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DISTRIBUTION OF YOUNG-OF-THE-YEAR PERCIDS IN EUROPEAN RESERVOIRS
Matena Josef, tech Martin, Drastik Vladislav, Frouzovi Jaroslava, Kratochvil
Michal, Kubecka Jan, Peterka Jifi, Vagek Mojmir.
Hydrobiological Institute, Academy of Sciences, Na sidkach 7, and Biological
Faculty USB, Branisovskd 31, 37005
Cesk6 Budejovice, Czech Republic. matena(hbu.cas.cz.
Introduction. Besides the cyprinids, perciform fish are
the most successful in colonizing European reservoirs.
This paper summarizes numerous published and
unpublished information on perch (Perca fluviatilis),
pike  perch   (Sander  lucioperca)  and  ruffe
(Gymnocephalus cernuus) distribution and behavior
during the first year of life. The emphasis is given on
the aspects of biology not mentioned in Urho (1996)
and Craig (2000).
Methods. Open water of five Czech reservoirs was
sampled by ichthyoplankton towed nets (Wanzenbock
et al., 1997; May-July), by 3*3 m frame trawl (August-
September) and by the SIMRAD EY 500 split-beam
echosounder. Littoral regions were sampled using fry
beach seine (10 m long, mesh size 1*1.35 mm). All
samplings were performed either during day time or
during 24 hours. All fish were counted and measured to
the nearest 0.5 mm. The results from three non-
stratified drinking water reservoirs in the Meuse
estuary, the Netherlands (Kube~ka et al., 1998) are
used for comparison.
Results and Discussion. J.Spawning, hatching and
embryonic period. The trigger of perch spawning is the
onset of thermal stratification when the surface
temperature reaches 80C (second half of April). The
spawning is one continuous event lasting about one
month with most activity at depth strata corresponding
to 8-12'C (Pivni~ka and 9vatora, 2001). Pikeperch and
ruffe spawn slightly later. Especially the spawning of
ruffe can be prolonged during May to July. The
spawning of pikeperch is usually considered as a single
event, but the appearance of very small (15-25 mm) fry
in August amongst contemporaries 50-150 mm long
may indicate portional spawning in Dutch reservoirs.
2. Pelagic larval period. Marine origin of percids
predetermined pelagic phase in the life cycle.
2.1. Surface-confined period. Larvae of perch and
pikeperch are positively phototactic. They leave
spawning grounds, enter open water and inhabit
surface layers during the day (Fig. 1) for week or two
(length 5-10 mm, end of May). Unpublished acoustic
studies show that in this period schooling aggregations
are missing (randomly distributed ichthyoplankton).
This period seems to be present in all studies compared
and the larvae seem to avoid littoral.
2.2. Period of more diverse distribution. In subsequent
period, social contacts and schooling develops
gradually, the behavior of larvae splits in several
modes:
0
0
2
_--   I
a 61
8 1
% of total abundance
I 10  20  30  40  50  60  70
-i-X~~~~~t ,,.....    . ....... -.  A  I I
-   - Lipno Perch
- Klicava Perch
-*-- Rimov Perch    I
| X Lipno Pikeperch I
I
Figure 1. Vertical distribution of percid fiy during
surface-confined period (end of May, day time).
%of total abundance
0        10      20
2
4
6
8
10
12
14
16
30
Figure 2. Vertical distribution of percid fry during the
period of bimodal distribution (day time, 0700-1900).
a) Surface distribution like in 2.1. This type of
distribution is often reported in the literature, but it
was never encountered during our studies when all
types of habitats were sampled. Such references have
to be treated with caution, as they may be result of
undersampling of deep pelagics. Pavlov et al. (1988)
show preference of pikeperch to the surface layers.
b) Bimodal vertical distribution with epi- and bathy-
pelagic perciform fry (Fig. 2). This type is typical for
deep stratified reservoirs. The cohort of all three
species splits into differently behaving groups:
Epipelagic fry (EPF), which spends the whole 24
hours in upper 4 meters together with the fry of
cyprinids and Bathypelagic fry (BPF) spending the day
in hypolimnion at depths 10-20 m with no measurable
light. No cyprinids are present in BPF. BPF formate 2-
3 m thick scattering layer, which can be followed by
the echosounders. In the evening, these fish migrate to
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