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

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

Diel vertical migrations of bathypelagic perch fry

M. Cˇ ECH*, M. KRATOCHVI´L, J.KUBECˇ KA,V.DRASˇ TI´K AND

J. MATEˇ NA

Hydrobiological Institute, Academy of Sciences of the Czech Republic, Na sa´dka´ch 7,

370 05 Cˇeske´ Budeˇjovice, Czech Republic and Faculty of Biological Sciences, University

of South Bohemia, Branisˇovska´ 31, 370 05 Cˇeske´ Budeˇjovice, 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 110 m in May and 125 m 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 3–4 m in May, 0–2 m 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 281% in May to 47% 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, L T ,119 mm) and epipelagic (average L T 146 mm) 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 L T 146 mm), was similar in

size to the bathypelagic fry (average L T 146 mm) while the less abundant group, of larger

epipelagic fry (average L T 344 mm), was similar in size to littoral perch fry (average L T

350 mm). 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 0–8 m)

at temperatures ranging predominantly from 7 to 11 C (Clady, 1976; Thorpe,

1977; O’Gorman, 1983; Craig, 1987; Treasurer, 1988). Their eggs develop for 10

to 20 days in temperatures ranging from 10 to 15 C and embryos hatch at sizes

between 4 and 6 mm (Thorpe, 1977; Whiteside et al., 1985). Smaller individuals

hatch 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

# 2005 The Fisheries Society of the British Isles

685


686 M. Cˇ ECH ET AL.

Soon after hatching the perch larvae migrate from the littoral into the pelagic

habitat. Before they return to the littoral area after metamorphosis, perch fry

stay 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) and

seem to be positively phototactic (Disler & Smirnov, 1977; Craig, 1987). At

this 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 and

reservoirs, making them less visible to predators (Faber, 1967).

Many authors have noted maximum abundances of perch fry in the

upper 4 m 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; Matěna, 1995a; Urho, 1996). There are also

records of perch fry from greater depths (Ward & Robinson, 1974; Cooper

et al., 1981; Perrone et al., 1983; Kubecˇka & Slad, 1990), but their distribution is

not sufficiently understood and few studies give possible evidence for diel

vertical migrations of perch fry in the depth range of 0–5 m (Ward & Robinson,

1974; Kelso & Ward, 1977). Older perch may undertake even bigger migrations

in 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, the

possible advantages of simultaneous acoustic and direct ichthyoplankton sampling

for the estimation of real fry abundances are shown.

STUDY

AREA

MATERIALS AND METHODS

The study was carried out in the canyon-shaped, meso- to eutrophic Slapy Reservoir,

Czech Republic (49 49 0 28 00 N; 14 25 0 58 00 E, 40 km south of Prague), which has an area

of 1392 ha (length 42 km, mean width 313 m), a volume of 269 10 6 m 3 ,andmaximum

depth of 58 m. The littoral zone involves


VERTICAL MIGRATIONS OF PERCH FRY 687

(a)

DAM

Prague

Slapy Reservoir

Zivohošt’ský Bridge

Stará

Zivohošt’

Nová

Zivohošt’

N

SEINING

AREA

TRAWLING AREA

TRIBUTARY

0 1 km

(b)

T1

RV-OO

R

T2

SB

F

W

IN

CB

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 71 ); RV-OO, research vessel Ota Oliva; R, 50 m rope; T2, transducer of

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

collecting bucket.

Reservoir was divided into three parts (littoral, depth


688 M. Cˇ ECH ET AL.

nominal angle of 71 . The transducer, beaming vertically, was held by a remotely

controlled aluminium plate on the frame construction in front of the research vessel

Ota 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 of

23 mm diameter (Foote et al., 1987). To detect all YOY fishes, including the smallest fish

larvae, the threshold for the primary noise filtering of the acoustic record during fieldwork

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 (2 m

in diameter) with a rectangular mesh-size of 1 135 mm (Wanzenbo¨ck et al., 1997). The

net had a 10 kg weight attached to the lower part of the frame and a styrofoam floater at

the surface. Sampling depth was adjusted by the length of the connecting line between the

upper part of the net frame and the floater. The ichthyoplankton net was towed along a

slightly curved trajectory (the net did not sample the area disturbed by the boat) c. 50m

behind the boat, usually for 5 min, at a velocity of 08–11ms 1 (3–4 km h 1 ). The

volume of water filtered (comprising in total 27 200 m 3 in May and 35 150 m 3 in June; a

total of 85 hauls were made with the ichthyoplankton net) was calculated from the real

sampling distance measured by Garmin eTrex Summit GPS and the opening area of the

net mouth. Several vertical tows of the net were made to verify the negligible contamination

of ichthyoplankton catches by fry from the upper layers when pulling the net from

the deeper layers to the surface. The accuracy of the depth of the towed ichthyoplankton

net was, in deeper layers (>4 m), checked by a commercial echosounder (Eagle Ultra

