Canadian Study Shows Effects of Sea Lice on Foraging Sockeye Salmon

A study published on the web of the Canadian Journal of Fisheries and Aquatic Sciences in late March says high abundances of sea lice affect the foraging abilities of out-migrating juvenile Fraser River sockeye salmon.

Pathogens threaten wildlife globally, but these impacts are not restricted to direct mortality from disease, said study authors Sean Godwin, Lawrence Dill, John Reynolds and Martine Krkosek.

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.
The authors conducted a competitive foraging experiment using out-migrating juvenile Fraser River red salmon to determine whether fish with high abundances of parasitic sea lice have reduced competitive abilities when foraging.

“Highly infected sockeye were 20 percent less successful at consuming food, on average, than lightly infected fish,” they concluded.

The authors noted that there are several potential mechanisms by which sea lice may reduce the competitive ability of juvenile sockeye salmon. These include, but are not limited to visual impairment, swimming impairment, stamina reduction and antagonistic behavior from larger or more dominant fish.

The study also noted that louse-loaded out-migrating sockeye are still very high in Johnstone Strait, and elevated sea louse abundances have been linked to salmon farms earlier on the sockeye migration route in the Discovery Passage.

“Understanding how pathogens like sea lice interact with fundamental ecological processes that determine fish survival is essential for effective marine conservation of populations vulnerable to pathogen infection,” they concluded.

“Competitive ability also increased with fish body size. Our results provide the first evidence that parasite exposure may have negative indirect effects on fitness of juvenile sockeye salmon, and suggest that indirect effects of pathogens may be of key importance for the conservation of marine fish.”

The abstract and links to the full article, available only to subscribers, are online at