We examined, using underwater snorkeling observations and field experiments, the influence of food availability on foraging behavior, habitat use, and survival of age-0 Atlantic salmon (Salmo salar) during the critical first-summer growth period. While most feeding attempts were directed at drifting invertebrate prey, we found a higher rate of benthic feeding forays than previously reported for salmon. Greater food abundance was associated with higher feeding foray rates, more time allocated to foraging, occupancy of higher microhabitat velocities, and greater first-year survival between two study streams. Experimental drift reduction reduced drift foray rates and triggered a change in behavior to increased benthic feeding. In contrast, within a single stream, greater predicted invertebrate drift in high-velocity microhabitats (1 m2) was unrelated to either microhabitat occupancy or drift foray rates of age-0 salmon. We suggest that, in some situations, salmonid foraging is related more directly to overall prey density than to the availability of high-velocity, high drift rate microhabitats. Differences in resource tracking (increased foraging, growth, or survival with increased food abundance) at different scales, along with the use of alternative predation modes, underscore the importance of considering behavior when linking food resources to growth and survival of stream salmonids.
Habitat structure versus food abundance: the importance of sparse vegetation for the common redstart Phoenicurus phoenicurus
