Does dispersal from landlocked trout explain the coexistence of resident and migratory trout females in a small stream?

The hypothesis that stream-resident females of brown trout Salmo trutta occ urring in sympatry with sea-migrant females in a small stream were immigran ts from an up-stream allopatric landlocked population was rejected. Genetic differentiation was not detected between the sympatric forms whereas they both diverged significantly from the land-locked population. suggesting a c ommon gene-pool for the sympatric migratory and stream-resident Forms. Assi gnment tests, based on microsatellite markers, to identify the population o f origin of individuals did not suggest pronounced dispersal from the landl ocked population into the down-stream population, However. it cannot be pre cluded that a modest degree of gene Row takes place from the landlocked pop ulation and that this may play a role in maintaining the two co-existing li fe-history forms among females in the down-stream population. (C) 2001 The Fisheries Society of the British Isles.