Effects of Salmon Carcass and Analog Additions on Resident Trout in Idaho

The nutrients delivered via salmon spawning runs can be an important subsidy for stream ecosystems of the Pacific Northwest.  However, the delivery of nutrient subsidies via Pacific salmon has declined or been eliminated in many watersheds of the Pacific Northwest.  Mitigation through nutrient augmentation seeks to increase food web productivity through bottom-up trophic processes.  Our objective was to determine how additions of salmon carcasses and salmon carcass analog (pelletized fish material) affects resident trout.  During summers 2008-2010 we treated nine tributaries of the North Fork Boise River (500m reaches) with salmon carcasses and salmon analog (n=3 of each, including control).  Trout growth rates and diets were collected at short (2-6 weeks) time scales while trout density and biomass was sampled annually, and periphyton standing stock was measured at both short and annual scales.  Treatments significantly increased periphyton chlorophyll-a at short time scales, but not between years. Carcass material comprised 50% of trout diets 2 weeks after application and decreased to 10% 6 weeks after application.  Analog material comprised 35% at 2 weeks and decreased to 10% 6 weeks post treatment.  Trout growth rates increased >2X over controls at short time scales (2-6 weeks) in all years.  Trout density and biomass did not significantly increase across years. Higher growth rates led to increased trout production 15-25% over two years.  High growth rates, due to direct consumption of treatment material and increased basal resources, contributed to increased trout production.  The increased trout production did not translate into higher density or biomass in following years, suggesting that production was exported from our study reaches.