Effects of experimental salmon carcass and analog additions on resident trout growth rates, abundance, and production

The nutrients delivered via salmon spawning runs is an important subsidy for stream ecosystems of the Pacific Northwest. However, this delivery has declined or been eliminated in basins across the Pacific Northwest. Nutrient mitigation seeks to increase productivity of algae, invertebrates, and fishes through bottom-up propagation of energy and nutrients. With the transfer of energy to higher trophic consumers, resident fishes are a focal species for nutrient mitigation strategies, with responses occurring at individual fish (growth rates), and population (abundance, production) scales. From summer 2008 through 2010 we conducted a large-scale field experiment across nine tributaries of the N. Fork Boise river, consisting of 500-m stream reaches treated with salmon carcasses (n=3) or pelletized salmon (n=3), and un-treated reference reaches (n=3). We hypothesized that additions of salmon carcass and analog pellets would increase growth rates of resident trout, which would translate to increased abundance, biomass, and production of their populations. Multi-pass depletions were conducted annually to quantify fish age, size structure, and population size, and collected fish were PIT tagged and released. Additional single passes 2, 4, and 6 weeks post-application were conducted to recapture fish to estimate growth rates. Nutrient mitigation treatments increased growth rates of resident fishes more than two times over controls by six weeks after treatment additions in 2008-2010. Moreover, increased growth rates translated into more than ~25% more trout production within the study reaches, though increased abundance of trout was not detected. Mitigation strategies should consider the mobility of trout, which can move longer distances than our 500m reach. Emigration of larger fish from the nutrient treatment reaches to mainstem habitats may play a role in population dynamics. The increased short-term responses suggest that direct consumption is important to fish production, as bottom up transfer from could not happen at such a short time scale