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 occurring in sympatry with sea-migrant females in a small stream were immigrants 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 common gene-pool for the sympatric migratory and stream-resident Forms. Assignment tests, based on microsatellite markers, to identify the population of origin of individuals did not suggest pronounced dispersal from the landlocked population into the down-stream population, However. it cannot be precluded that a modest degree of gene Row takes place from the landlocked population and that this may play a role in maintaining the two co-existing life-history forms among females in the down-stream population. (C) 2001 The Fisheries Society of the British Isles.