Classic) equipped with a transducer of 60 beam width mounted on the supporting small

boat 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 2 m with rectangular mesh-size of

1 135 mm (only three to six seinings per one diurnal period were made, 150 m 3 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 (n May ¼ 305, n June ¼ 947 YOY fishes) and ichthyoplankton

(n May ¼ 7422, n June ¼ 4313 YOY fishes) catches were immediately preserved in

6–10% formaldehyde, in the field. All fish larvae and juveniles were determined to species

according to the keys of Koblickaya (1981) and into developmental stages according

to the keys of Pinder (2001), measured (total length, L T ) and grouped into 1 mm L T

classes.

The acoustic data were analysed using the automatic tracking facilities of the new postprocessing

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 water

column was divided into 1 m-thick layers down to a depth of 16 m below the water

surface. Below this depth, no fry were observed. The uppermost 2 m of the water column

were not accessible to acoustic analysis due to the near field of the transducer (097 m),

possible avoidance, and the low sampling volume at closer ranges, and the fry abundance

had to be reconstructed using net catches. For each of the other 14 1 m- thick layers, the

abundance of fry was then calculated for each 10 min of the acoustic record. This was

done using echointegration, and by scaling the echointegrated energy with the average

backscattering cross section (MacLennan & Simmonds, 1992). The backscattering cross

section came from the analysis of a single target population, and its quality was further

improved by tracking. Outside the shoals, the proportion of sizeable single targets in the

total fry volume scattering strength (s v , MacLennan et al., 2002) ranged between 70–

100%, of which >80% satisfied the tracking criteria. All sizing of acoustically detected

fry and setting of the size limits was done using the perch fry TS L 1 T relationship for

their dorsal aspect (Frouzova´ & Kubecˇka, 2004).

To examine whether the depth of bathypelagic perch fry was controlled by light

intensity, the depth of the main layer was defined as the 1 m thick layer of the water

column with the highest abundance of migrating, non-shoaling perch fry. In addition, to

compare the selective advantage of shoaling behaviour (Magurran, 1990) to bathypelagic

# 2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 685–702


VERTICAL MIGRATIONS OF PERCH FRY 689

perch fry, the depth of the shoal (acoustic fish shoal; Fre´on et al., 1992) was also defined

as the 1 m thick layer of the water column where the centre of a perch fry shoal was

present. For every shoal, its physical dimensions were measured: (1) the length of a fry

shoal was calculated from the number of acoustic emissions produced per second (system

ping rate) and the time spent by the shoal in between the borders of the emitted

ultrasound cone and the speed of the research vessel; (2) the height of the shoal was

calculated 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 of

the water column (0–1 and 1–2 m below the water surface) was reconstructed from the

results of the towed ichthyoplankton net and the ratio between estimated ichthyoplankton

net abundance of fry and acoustic abundance of fry in deeper layers sampled

simultaneously by the net and the acoustics.

In May, the L T , of perch larvae and juveniles caught in the pelagic zone of Slapy

Reservoir ranged from 83 to236 mm. For the post-processing procedure, the TS threshold

was set at 70 dB (62mm L T ), to avoid acoustic under-estimation of perch fry

abundance 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 and

descent in the water column (Cˇech & Kubecˇka, 2002). To exclude infrequently occurring

larger fish, targets > 57 dB (260mm L T ) were manually erased from the analysis,

using the erase function of Sonar5 (Balk & Lindem, 2003). In June, the L T of perch

larvae and juveniles caught in the pelagic zone of Slapy Reservoir ranged from 89 to

400 mm. Consequently, the TS threshold was set to 68 dB (77mm L T ). Targets

> 53 dB (405mm L T ) were again erased from the analysis. The other configuration of

the 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

(0–197% in abundance; the zero value occurred at dusk) except at dawn when

perch dominated this habitat [911%; Fig. 2(a)]. In contrast, in June perch

strongly dominated the fry assemblage of the littoral zone for most of the day

(907–966% in abundance), except at dusk, when no perch fry were again

recorded in this habitat [Fig. 2(b)]. The distribution of fry in littoral zone was

rather variable due to local aggregations; absolute fry densities were from tens

to hundreds of individuals 1000 m 3 (Fig. 2).

