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Table 1. Average overestimation index (01) and
proportion in catch (%) for total number of percids
and for particular perciform species.
Reservoirs        Dutch     Czech    Both
Percids total: 01  1.17     2.60      1.63
% in gillnets     66       43        52
% in seines       56        17       32
Perch: 01          1.91     2.62      2.28
% in gillnets     29       28        29
% in seines       15        1 1      13
Pikeperch: 01      2.50     1.44      1.60
% in gillnets      2        4        3
%inseines          1        3        2
Ruffe: 01          0.86     3.53      1.16
% in gillnets     35        11       20
%inseines         40        3        17
25 -I
x
0)
r_
V
0
4o
cm
E
4-
0)
a)
0
20
15
10
5
0
.
* Realvahes
o Modelvahes
0
.0
.5-
.0.
0      20     40
60
..   .       1
80        1 oo
% of percids from seines
Figure 2. Dependence of value of overestimation
index on proportion of percids in reference
community.
Overestimation index of percids varied a lot
between two groups of reservoirs. No obvious
similarity for the same species or the same reservoir
was found. The results suggest that the patterns of
gillnet  overestimation  of  percids  are  more
complicated than simple mechanical selectivity. The
highest difference was found with ruffe. It is
interesting, that ruffe was slightly underestimated by
gillnets in Dutch reservoirs, while it was very much
selected in the Czech reservoirs. Significant
proportion of more active perch in Dutch reservoirs
may be responsible for the discrepancy. Another
explanation would be habitat selection of ruffe and
ability of fishing gears to cover microhabitats. On the
flat concrete bottom and shores of Dutch reservoirs,
the effectiveness of gillnets and seine nets could be
comparable. Contrary to this, the Czech reservoirs
have natural beaches, where the benthic gillnets can
follow the roughness of the bottom, whereas hauls
with seine nets over such a bottom leave many gaps
to escape. We should also consider selectivity of
seining due to physical obstructions and various
ecology of species (Parsley et al., 1989; Pierce et al.,
1990). Two types of reservoirs differed in species
proportions and many aspects of fish biology
(Kubecka et al., 1998) and the patterns of net
selectivity may differ between them.
The   results  showed  that   the  positive
overestimation of percids by gillnets and higher
probability of capture usually take place in reservoir
surveys. Selectivity of gillnets is complicated by both
mechanical and behavioral aspects of fish capture as
well as by the fish community structure. However,
multimesh gillnet is a very robust and frequently used
sampling gear and in many cases it is the only
method capable of fair sampling of all habitats of the
reservoir. Also due to constancy of this gear, the long
time gillnet data are highly valuable. Therefore, we
believe that correcting the selective gillnet catch by a
sound correction factor is a challenging task worth
intensive investigation. Future studies should include
more detailed analysis of the capture mechanism,
more extensive catch statistics, and experiments with
gillnet selectivity in tailored fish communities.
Acknowledgements. This study was supported by
the grant No. 206/02/0520 of the Grant Agency of the
Czech Republic, grant No.K6005114, S6017004 and
A 6017201 of the Czech Academy of Sciences,
Project No. 123100004 of the Ministry of Education
and   Waterworks   Brabantese  Biesbosch,  the
Netherlands.
References. Kipling, C. 1963. Some estimates
of theoretical minimum expected sizes of perch in gill
nets. ICNAF Special Publication 5: 128-130.
Kubecka, J. et al. 1998. Composition and
biomass of the fish stocks in various European
reservoirs. Internat. Rev. Hydrobiol. 83: 559-568
Kurkilahti, M. 1999. Nordic Multimesh Gillnet -
Robust Gear for Sampling Fish Populations.
Academic Dissertation, University of Turku, Finland.
Helsinki, 1 13 pp.
Parsley, M.J., D.E. Pamler and R.W.
Burkhardt.1989. Variation in Capture Efficiency of a
Beach Seine for Small Fishes. N. Am. J. Fish.
Manage. 9: 239-244.
Pierce, C.L., J.B. Rasmussen and W.C. Leggett.
1990. Sampling littoral fish with a seine: corrections
for variable capture efficiency. Can. J. Fish. Aquat.
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Reis, E.G. and M.G. Pawson. 1999. Fish
morphology and estimating selectivity by gillnets.
Fisheries Research 39: 263-273.
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