Effects of Physical Habitat and Ontogeny on Lentic Habitat Preferences of Juvenile Chinook Salmon

We experimentally tested the habitat preferences of juvenile Chinook salmon Oncorhynchus tshawytscha to evaluate whether habitat availability was limited for a lake‐rearing population in Lake Washington (Seattle, Washington). A segment of this population resides in shallow (<1 m deep), nearshore areas of the lake during January–May and migrates to saltwater at age 0. During the shallow, nearshore phase, fish are exposed to varying degrees of bottom slope, substrate, and cover (e.g., overhead docks and woody debris) formed by the natural and modified shorelines of this highly urbanized system. The effects of these three habitat variables on patch use or electivity were tested in combination with ontogeny and the presence or absence of piscivores. Fry and presmolts avoided steeper bottom slopes, but presmolt responses were strongest. Responses to substrate and cover options were weak, although fry exhibited some coherent preference for finer substrates. The habitat preferences displayed by both life stages corroborated the observations from nearshore field surveys in Lake Washington. No direct effects on habitat preference from diel period or piscivore presence were evident. These results, combined with field observations, suggest that juvenile Chinook salmon may risk exposure to predation in order to utilize preferred habitat and to forage at a high rate. Therefore, nearshore habitat restoration projects aimed at increasing preferred juvenile Chinook salmon habitat should consider this risk‐prone behavior. Future experiments in larger experimental arenas could clarify the importance of heterogeneous nearshore habitat and further examine predation effects on the productivity of lake‐rearing juvenile Chinook salmon.