The effect of habitat structure on trophic level transfer efficiency and adaptive tradeoffs by juvenile salmonids

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Physical habitat structure is a key ecosystem attribute that influences both basal system productivity and the transfer of energy to higher trophic levels. This is especially true in rivers and streams, where different species of juvenile salmon appear to have evolved divergent adaptations to physical habitat extremes along environmental gradients related to stream slope, velocity, and substrate size. Despite extensive research on juvenile salmon habitat relationships, the mechanisms whereby habitat structure constrains adaptations and production of drift-feeding fish remain poorly understood. Most commonly used models for assessing the effects of flow and habitat on productive capacity in streams are based on correlative Habitat Suitability Curve relationships, which are limited in their predictive ability, and there is a need for a more mechanistic approach to modeling fish production in streams. We propose a series of experiments that will rigorously evaluate the effects of physical habitat on i) adaptive constraints for drift-feeding fish, ii) transfer efficiency of energy to the fish trophic level along a longitudinal downstream gradient, and iii) bottom-up effects of habitat structure on fish production mediated through increased production of invertebrate prey. Research will be carried out in both natural and artificial stream channels, and will permit assessment of the underlying mechanisms whereby different habitat configurations affect production of drift-feeding fish. Data from these experiments will be used to develop and parameterize a general bioenergetic model that will have broad application in stream fisheries ecology as a novel research/management tool for predicting the effects of changes in habitat, temperature, flows, and prey abundance on juvenile salmon productive capacity. Collectively, this information will be used to inform stream habitat management and conservation activities in the face of human development, and to guide and optimize restoration of degraded streams.