Sea lice, sockeye salmon, and foraging competition: Lousy fish are lousy competitors

Pathogens threaten wildlife globally, but these impacts are not restricted to direct mortality from disease. For fish, which experience periods of extremely high mortality during their early life history, infections may primarily influence population dynamics and conservation through indirect effects on ecological processes such as competition and predation. We conducted a competitive foraging experiment using outmigrating juvenile Fraser River sockeye salmon (Oncorhynchus nerka) to determine whether fish with high abundances of parasitic sea lice (Caligus clemensi and Lepeophtheirus salmonis) have reduced competitive abilities when foraging. Highly infected sockeye were 20% less successful at consuming food, on average, than lightly infected fish. Competitive ability also increased with fish body size. Our results provide the first evidence that parasite exposure may have negative indirect effects on the fitness of juvenile sockeye salmon and suggest that indirect effects of pathogens may be of key importance for the conservation of marine fish.