Bottleneck effects and the depression of genetic variability in hatchery stocks of Penaeus japonicus (Crustacea, Decapoda)

Aquaculture of Penaeus japonicus is developing in Italy at a production level. Genetic analysis of the founder stock and five subsequent hatchery generations revealed a constant reduction in levels of allozyme polymorphism. Average heterozygosity decreased from 0.102 to 0.039. The magnitude of the reduction in heterozygosity was much higher than expected from the numbers of breeders placed into spawning tanks at each reproductive cycle. We estimated, under the assumption of neutrality, that the effective number of parents contributing to each broodstock might have been as low as four, although the number of shrimp pairs held in spawning tanks varied from 50 to 300 after an initial bottleneck occurred in the first generation. This discrepancy may be explained as the combined effect of some common farming practices and it points out the importance of a careful check of the number of spawners actually contributing to each reproductive cycle. © 1986.