GENETIC DIFFERENTIATION OF SOCKEYE-SALMON SUBPOPULATIONS FROM A GEOLOGICALLY YOUNG ALASKAN LAKE SYSTEM

The Tustumena Lake drainage in southcentral Alaska is glacially turbid and geologically young (< 2,000 years old). Previous field studies id entified at least three subpopulations of sockeye salmon Oncorhynchus nerka at Tustumena Lake, based on the distribution and timing of spawn ers. The subpopulations included early-run salmon that spawned in six clearwater tributaries of the lake (mid August), lake shoreline spawne rs (late August), and late-run fish that spawned in the lake's outlet, the Kasilof River (late September). Our objective was to determine th e degree of genetic differentiation among these subpopulations based o n restriction enzyme analyses of the cytochrome b gene of mitochondria l DNA and analyses of four polymorphic allozyme loci. Mitochondrial DN A haplotype frequencies for outlet-spawning sockeye salmon differed si gnificantly from those of all other subpopulations. The most common (3 6%) haplotype in the outlet subpopulation did not occur elsewhere, thu s suggesting little or no gene flow between outlet spawners and other spatially close subpopulations at Tustumena Lake. Allele frequencies a t two allozyme loci also indicated a degree of differentiation of the outlet subpopulation from the shoreline and tributary subpopulations. Allele frequencies for three tributary subpopulations were temporally stable over approximately 20 years (based on a comparison to previousl y published results) despite initiation of a hatchery program in two o f the tributaries during the intervening period. Collectively, our res ults are consistent with the hypothesis that significant genetic diffe rentiation has occurred within the Tustumena Lake drainage since degla ciation approximately 2,000 years ago.