichthyoplankton net (2m in diameter) and the ratio

between estimated ichthyoplankton net fry abundance and acoustic fry abundance in deeper layers.

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[c. 1894 of 9600 individuals 1000m�2, Fig. 9(b)] of the population of pelagic perchfry (bathypelagic) performed DVM. The remaining 71�9% of the population ofpelagic perch fry (epipelagic) stayed in the epilimnion, mainly in the upper 2mof the water column [Fig. 9(a), (b)]. In June, the group of migrating bath-ypelagic perch fry comprised only 4�7% [c. 586 of 14 210 individuals 1000m�2,Fig. 9(d)] of the pelagic fry and 95�3% belonged to the non-migrating epipelagicfry [Fig. 9(c), (d)]. Around dawn in both months, the bathypelagic perch frysegregated from the epipelagic fry left in the upper layers of the water column,and stayed in the hypolimnion during the daylight hours. During dusk, thebathypelagic fry reached the epilimnion again and joined together with theepipelagic perch fry. In May, the whole pelagic stock of perch fry thendescended slightly and spent the night around the thermocline. In June thisdescent was not so evident and most of the pelagic stock of perch fry stayed inthe upper 2m of the water column.Apart from the different use of space in time, a difference between

bathypelagic and epipelagic perch fry was also recorded in their sizes. In May,bathypelagic perch fry (average 11�9mm LT) were significantly smaller than theepipelagic fry (average 14�6mm LT) [ANOVA, F1,584, P< 0.001, Fig. 10(a)].

30

30

25

20

15

(a)

(b)

Fre

quen

cy (

%)

10

5

0

25

20

15

10

5

LT (mm) Target strength (dB)

0 6–7

0

–67

–65

–63

–61

–59

–58

–57

–56

–55

–54

–53

8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44

FIG. 10. Frequency distribution of total length of perch fry (day catches; period 0600–2000 hours) in (a)

May and (b) June in Slapy Reservoir. BATHY-AC, (1) bathypelagic fry (all targets tracked at

depths 10–16m in period 1000–1400 hours exclusively). For acoustic records, the x axis gives the

original target strength TS (non-linear) and the corresponding total lengths estimated using a TS

and LT conversion according to Frouzova & Kubecka (2004). BATHY-IN (&), bathypelagic

perch fry from ichthyoplankton catches (10–16m below the water surface, in the period 1000–

1400 h exclusively); EPI-IN (2) ( ), epipelagic perch fry from ichthyoplankton catches (0–4m

below the water surface); LITT-SN (&), littoral perch fry from seine catches.

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In June, the epipelagic perch fry showed a clear bimodal character. There was agroup of small epipelagic fry ranging from 10 to 22mm LT (average 14�6mmLT, comprising 73�8% of the epipelagic perch fry) and a group of big epipelagicfry ranging from 25 to 41mm LT (average 34�4mm LT, 26�2% of epipelagic fry)[Fig. 10(b)]. The group of larger epipelagic perch fry was caught exclusively inthe upper 2m of the water column. Small epipelagic fry did not differ signifi-cantly in size from the bathypelagic fry (average 14�6mm LT) (ANOVA, F1,273,P¼ 0�93) but, in contrast, the big epipelagic fry were almost the same size asthose in the littoral with sizes ranging from 20 to 43mm LT (average 35�0mmLT) [ANOVA, F1,204, P¼ 0�20, Fig. 10(b)].

DISCUSSION

In Slapy Reservoir, there are three ecological groups of YOY perch present atthe same time, differing in their distribution patterns and size structures, and theswitch to a demersal mode of life was far less apparent than usually reported(Coles, 1981; Whiteside et al., 1985; Treasurer, 1988; Urho, 1996). The littoralregion of the canyon-shaped Czech reservoirs is favoured by fry (Duncan &Kubecka, 1995; Matena, 1995b). The littoral, however, comprises a negligiblepart of Slapy Reservoir volume (<<1%), and so has limited carrying capacityand harbours restricted absolute numbers of fry (although relative densities arecomparable with open water). This is why it is very unlikely that eventualhorizontal migration of perch fry between littoral and open water (Gliwicz &Jachner, 1992) would influence the changes of numbers and distribution in theopen water (there was never enough of littoral fish to change the proportions inmuch larger volumes of open water). Instead, the majority of perch fry utilizethe pelagic zone for a longer period. Most of the fry stay in the epilimnion,where it is difficult to provide a direct estimate by acoustic methods (MacLennan& Simmonds, 1992). Under these conditions, the use of data reconstructedaccording to the catch of an ichthyoplankton net seemed to be essential to obtainestimates of fry abundance.This study for the first time elucidates in detail the phenomenon of scattering

