Application of microsatellite DNA variation to estimation of stock composition and escapement of Nass River sockeye salmon (Oncorhynchus nerka)

Spawning escapements of individual Pacific salmon stocks returning to remot e spawning locations throughout large river systems can in theory be estima ted by mixed-stock analysis of appropriately weighted samples from test fis heries near the river mouth. However, the feasibility of this approach has usually been limited by practical difficulties in identifying closely relat ed populations within the same watershed. Microsatellite DNA techniques off er new promise for overcoming these limitations as illustrated for sockeye salmon (Oncorhynchus nerka) in the Nass River of northern British Columbia. Variation at six microsatellite DNA loci (Omy77, Ots3, Ots100, Ots103, Ots 107, and Ots108) was surveyed from about 1400 fish from nine stocks in the Nass River drainage as well as from 249 fish in a test fishery conducted in the lower river during 1996. Five stocks were surveyed in more than one ye ar, and variation in allele frequencies among stocks was, on average, about 10 times greater than annual variation within stocks. Allele frequencies o f stocks where the juveniles do not rear in lakes ("riverine" or "sea type" ) were more similar to each other compared with frequencies from lake-reari ng stocks. Significant differences in allele frequencies were observed amon g most stocks at all loci. About 4.5% of observed variation over all loci w as attributable to stock differentiation. Simulated mixed-stock samples sug gested that the six microsatellite DNA loci surveyed should provide the abi lity to provide relatively accurate and precise estimates of stock composit ion when utilized for fishery management applications. The estimated propor tion of Meziadin Lake sockeye salmon in the 1996 test fishery was about 73% , in close agreement with an estimate derived from direct tagging of fish i n the test fishery.