Recolonization in action: Effects of salmon carcass density on experimental stream ecology

The importance of salmon carcasses to stream ecosystems is well documented, as they provide food and nutrients to otherwise oligotrophic systems. Their role in promoting salmon recolonization of historically accessible habitat, however, is unknown. The upper Cedar River, Washington was blocked to upstream movement of native chinook and coho salmon in 1905. In 2003, the first returning salmon were allowed passage via a fish ladder. This study evaluates how salmon carcasses may impact resident food webs and ecosystem processes at current and future run sizes. We hypothesized that increasing carcass density will: increase primary productivity, invertebrate density, growth rates of resident fish, N¹⁵ concentrations in aquatic organisms, and nutrient export. We tested these hypotheses in 16 experimental streams, and eight salmon carcass densities ranging from zero to 4.0 kg/m², each with two replicates. Carcass material was added gradually in accordance with historic spawner abundance surveys. The channels offered pool and riffle habitats and a variety of cover types. Each stream was stocked with one cutthroat, one coho, and one sculpin to represent the most prevalent fish species in Cedar River tributaries. We observed peaks of salmonid growth rate and invertebrate density at intermediate carcass densities (0.5 – 1 kg/m²).