Adaptive peaks and alternative foraging tactics in brook charr: evidence of short-term divergent selection for sitting-and-waiting and actively searching

Some recently emerged brook charr (Salvelinus fontinalis) inhabiting still- water pools along the sides of streams are sedentary and eat crustaceans fr om the lower portion of the water column. Others are more active and oat in sects from the upper portion of the water column, We provide evidence that this divergent foraging behavior reflects short-term divergent selection br ought about by intraspecific competition in the presence of alternative foo d sources. Rates of encounters and interactions between individuals were de nsity dependent, and encounter and interaction events were closely timed wi th prey capture attempts. In addition, aggressive fish made more foraging a ttempts per minute than nonaggressive fish. Aggressive fish were also eithe r inactive or very active, while nonaggressive fish exhibited intermediate levels of activity. Growth rate potential, an important component of fitnes s during the early life stages of brook charr, was assessed using tissue co ncentrations of RNA and found to be highest for sedentary fish and for acti ve fish making frequent foraging attempts, and lower for fish exhibiting in termediate levels of activity. Our findings support contentions that indivi dual behavior plays an important role during initial steps in the evolution of resource polymorphisms.