TABLE I. Tracking variables: minimum track length, minimal number of detections to

track 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 8–16

Minimum track length (ping) 2 3 4 1 3 4 1 4

Maximum ping gap (ping) 0 0 0 0 0 2

Gating range (m) 007 007 007 007 007 007

# 2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 685–702


690 M. Cˇ ECH ET AL.

(a)

100

25

±22

111

±129

211

±226

224

±189

80

60

40

20

Abundance (%)

0

(b)

504

±1114

100

DAY DUSK NIGHT DAWN

67

±115

528

±635

98

±62

80

60

40

20

0

DAY DUSK NIGHT DAWN

Period

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 1000 m 3 .

In both May and June, perch strongly dominated the fry stock of the pelagic

zone in Slapy Reservoir (Fig. 3). In May, the bathypelagic fry community (10–

16 m below the water surface in the period 1000–1400 hours) was composed of

955% perch, 32% zander Sander lucioperca (L.) and 13% ruffe Gymnocephalus

cernuus (L.). In June, the daytime bathypelagic fry community was composed of

921% perch, 57% ruffe and 22% 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 echogram

were related almost exclusively to perch. Consecutive acoustic analyses revealed

that, in both months, during the night most of the acoustic biomass of pelagic

perch fry (>93% s v in May and >98% s v in June) was concentrated in the upper

5 m of the water column (Fig. 4). Night s v values were higher than day ones

suggesting that some of the epipelagic fry descended deeper and could be

detected by the echosounder. In May, the night peak of acoustic biomass was

in the 3–4 m layer, just above the thermocline [Figs 4(a) and 5(a)]. In June, the

# 2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 685–702


VERTICAL MIGRATIONS OF PERCH FRY 691

(a)

100

268

±45

156

±53

63

±8

953

±212

345

±71

80

60

40

20

Abundance (%)

0

100

BATHY-

DAY

(b)

21

±4

EPI-DAY

147

±58

EPI-

DUSK

161

±26

EPI-

NIGHT

866

±327

EPI-

DAWN

162

±31

80

60

40

20

0

BATHY-

DAY

EPI-DAY

EPI-

DUSK

EPI-

NIGHT

EPI-

DAWN

Period

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

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

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

peak of acoustic biomass was in the 2–3 m depth layer. Cross referencing the

acoustic results with the ichthyoplankton catches, however, revealed that, in

June, the maximum perch fry occurrence was even closer to the water surface

(depth layer 0–2 m, i.e. in the blind zone of the echosounder), which might be

induced by a less steep temperature stratification [Figs 4(b) and 5(b)]. During

dawn, the acoustic biomass for the depth range 2–16 m dropped suddenly and

the peak of acoustic biomass of perch fry started to descend into the deeper

layers. After reaching the depth maximum during the noon period (13–14 m in

May, 11–12 m in June), the peak of acoustic biomass started to ascend slowly

towards the surface during the afternoon and dusk. After dusk (in June already

# 2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 685–702


692 M. Cˇ ECH ET AL.

(a)

0·0000018

0·0000016

0·0000014

s v (m 2 m –3 )

0·0000012

0·0000010

0·0000008

0·0000006

0·0000004

0·0000002

0

2·5

4·5

6·5

8·5

10·5

Depth (m)

12·5

14·5

0000–0200

0400–0600

1000–1200

1400–1600

2000–2200

Period

(b)

0·0000008

0·0000007

0·0000006

s v (m 2 m –3 )

0·0000005

0·0000004

0·0000003

0·0000002

0·0000001

0

2·5

4·5

6·5

8·5

10·5

Depth (m)

12·5

14·5

0000–0200

0400–0600

0800–1000

1200–1400

1800–2000

2200–2400

Period

FIG. 4. Diurnal fluctuation of acoustic biomass (s v ) of pelagic fry in 1 m 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].

during dusk), the value of acoustic biomass analysed for the whole water

column again increased dramatically. The sequence of echograms [Fig. 6(a),

(c)] revealed that around dawn, the bathypelagic perch fry left the upper layers

of the water column (epilimnion and metalimnion) and, as a scattering layer,

migrated into the deeper layers (hypolimnion). During the afternoon, the layer

# 2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 685–702


VERTICAL MIGRATIONS OF PERCH FRY 693

(a)

