Community context and the influence of non-indigenous species on juvenile salmon survival in a Columbia River reservoir

Non-indigenous species (NIS) have been called biological pollutants, which implies that reducing their numbers should reduce negative impacts. To test this hypothesis, we used food web models, parameterized with data from field studies, to ask how reducing the number of NIS co-occurring with endangered salmon would affect salmon mortality. Our analyses indicate that predation on Upper Columbia River spring chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss juveniles was affected very little by NIS reduction. The effects of removing NIS were partly or totally offset by indirect food web interactions, and were subtle compared to effects of native predator management. We predict that the most effective way of reducing predation on salmon smolts will involve managing native predators and targeted removals of specific NIS. Minimizing impact of established NIS thus entails not only reducing NIS prevalence, but also considering background management practices and community context.