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

We used six polymorphic microsatellite loci to investigate a potential meta population system in bull trout (Salvelinus confluentus) from five spawning localities in the Lightning Creek drainage, a tributary to Lake Pend Oreil le, Idaho. The number of spawners as estimated by redd counts is low in all populations sampled. Analytic viability models indicate that local isolate d populations of these sizes are unlikely to persist. We tested two hypothe ses: (1) these are remnant populations that are vulnerable to local extinct ion, 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 bas in an significantly differentiated (P<0.005), and the allele frequencies ap pear to be stable among temporally separated subsamples within locations. I t is therefore unlikely that extensive dispersal has linked tributaries in a manner consistent with a metapopulation structure. The low number of spaw ning individuals combined with the degree of isolation indicated by the gen etic data suggest that extinction of the tributary populations is probable if temporal variability and small size is prolonged. However, these populat ions 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.