Bioenergetics and Migration of Snake River Fall Chinook Salmon

Density-dependent mechanisms are a major regulatory force acting on the growth, behavior, and survival of riverine fishes.  Density-dependence could be a concern in hatchery-managed systems because artificial increases in abundance may exceed the carrying capacity of stocked habitats.  We investigated a 17-year time series of information on naturally-reared and individually PIT-tagged subyearling fall Chinook salmon (n=5,070) in the Lower Snake River, Idaho.  Our objectives were to: (1) quantify temporal changes in individual fish weight and time spent in free-flowing and reservoir habitats, (2) compare empirical fish weights at each recapture to those expected from a bioenergetics model, and (3) use the model to determine per capita consumption with respect to maximum consumption in both free-flowing and reservoir habitats in association with river flows and hatchery-induced changes in fish abundance.  In free-flowing reaches, weights at recapture were similar to those expected by the bioenergetics model that assumed maximum consumption, yet as fish abundances increased in the reservoir, weights at recapture were less than expected at maximum consumption. The effect of abundance on weight at recapture was negative and non-linear, supporting the presence of a threshold, whereby large decreases in per capita consumption may be expected when > 25,000 subyearlings per day were in the reservoir.  Increases in river flow were also associated with higher per capita consumption and weight at recapture at the dam.  The measured effect of fish abundance on per capita consumption was likely attenuated, however, by an increased emigration response (i.e., increased travel rate) from the study area.  Our findings support the conclusion that hatchery-induced changes in abundance have altered growth potential during rearing, and in turn, lead to recent reductions in fish size and earlier emigration from reservoir habitats.