Homing behaviour facilitates subtle genetic differentiation among river populations of Alosa sapidissima: microsatellites and mtDNA

Significant but subtle differentiation was detected for both microsatellite DNA and mitochondrial DNA among four populations of American shad Alosa sa pidissima. The data indicate that straying among rivers is sufficient to pe rmit only marginal population differentiation in this species, but suggest that individual river populations should be managed as distinct stocks. Com parison of the Hudson and Columbia populations, the latter derived from the former over 100 years ago, revealed only a slight reduction in microsatell ite DNA variation for the founded population but halving of mitochondrial D NA, consistent with the haploid maternal inheritance of the latter marker. The depleted and endangered James River (Virginia) population and two other Atlantic coast populations exhibited similar levels of microsatellite DNA variation, but mtDNA diversity in the James River was marginally lower than in other Atlantic populations, again consistent with the low effective pop ulation size of mtDNA. (C) 2000 The Fisheries Society of thy British Isles.