Temperature (° C)

O 2 (mg l –1 )

5 15 25 0 5 10

0

2·5

5·0

7·5

10·0

12·5

15·0

17·5

Depth (m)

20·0

0

(b)

Light (µmol m –2 s –1 )

0 1000 2000

2·5

5·0

7·5

10·0

12·5

15·0

17·5

20·0

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.

of bathypelagic fry migrated in the opposite direction and around dusk entered

the epilimnion. It was evident that changes in the light intensity were followed

by changes in the depth of the main layer of bathypelagic perch fry. The

increase in light intensity during dawn was followed by the descent of

bathypelagic perch fry into the deeper layers while decreasing light intensity

during late afternoon and dusk was followed by the ascent of these fry towards

the surface [Fig. 6(b), (d)]. The maximum amplitude of the DVM of bathypelagic

perch fry was 11 m in May and 125 m in June and the depth of the main layer

of bathypelagic fry was therefore strongly controlled by the light intensity

(regression analysis May; F 1,27 , P < 0001, r 2 ¼ 084; regression analysis June;

F 1,36 , P < 0001, r 2 ¼ 093; Fig. 7).

In contrast to May, in June, shoaling behaviour was observed in some of the

bathypelagic perch fry and the shoals (n ¼ 58) were significantly higher in the

# 2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 685–702


1

694 M. Cˇ ECH ET AL.

(a)

3

5

7

9

11

13

15

17

19

21 1620 1904 2046 2107 0016 0030 0454 0517 0828 1214

Depth (m)

1

3

5

7

9

11

13

15

17

19

21

(c)

1615 1910 2044 2140 2217 0059 0410 0535 0930 1204

Time (hours)

0

2

4

6

8

10

12

14

16

(b)

1212

1256

1357

1559

1902

2010

2105

0057

0159

0258

0454

0842

0935

(d)

140

120

100

80

60

40

20

Light intensity (10 3 lx)

0

2

4

6

8

10

12

14

16

11570

140

120

100

80

60

40

20

0

1213

1634

1740

1840

2056

2151

0043

0142

0409

0459

0557

0943

1047

1144

Depth of main layer (m)

Time (hours)

FIG. 6. Ilustration of the diel vertical migration of perch fry in Slapy Reservoir in (a), (b) May and (c), (d) June 2002. (a), (c) Sequence of raw 20 LogR TVG (for

definition see MacLennan & Simmonds, 1992) echograms representing day, dusk, night, dawn and day again and (b), (d) dependence of the depth of the main

layer (– –) on the light intensity (-- --) measured at the water surface. Traces below the fry layer (May, time 0016, 0030, 0454 hours) correspond to chironomid

pupae entering upper layers of the hypolimnion during night-time.

# 2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 685–702


VERTICAL MIGRATIONS OF PERCH FRY 695

Depth of main layer (m)

0

2

4

6

8

10

12

14

16

0 1 2 3 4 5 6

Light intensity (log 10 lx)

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

in May ( , – –) (y ¼ 238x þ 142; r 2 ¼ 084, P < 0001) and June ( ,—)(y ¼ 348x–486, r 2 ¼ 093,

P < 0001) in Slapy Reservoir.

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 was

also strongly controlled by light intensity (regression analysis, y ¼ 258x 200;

r 2 ¼ 072, F 1,56 , P < 0.001). The length of bathypelagic perch fry shoals ranged

from 1 to 47 m; the average length of a shoal was 26 m; the height of the shoals

ranged from 037 to 131 m with an average height of 067 m. The distribution

was highly asymmetric, with only a few shoals >1 m in height.

Average fry (all species) densities in every depth layer (layers 0–2 m corrected

by the netting results) provided an estimate of total fry numbers and the

proportions of epi- and bathypelagic fry (Fig. 9). Day and night abundances

in whole water column differed significantly, especially in the June data [1980

and 14 210 individuals 1000 m 2 , Fig. 9(c), (d)] which probably indicated daytime

net avoidance in the surface layers. This is why the night results integrated

Depth (m)

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)

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).

