Favorable foraging locations for young atlantic salmon: application to habitat and population restoration

Declines in the populations of salmonid fishes have generated major interest in conservation and restoration of wild populations and river habitats. We used a foraging‐based model, combined with field observations and surveys, to predict individual habitat use, and to assess the effects of stream habitat conditions and management practices on the potential for reestablishing Atlantic salmon, Salmo salar. Using a model based on a simple trade‐off between increasing prey encounter rate and decreasing salmon capture success with increasing stream current velocity, we predicted favorable foraging locations for salmon in their first (age‐0) spring and summer. We tested, in six streams, whether (1) salmon preferred locations (=microhabitats) that were predicted to yield high consumption rates, (2) salmon growth and survival was greater in streams with a greater proportion of preferred, profitable, microhabitats, and (3) stream habitat remediation (introduction of large in‐stream structures such as large woody debris) increased the availability of microhabitats found to be preferred by salmon, and energetically profitable. Salmon early in their first season (May–June) were predicted to obtain the highest consumption rates (within 10% of maximum) in microhabitats with a narrow range of relatively slow current velocities (0.08–0.18 m/s). In contrast, later in the season (July–August) fish were predicted to obtain highest consumption rates over a wide range of fast current velocities (0.21–0.57 m/s). Salmon in both the early and late seasons showed strong preferences (use in proportion to availability) for microhabitat in velocity categories predicted to provide high consumption. Streams with the greatest proportion of preferred early‐season, but not late‐season, microhabitats retained a higher proportion of salmon as measured at the end of the first summer. Stream habitat remediation increased the amount of preferred early‐season microhabitat and did not negatively affect invertebrate prey abundance, or the amount of preferred late‐season microhabitats. Thus, the availability of favorable foraging areas for juveniles significantly improves the retention of salmon during the critical first summer, and stream remediation provides better foraging habitat during this important period. Our results are encouraging for broader application to identify sites that show promise for salmon reintroduction, and to help guide restoration of particular sites to provide suitable habitat.