The Coastal cutthroat trout (Oncorhynchus clarki clarki) : genetic population structure, migration patterns, and life history traits

Abstract

Coastal cutthroat trout (Oncorhynchus clarki clarki) populations have undergone major range wide declines over the past two decades. This study was initiated in light of their declining status in an effort to augment the paucity of existing data, both in terms of their general ecology and genetic population structuring. Multiplexed groups of fluorescently labeled microsatellite primers were used in an analyses of allelic variation, both on a statewide and microgeographic scale. Analysis of six loci for 13 anadromous populations from throughout Washington revealed high within population variability (He = 71%; mean # alleles/locus = 24), significant differences in genotypic frequencies for single-locus pairwise comparisons between all populations, and substantial population subdivision (Fst = 0.121, Rst - 0.093). Similarly, analysis of allelic variation at 10 loci for 10 populations from within Hood Canal revealed high within population variability (He = 69%; mean # alleles/locus = 17), significant differences in genotypic frequencies between all populations across all loci, and in 304 of 450 single-locus pairwise comparisons, and significant population subdivision (Fst = 0.030, Rst = 0.029); all of which suggest that individual creeks form the basis for distinct breeding units in this subspecies. Mantel tests supported an isolation by distance model of population structure within Hood Canal using both Fst (P=0.015) and the Cavalli-Sforza and Edwards’ (1967) chord distance (P=0.001) as measures of genetic distance, and for the latter on a statewide scale (P=0.0l). Estimated levels of gene flow from direct observations, through a 3-year tag-recapture study, were similar to those estimated indirectly from allele frequency data, being on average approximately 5 to 10 effective migrants per generation between neighboring populations. Intensive ecological study of the Big Beef Creek population (Hood Canal, WA) revealed: differing migration patterns between the sexes, with males migrating into freshwater sooner and remaining longer than females; significant correlation between the upstream migration date for individuals across years (R^2 = 0.43, P=0.002); and evidence that some individuals likely overwintered at sea. Survival estimates averaged 54% for the overwintering period in freshwater, and 12-24% and 33-59% during salt water migrations for first and second-time migrants, respectively.