The growth of asp (Aspius aspius L.) in the Lake Balaton and the selective effects of commercial fisheries on population structure

Bíró, Péter and Fűrész, György (1976) The growth of asp (Aspius aspius L.) in the Lake Balaton and the selective effects of commercial fisheries on population structure. A Magyar Tudományos Akadémia Tihanyi Biológiai Kutatóintézetének évkönyve, 43. pp. 47-67. ISSN 0365-3005

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Abstract

Length and age distribution of asps (496 specimens in 1974, and 217 specimens in 1975), caught at five different areas of Lake Balaton, as well as their growth have been studied. Mortality and production of the harvestable part of population and the selective effects of commercial fisheries on the population structure have also been studied. 1) Standard length of 80—85 per cent of asps caught in Lake Balaton ranged between 32 and 46 cm, on an average it was 42.2 cm. Length distribution or fish originated from different areas of the lake was asymmetrical. 2) The b regression coefficient included in the length-weight relationshipwas calculated to be 2.5985. 3) The linear regressionbetween the standard lengths and the lateral radii of scales cuts 7.9 mm from the abscissa which indicates the size measurable in time of “key-scale” formation. Annual marks on the scales of fish in age groups 3+ to 11+ are definitely differentiated from each other, and the annual growth of lateral radii of scales is uniform. The growth both in measured and back-calculated standard lengths is also uniform. Different rates of growth were observed in age groups up to 6+ and then from 7+ upward especially in the Keszthely Bay. Maximum standard length that could be attained in Lake Balaton on an average was estimated to be L∞ = 68.2 cm, and coefficient of rate of growth proved to be K = 0.1518. Their areal deviations have been insignificant. 4) The overwhelming majorityage groups 4+ to 6+. Instantaneous total mortality coefficient calculated for the most intensively exploited part of the population (age groups 4+ to 9+) on an average was Z = 0.6460. Its areal deviations have been significant. The assessed rate of survival on an average was 52 per cent, hence the average annual mortality was 48 per cent. 5) Average biomass of asps in the sample covering age groups 4+ to 10+ was was B = 526.7 kg which produced 150.6 kg a year, hence the P/B ratio on an average amounted to 28.6 per cent. 6) 50 per cent length retention of 1000 m seine nets used in Lake Balaton was found at 38.7 cm standard length of the fish, which has been accompanied by a mean age of 4.3 year. The regressions between the instantaneous total mortality coefficients (Z) and the 50 per cent selectivity index obtained for mean standard length (L), as well as between the average annual mortality (A) and the selectivity index calculated for mean age (f) are highly significant (P<<0.001).

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