layers observed in several perch dominated lakes and reservoirs in the CzechRepublic (V. Hruska, unpubl. data; J. Kubecka, unpubl. data). In the case ofDVM by perch fry in Slapy Reservoir, some interesting questions still remainunanswered. One question is, whether DVM of perch fry and the amplitude ofthis movement is definitely under the direct control of light intensity as seems tobe indicated by statistical analysis in this study and as was demonstrated foroverwintering perch by Eckmann & Imbrock (1996) or whether the twilight isonly a cue and all this migration behaviour is genetically coded as was describedby Gaudreau & Boisclair (1998, 2000) in the case of dace Phoxinus eos(Cope)�Phoxinus neogaeus Cope. In Slapy Reservoir direct regulation bylight appears to be more likely, since it was observed that the layer ofbathypelagic fry rose slightly every time clouds covered the sun, even for severalminutes, and vice versa.A distinct portion of daily migrating pelagic perch fry (bathypelagic) moved

from warm epilimnetic water during the night (18� C in May, c. 23� C in June)to cold hypolimnetic water during the day (9�5� C in May, 11� C in June). Wang

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& Eckmann (1994), however, have described the temperature preference ofperch larvae as between 16 and 26� C and Ross et al. (1977) more preciselybetween 22 and 24� C. The results of Kudrinskaya (1970) have suggested that attemperatures <16� C, 80–90% of perch larvae stop feeding and their growthrate decreases up to five times. Karas (1987) has shown mortality of perchlarvae, if the temperature drops to <10–12� C. Gut content of bathypelagicperch fry in Slapy Reservoir was low compared to epipelagic and littoralperch fry (M. Kratochvıl & J. Peterka, pers. data). Why does the migratingportion of the pelagic perch fry population spend the daylight hours attemperatures that do not allow them to feed and grow?Seeking refuge from predators in the dark hypolimnion seems to be the most

obvious explanation of DVM of bathypelagic fry. Despite the fact perch frytransparency is an excellent strategy for staying invisible to horizontally scan-ning eyes, Thetmeyer & Kils (1995) have shown that in some cases transparentorganisms may be visible and detectable by predators due to light absorbanceby opaque parts of the body or light scattered in the tissues. In addition, thenewly discovered sinusoidal swimming pattern of common reservoir fishes as aswim-search behaviour is probably an efficient way to sense transparent prey(Cech & Kubecka, 2002). The position of sporadic shoals of bathypelagic perchfry, probably created as an antipredator defence (Magurran, 1990; Eklov &Persson, 1995), clearly above the layer with the highest density of non-shoalingbathypelagic fry does also suggest a possible role of predation threat. The otherreason for DVM could be alternative optimization of energetic loses by areduction of metabolism in cold hypolimnetic water (Northcote et al., 1964;Levy, 1990).A number of studies document the DVM as a trade-off between well fed and

living dangerously or hungry and hidden (Clark & Levy, 1988; Goldspink, 1990;Levy, 1990; Appenzeller & Leggett, 1995; Eckmann & Imbrock, 1996). Undercertain circumstances fish cohorts usually adopt one of the strategies at a time.In Slapy Reservoir, the same cohort of YOY perch seems to be able to adaptseveral survival strategies at a time (epipelagic, bathypelagic and littoral). Thistype of ecological differentiation may be useful way for survival in large waterbodies where all types of habitats are available and at least one of ecologicalgroup can succeed.

We thank J. Frouzova, Z. Prachar, M. Prchalova, P. Stafa and M. Vasek for help indata collection, M. Burgis for careful reading and correcting the English and twoanonymous referees for helpful comments to the manuscript. The study was supportedby the Grant Agency of the Czech Academy of Sciences (projects No. S 6017004, A6017201, A 6017901 and K 6005114).

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