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

Abstract

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.Résumé : Les pathogènes sont une menace pour les espèces sauvages à l'échelle planétaire, mais leurs impacts ne se limitent pas à la mortalité directe découlant de maladies. Pour les poissons, caractérisés par une période de mortalité extrêmement élevée au début de leur cycle de vie, les infections pourraient influencer principalement la dynamique des populations et la conservation par le biais d'effets indirects sur des processus écologiques comme la concurrence et la prédation. Nous avons mené une expérience d'approvisionnement concurrentielle en utilisant des saumons rouges (Oncorhynchus nerka) juvéniles du fleuve Fraser dans leur migration vers la mer, afin de déterminer si les poissons présentant une forte abondance de poux du poisson (Caligus clemensi et Lepeophtheirus salmonis) parasitiques présentaient également une capacité concurrentielle réduite durant l'approvisionnement. Les saumons rouges très infectés avaient un taux de succès de consommation de nourriture 20 % plus faible, en moyenne, que les poissons légèrement infectés. La capacité concurrentielle augmentait également avec la taille du corps du poisson. Nos résultats sont les premiers à indiquer que l'exposition aux parasites pourrait avoir des effets négatifs indirects sur l'aptitude des saumons rouges juvéniles et donnent à penser que les effets indirects des pathogènes pourraient être d'importance capitale pour la conservation des poissons de mer. [Traduit par la Rédaction]

Reduced growth in wild juvenile sockeye salmon Oncorhynchus nerka infected with sea lice

“…clemensi rather than L . salmonis , which is consistent with previous reports (Price et al ., ; Godwin et al ., ). As only six of the 369 lice were L .…”

“…While direct mortality from sea lice is probably very low for O . nerka (Jakob et al ., ), the indirect effects of louse parasitism on these fish are unknown apart from an association with reduced competitive ability in highly infected fish (Godwin et al ., ). Oncorhynchus nerka are of particular interest because of their iconic status, their importance to fisheries (FAO, ), and their complex multi‐host dynamics with sea lice; C .…”

“…nerka migrating northward along the south coast of British Columbia, however, are primarily infected by C . clemensi (Price et al ., ; Godwin et al ., ) and are post‐smolts (having spent at least 1 year in fresh water before migrating to the sea) rather than young‐of‐the‐year. While direct mortality from sea lice is probably very low for O .…”

Sea trout adapt their migratory behaviour in response to high salmon lice concentrations

“…Based on earlier studies (Gjelland et al, 2014;Godwin et al, 2015;Skaala et al, 2014;Vollset et al, 2016) , we expected to see higher predation and other mortality in the untreated group and in the high infestation years. In addition to trout adapting their behaviour to infestation pressure as discussed above, the results of this study might be also partly explained by both the high individual variability in behaviour among the tagged trout, and relatively large emigrated and unknown fate groups, that led to relatively small-sized remaining fate groups.…”

Lessons from sea louse and salmon epidemiology

“…Farmed fish are often exposed to pathogens from sympatric wild fish populations, but are sheltered from many of the natural processes (e.g. predation [5], competition [6], migration [7,8]) that can remove infected individuals from populations. Farmed host populations are typically held at higher densities than populations in the wild, increasing opportunities for infections to spread.…”