Impact of artificial stocking and its intensity on genetic diversity, genetic structure, and effective number of breeders in masu salmon Oncorhynchus masou

Artificial stocking, an important tool in fishery resource enhancement and wildlife management, exerts negative effects on wild conspecifics. However, the factors determining the magnitude of these effects remain unclear. Here, we investigated how stocking influences genetic diversity, genetic structure, and effective number of breeders (N_b) in masu salmon Oncorhynchus masou and how these effects vary with the stocking intensity, such as the number of fish stocked and proportion of non-native fish. We examined 15 microsatellites in 1989 individuals from 33 masu salmon populations and four hatchery stocks in Hokkaido, Japan. Stocking increased the allelic richness (Ar); however, this effect decreased with increasing stocking intensity. Lower genetic differentiation (F_ST) was observed among populations affected by stocking than among native populations. N_b correlated positively with the number of fish stocked and proportion of juveniles, but negatively with the proportion of non-native fish. Overall, this study shows that the genetic impact of stocking depends on various factors, such as the origin of stocked fish and stocking intensity.