Page 121

ARE PERCID FISH OVERESTIMATED BY GILLNET SAMPLING?
Marie Prchalova' and Jan Kubecka, Faculty of Biological Sciences, University
of South Bohemia, Branigovskd 31,
370 05 Ceske Budejovice, Czech Republic and Hydrobiological Institute, Czech
Academy of Sciences, Na
Sadkach 7, 370 05 Cesk6 Budejovice, Czech Republic, marie.prchalovaCa)bf.Icu.cz
Introduction. Common percid fish - perch (Perca
fluviatilis), pikeperch (Stizostedion lucioperca) and
ruffe (Gymnocephalus cernuus) - possess a
structured body surface and have a relatively firm
body structure (Kipling, 1963). This, together with a
potential difference in activity, may result in higher
probability of capture and retention in gillnets
comparing with smooth bodied fish like cyprinids or
salmonids. Due to this, percid fish can be
overestimated by gillnets.
To verify potential bias, we compared fish
species composition based on catches with gillnets
and a different fishing gear. As a reference gear,
shore seines at the same time and site was used.
Methods.    Suitable data for observing potential
overestimation of percid fish were taken from three
Dutch water-supply reservoirs (De Gijster, Honderd
en Dertig and Petrusplaat) and two Czech reservoirs
(Rimov and Staviste) during 1998 - 2002.
Nordic multimesh benthic gillnets were used for
the studies. Sets included 13 different mesh sizes - 6,
8, 10, 12.5, 16, 19.5, 24, 29, 35, 43, 55, 70 and
90 mm (knot to knot). Each panel was 1.5 m high and
25 m long. Benthic gillnets were set for 12 hours
overnight in the littoral in the depth 2 - 3 m. Hauls
with 50 m shore seines with mesh size 10 mm (knot
to knot) were carried out during the same night and
on the place as close to the gillnets as possible to not
disturb fish.
Absolute numbers of caught fish from both gears
were converted to a ratio of number of percids to the
sum of numbers of percids and cyprinids. Cyprinids
created major part of abundance in selected reservoirs
and they represent a close group considering the
morphology. This ratio was tested with multifactor
analysis of variance (MANOVA, p-level of 0.05)
after arcsin transformation. An overestimation index
of percids from gillnets was determined as a ratio of
the proportion in gillnets / proportion in seine. Values
above I mean overestimation and values below I
signify underestimation of percids by gillnets.
Results.   Percids (perch, pikeperch, ruffe) and
cyprinid fishes (mainly roach Rutilus rutilus,
common bream Abramis brama and bleak Alburnus
alburnus) comprised 90 - 100% of both gillnet and
seine catches in all reservoirs (Fig. 1). The rest of
catches were represented by a sporadic catch of pike
(Esox lucius), trout (Salmo trutta), eel (Anguilla
anguilla) and smelt (Osmerus eperlanus, in Dutch
reservoirs only). We compared 26 couples of ratios of
the number of percids to the sum of percids and
cyprinids - 10 from Dutch reservoirs, 15 from the
Rkimov reservoir and one from the Staviste reservoir.
A higher proportion of percid fish in gillnets was
found in 77% of cases. Statistical analysis showed
significant differences between proportions of percids
100 -I
80 -
60-
40 .
20
Dutch   Dutch   Czech   Czech
gillnets  seines  gillnets  seines
* others
m bream
o roach
o ruffe
E perch
from two compared fishing gears (p-level 0.0 142).
Figure 1. Average species composition from gillnets
and seines in Dutch and Czech reservoirs.
In individual catches, the overestimation index
varied between 0.7 and 16. Table I shows the
weighted average of the overestimation index for
total number of percids and for particular perciform
species. With only one exception - ruffe in Dutch
reservoirs perciform species were overestimated by
gillnets. The highest overestimations were gained for
ruffe and perch in Czech reservoirs and for pikeperch
from Dutch reservoirs.
Overestimation index of percids was found to be
adversely related to the proportion of percids in the
community from the reference catch (Fig 2). In high
percid densities, the overestimation index cannot be
high by definition. This may partly explain higher
values of overestimation indexes in Czech reservoirs
(Table 1). After log-transformation, the relationship
in Figure 2 can be described by linear function
(Log y= -0.74 Log x + 1.33; R2 0.75) and has a
hyperbolic course similar for all three perciform
species.
Discussion.  The catchability of fish by gillnets is
affected by factors, which are related either to the
characteristics of the net or of the fish (Reis &
Pawson, 1999). Many works were oriented to the
significance of nets parameters (Kurkilahti, 1999),
but less attention was paid to the fish attributes.
121