Prey Selection of Subyearling Atlantic Salmon and Rainbow Trout Coexisting in a Natural Stream

ABSTRACT

Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) are similar ecologically but share no coevolutionary history favoring the evolution of niche-partitioning mechanisms. Therefore, interspecific competition may occur as each species becomes naturalized throughout the other's native range. As a first step towards investigating competitive interactions, we tested hypotheses regarding prey selection in allopatric and sympatric populations of Atlantic salmon and rainbow trout in a natural stream. Diet samples from 184 fish and contemporary drift samples were collected during a 24-hour period in mid-summer. Diet overlap was low between salmon and trout in allopatry, but high in sympatry. Salmon consumed more prey and a greater proportion of baetid mayflies than did trout, which fed more frequently on chironomid larvae and aerial-terrestrial invertebrates. Both species consumed larger prey when allopatric than when sympatric. Salmon in allopatry fed as generalists and trout in allopatry fed as specialists, but diets converged when in syinpatry. Feeding habits of both salmonines seemed to be affected by the other's presence, although more notably for trout. Results were more consistent with predictions based on optimal foraging theory than on niche-partitioning theory.