Genetic Structure of Anadromous Cutthroat Trout (Salmo clarki clarki) Populations in the Puget Sound Area: Evidence for Restricted Gene Flow

Abstract

Genetic relationships among anadromous populations of coastal cutthroat trout (Salmo clarki clarki) in the Puget Sound area (United States) were investigated by electrophoretic methods between successive year classes within streams (populations), among streams within drainages, between drainages within each of two regions separated by Puget Sound, and between the two regions. Average allele frequencies for fish from the two regions differed by approximately 0.10 at several polymorphic loci, suggesting that gene flow between the two regions is restricted. Despite this divergence, the sampled populations were all very similar genetically (Nei's I > 0.97), possibly reflecting a common ancestral invasion following Pleistocene glaciation. The total gene diversity (expected heterozygosity) was 0.101 and was partitioned as follows: between regions, 1.24%; between drainages, 1.01%; among streams, 2.41%; between year classes, 1.17%; within individual year class samples, 94.2%. In addition, stream effects were significant in 7 of 12 intralocus ANOVA comparisons, suggesting that populations were structured genetically at this level of subdivision. This latter result may be reflecting the homing instinct of spawning adults.