| Popis: |
Asian carps (bighead carp Hypophthalmichthys nobilis × silver carp H. molitrix) had been introduced into Lake Balaton (Hungary) from 1972 to increase fishery yields and as a biological control of algal blooms. Although the stocking was stopped by 1983, the biomass of Asian carps is still high in the lake, constituting about one-third of the total fish biomass. Our analyses revealed that the present Asian carp stock cannot have originated directly from the last official stockings. The origin of the present stock is undefined, and there are two potential explanations for the source of recruitment. First, it is possible that Asian carps escape from the nearby fishery ponds and get into Lake Balaton periodically. The second explanation is that Asian carps spawn in the lake. However, preceding studies suggested that Asian carps are not able to reproduce in lacustrine environments. In this study, our aim was to contrast the pro and contra evidences of natural Asian carp reproduction in Lake Balaton. We studied the reproductive parameters and genetic pools of 58 Asian carps, sampled between March−November 2011 and 2013. Mature vitellogenic oocytes and post-ovulatory follicles were found in the posterior segments of gonads. This suggests that a fraction of mature eggs was released from the ovaries during the year, but does not necessary imply successful reproduction. Considerable seasonal variations in gonado-somatic indexes (GSI) were observed, fluctuating between 6−28% in 2011 and 12−29% in 2013. However, the within-year dynamics of GSI changes suggest that fish do not release significant amounts of milt during the potential spawning period. Rather, the resorption of nutrients allocated to eggs seems probable during autumn. Genetic analyses showed remarkable deviation from Hardy-Weinberg equilibrium within the stock, which indicates that Asian carps do not constitute a closed, self-sustaining population in Lake Balaton. |
