Genetic population structure within streams: microsatellite analysis of bull trout populations

Abstract– We used six polymorphic microsatellite loci to investigate a potential metapopulation system in bull trout ( Salvelinus confluentus ) from five spawning localities in the Lightning Creek drainage, a tributary to Lake Pend Oreille, Idaho. The number of spawners as estimated by redd counts is low in all populations sampled. Analytic viability models indicate that local isolated populations of these sizes are unlikely to persist. We tested two hypotheses: (1) these are remnant populations that are vulnerable to local extinction, or (2) these populations are interconnected by migration and are being maintained at larger effective sizes than indicated by the redd counts (i. e. metapopulation dynamics). All populations within the Lightning Creek basin are significantly differentiated ( P > 0.005), and the allele frequencies appear to be stable among temporally separated subsamples within locations. It is therefore unlikely that extensive dispersal has linked tributaries in a manner consistent with a metapopulation structure. The low number of spawning individuals combined with the degree of isolation indicated by the genetic data suggest that extinction of the tributary populations is probable if temporal variability and small size is prolonged. However, these populations contain amounts of genetic variation similar to populations throughout the range of bull trout. Therefore, if the apparent demographic trends are reversed, these populations may recover without suffering the detrimental effects of a severe bottleneck.