Pulsed Food Subsidies Across a Habitat Mosaic Shape Growth Opportunities for Rearing Salmon in a Glacial Alaskan River

To grow large enough to survive, juvenile salmon must rear in habitats with favorable combinations of food, water temperature, and other limiting factors.  Glacial rivers with large salmon runs are heterogeneous in physical habitat and the supply of riverine, terrestrial, and marine-derived food resources, yet the interplay and relative importance of these factors for salmon growth remain poorly understood.  We compared the diet and growth patterns of juvenile Chinook and Coho salmon among glacially influenced mainstem habitats, sloughs, and clear-water tributaries in the Susitna River, Alaska.  We investigated the influence of food availability, food quality, water temperature, and other physical variables on feeding and growth rates using bioenergetics and growth-rate potential models.  Sloughs and tributaries contained the highest invertebrate densities, and some also had salmon spawning in pulses during midsummer-fall.  Juvenile salmon in these habitats shifted from feeding on invertebrates to salmon eggs if they surpassed a gape limitation threshold during spawning runs.  Initial bioenergetics results suggested salmon growth was more limited by food than temperature in most habitats.  Understanding how environmental factors interact to provide a mosaic of growth opportunities within complex watersheds is important for managing fish stocks and their habitats, particularly in the face of environmental change.