The growth of bleak (Alburnus alburnus L.) (Pisces, Cyprinidae) in Lake Balaton and the assessment of mortality and production rate

Biró, Péter (1975) The growth of bleak (Alburnus alburnus L.) (Pisces, Cyprinidae) in Lake Balaton and the assessment of mortality and production rate. A Magyar Tudományos Akadémia Tihanyi Biológiai Kutatóintézetének évkönyve, 42. pp. 141-158. ISSN 0365-3005

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Abstract

Length- and age-distribution of 1112 bleaks, as well as the growth of 294 specimens collected during consecutive months of 1968 have been studied. The seasonal variations in length-weight relationship, the mortality, average biomass and production of the population were studied. It was established that: 1. The population showed in spring a bimodal distribution by standard length, which was transformed to asymmetrical. 2. The length-weight relationship showed significant seasonal variation, which is evidently in connection with ripening cycle of gonads, with spawning as well as changes in condition. 3. Regression between the total caudal radii of scales and standard lengths was linear. The straight line cuts 8.7 mm from the abscissa. On the basis of standard lengths calculated from the annuli of scales, the growth of bleak is usually slow in Lake Balaton, its annual increase in length is small. Exponential growth in standard length could be well represented by Bertalanffy’s model. The observed stunted growth of bleak as compared to previous data may be probably the consequence of food scarcity of the littoral zone and the result of food competition with other fish species. 4. Age-structure of the stock is asymmetrical and its overwhelming majority consisted of age-groups 1+ an d 2+ in 46 and 34.7 per cent, respectively. One-summer-old (0+) fishes were present only in 13.7 per cent, and the ratios of older ones, belonging to age-groups 3+ — 5+, were between 0.3 — 4.1 per cent. Based on decrease in logarithmic number of specimens, the instantaneous total m o rtality coefficient proved to be Z = 1.33, the rate of survival was S = 26 per cent and that of annual mortality A = 74 per cent. These values have changed in different age-groups. The ratio of average biomass and production on an average was P/B = 71.8 per cent. It was found to be highest in age-group 1+ (99.3 per cent), while in others (2+ to 5+) it varied between 24—60 per cent.

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