# 2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 685–702


696 M. Cˇ ECH ET AL.

0·5

2·5

(a)

5380

*

epilimnion

(b)

*

9600

epilimnion

4·5

6·5

8·5

10·5

12·5

14·5

thermocline

hypolimnion

thermocline

hypolimnion

Depth (m)

16·5

0 500 1000 1500 2000

(c)

1980

0·5

*

2·5

epilimnion

0 1000 2000 3000 4000 5000

(d)

14210

*

epilimnion

4·5

6·5

8·5

10·5

12·5

14·5

thermocline

hypolimnion

thermocline

hypolimnion

16·5

0 500 1000

0 2000 4000 6000 8000

Abundance (individuals 1000 m –3 )

FIG. 9. Comparison of mean þ S.D. pelagic fry abundance (all species) in the upper 16 m 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 1000 m 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, 1–2 m below the water surface)

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

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

for all depths [Fig. 9(b), (d)] were more likely to provide the total estimate of the

pelagic fry cohort and the share of bathypelagic fry on the total cohort should

have been related to night rather than day total abundance. During May 281%

# 2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 685–702


VERTICAL MIGRATIONS OF PERCH FRY 697

[c. 1894 of 9600 individuals 1000 m 2 , Fig. 9(b)] of the population of pelagic perch

fry (bathypelagic) performed DVM. The remaining 719% of the population of

pelagic perch fry (epipelagic) stayed in the epilimnion, mainly in the upper 2 m

of the water column [Fig. 9(a), (b)]. In June, the group of migrating bathypelagic

perch fry comprised only 47% [c. 586 of 14 210 individuals 1000 m 2 ,

Fig. 9(d)] of the pelagic fry and 953% belonged to the non-migrating epipelagic

fry [Fig. 9(c), (d)]. Around dawn in both months, the bathypelagic perch fry

segregated from the epipelagic fry left in the upper layers of the water column,

and stayed in the hypolimnion during the daylight hours. During dusk, the

bathypelagic fry reached the epilimnion again and joined together with the

epipelagic perch fry. In May, the whole pelagic stock of perch fry then

descended slightly and spent the night around the thermocline. In June this

descentwasnotsoevidentandmostofthe pelagic stock of perch fry stayed in

the upper 2 m 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 119mm L T ) were significantly smaller than the

epipelagic fry (average 146mm L T ) [ANOVA, F 1,584 , P < 0.001, Fig. 10(a)].

Frequency (%)

30

(a)

25

20

15

10

5

0

30

(b)

25

20

15

10

5

0

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

44

–70

–67

–65

–63

–61

–59

–58

–57

–56

–55

–54

–53

L T (mm)

Target strength (dB)

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 10–16 m 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 L T conversion according to Frouzová & Kubečka (2004). BATHY-IN (&), bathypelagic

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

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

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

# 2005 The Fisheries Society of the British Isles, Journal of Fish Biology 2005, 66, 685–702


698 M. Cˇ ECH ET AL.

In June, the epipelagic perch fry showed a clear bimodal character. There was a

group of small epipelagic fry ranging from 10 to 22 mm L T (average 146mm

L T , comprising 738% of the epipelagic perch fry) and a group of big epipelagic

fry ranging from 25 to 41 mm L T (average 344mm L T ,262% of epipelagic fry)

[Fig. 10(b)]. The group of larger epipelagic perch fry was caught exclusively in

the upper 2 m of the water column. Small epipelagic fry did not differ significantly

in size from the bathypelagic fry (average 146mm L T ) (ANOVA, F 1,273 ,

P ¼ 093) but, in contrast, the big epipelagic fry were almost the same size as

those in the littoral with sizes ranging from 20 to 43 mm L T (average 350mm

L T ) [ANOVA, F 1,204 , P ¼ 020, Fig. 10(b)].

DISCUSSION

In Slapy Reservoir, there are three ecological groups of YOY perch present at

the same time, differing in their distribution patterns and size structures, and the

switch to a demersal mode of life was far less apparent than usually reported

(Coles, 1981; Whiteside et al., 1985; Treasurer, 1988; Urho, 1996). The littoral

region of the canyon-shaped Czech reservoirs is favoured by fry (Duncan &

Kubečka, 1995; Matěna, 1995b). The littoral, however, comprises a negligible

part of Slapy Reservoir volume (


VERTICAL MIGRATIONS OF PERCH FRY 699

& Eckmann (1994), however, have described the temperature preference of

perch larvae as between 16 and 26 C and Ross et al. (1977) more precisely

between 22 and 24 C. The results of Kudrinskaya (1970) have suggested that at

temperatures


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