L. Forró - É. Murai szerk.: Miscellanea Zoologica Hungarica 6. 1991 (Budapest, 1991)

Csányi, B.: Changes of the fish assemblages in the Kis-Balaton Reservoir between 1985 and 1989

METHOD AND MATERIAL Due to several theoretical and practical difficulties the quantitative estimation of the statistic variables of the fish populations was intentionally avoided. First, because the description of the spatial structures in a population requires a fractional and systematic sampling of the field area under study (CHESSEL 1978). On the other hand, regarding the aleatory character of the fish samples, effective sampling of an aggregate population, statistically speaking, is ensured only by the use of the smallest sample size possible (GÉRARD & BERTHET 1971). Both of these requirements (small sized samples, fractional sampling procedures) need huge amount of effort, time and cost. Considering the reservoir as an open system for fishes, a sampling strategy suitable for the problem in question was chosen. A combination of different fishing methods was thought most feasible for attaining the research objectives within the constraints of the present research program. An electrofishing equipment, a 20 m long trawling net and a small lifting net (2 mm mesh size) with a long handle were used for sampling the juveniles and the small sized species. The adult stock was captured by trawling, trapping and gill nets with various length (10-50 m) and mesh size (30-55 mm). The electrofishing apparatus was also effective to catch the adult fish specimen. The experimental fishing took place in the various basins of the reservoir. Both the open water areas and the littoral zones, having different aquatic plant communities, were investigated approximately with the same fishing effort: the fish sampling was repeated 7-11 times a year with the duration of 3-5 days at each case. Only the yearly average relative frequency changes were analysed. RESULTS The validity of population statistics in fish studies depends on sampling methods. Studies dealing with fish population dinamics usually elaborate methodology to estimate the size of age groups, particularly the juvenile cohorts (e.g.Point Abundance Sampling, NELVA et al. 1979, COPP 1990). Several effective capture technics were developed which can be simultaneously used (MANN & PENCZAK 1986) in fish surveys. BERINKEY (1972) indicated the presence of 15 fish species in the surrounding catchment area of the River Zala. Faunistical research was carried out in the Kis-Balaton Reservoir and the catchment area of the Lake Balaton between 1986 and 1990 by BÍRÓ (1990) including the description of the allometric growth of different species. His results concerning the colonization are different from ours: two very common species (the bitterling and the Pseudorasbora sp.) was absent and very rare, respectively, during 1988. Another difference between his and our experiences is that we have never found Leuciscus cephalus in the reservoir area during the study period. The results of our study led to the following conclusions: the drastic structural change of the community requires regulation both in fishing and species replacement. The yearly average relative frequency data and the abundance class values (established on logarithmic scale) can properly describe the characteristic changes in the fish stock due to different impacts (LEGENDRE & LEGENDRE 1983). The number of taxa found in 1985 was 13 that has increased to 19 in the next