Microsatellite DNA population structure and stock identification of steelhead trout (Oncorhynchus mykiss) in the Nass and Skeena Rivers in northern British Columbia

Population structure and the application to genetic stock identification fo r steelhead (Oncorhynchus mykiss) in the Nass and Skeena Rivers in northern British Columbia was examined using microsatellite markers. Variation at 8 microsatellite loci (Oki200, Omy77, Ots1, Ots3, Ssa85, Ots100, Ots103, and Ots108) was surveyed for approximately 930 steelhead from 7 populations in the Skeena River drainage and 850 steelhead from 10 populations in the Nas s River drainage, as well as 1550 steelhead from test fisheries near the mo uth of each river. Differentiation among populations within rivers accounte d for about 1.9 times the variation observed among years within populations , with differences between drainages less than variation among populations within drainages. In the Nass River, winter-run populations formed a distin ct group from the summer-run populations. Winter-run populations were not a ssessed in the Skeena River watershed. Simulated mixed-stock samples sugges ted that variation at the 8 microsatellite loci surveyed should provide rel atively accurate and precise estimates of stock composition for fishery man agement applications within drainages. In the Skeena River drainage in 1998 , Babine River (27%) and Bulkley drainage populations (31%) comprised the m ain components of the returns. For the Nass River in 1998 steelhead returni ng to Bell-Irving River were estimated to have comprised 39% of the fish sa mpled in the test fishery, with another 27% of the returns estimated to be derived from Cranberry River. The survey of microsatellite variation did no t reveal enough differentiation between Nass River and Skeena River populat ions to be applied confidently in estimation of stock composition in marine fisheries at this time.