Genetic effects of long-term stock enhancement programs

Abstract

Since the 1980s, there has been growing concern about the deleterious genetic effects of hatchery fish on wild populations. The general awareness of the loss of genetic variation in hatchery-produced juveniles has increased. However, the genetic effects of hatchery fish on wild populations have not yet been fully evaluated, and their long-term effects remain largely unknown, with no data available for marine species. To address this issue, we have reviewed the scientific literature on large-scale marine fish stocking programs worldwide, conducted over multiple generations, in which the catches and genetic diversity of wild and hatchery-released fish were monitored. However, we found no published work apart from two Japanese reports of different types of stocking: red sea bream in Kagoshima Bay (KB) and Pacific herring in northern Japan. We investigated the genetic effects of hatchery fish on wild populations using a comparative analysis of these two stocking programs. The loss of rare alleles was observed in red sea bream in the inner KB and for Pacific herring in Honshu. A bottleneck test and numerical simulations revealed that the loss of rare alleles in the inner KB was caused by the extensive releases of juveniles produced from captive broodstock used to rear multiple generations, whereas a bottleneck at the southern limit of the species was responsible for these effects in herring in Honshu. There was no evidence that the herring release programs using native brood stock every year had a genetic impact. The recapture rate for one-year-old red sea bream decreased consistently, suggesting a decline in the survival rate of the hatchery fish. Nevertheless, the wild catch of red sea bream in KB has generally remained above the catch level at the commencement of the release program, with no evidence of fitness decline in the wild population. The genetic impact of hatchery fish on red sea bream localized in the bay was much smaller than that of the evolutionary forces affecting Pacific herring through their life history and environmental and fishing selection pressures. We discussed on precision of our divergence estimates in relation to the number of loci examined and how genetic diversity could be maintained in hatchery and stocked populations.