Sea louse infection of juvenile sockeye salmon in relation to marine salmon farms on Canada's west coast

Abstract

Background: Pathogens are growing threats to wildlife. The rapid growth of marine salmon farms over the past two decades has increased host abundance for pathogenic sea lice in coastal waters, and wild juvenile salmon swimming past farms are frequently infected with lice. Here we report the first investigation of the potential role of salmon farms in transmitting sea lice to juvenile sockeye salmon (Oncorhynchus nerka).

Methodology/principal findings: We used genetic analyses to determine the origin of sockeye from Canada's two most important salmon rivers, the Fraser and Skeena; Fraser sockeye migrate through a region with salmon farms, and Skeena sockeye do not. We compared lice levels between Fraser and Skeena juvenile sockeye, and within the salmon farm region we compared lice levels on wild fish either before or after migration past farms. We matched the latter data on wild juveniles with sea lice data concurrently gathered on farms. Fraser River sockeye migrating through a region with salmon farms hosted an order of magnitude more sea lice than Skeena River populations, where there are no farms. Lice abundances on juvenile sockeye in the salmon farm region were substantially higher downstream of farms than upstream of farms for the two common species of lice: Caligus clemensi and Lepeophtheirus salmonis, and changes in their proportions between two years matched changes on the fish farms. Mixed-effects models show that position relative to salmon farms best explained C. clemensi abundance on sockeye, while migration year combined with position relative to salmon farms and temperature was one of two top models to explain L. salmonis abundance.

Conclusions/significance: This is the first study to demonstrate a potential role of salmon farms in sea lice transmission to juvenile sockeye salmon during their critical early marine migration. Moreover, it demonstrates a major migration corridor past farms for sockeye that originated in the Fraser River, a complex of populations that are the subject of conservation concern.

Figure 1. Sockeye salmon collection sites relative to salmon farms.

Legend: Downstream boundary encircles all sockeye collection sites situated downstream of at least one salmon farm given the direction of prevailing oceanic flow and migration direction; all other collection sites are considered upstream.

Figure 2. Annual estimates of Caligus clemensi abundance on sockeye salmon.

Legend: North Coast region is without salmon farms, Discovery Islands upstream region encompasses sockeye collection sites upstream of all salmon farms given the direction of prevailing oceanic flow and migratory direction, and Discovery Islands downstream represents all collection sites downstream of farms for 2007 (solid line) and 2008 (dotted line). Estimates were obtained by back-transforming least-squares means; error bars, by back-transforming the least-squares means ±1 standard error.

Figure 3. Annual estimates of Lepeophtheirus salmonis abundance on sockeye salmon.

Legend: North Coast region is without salmon farms, Discovery Islands upstream region encompasses sockeye collection sites upstream of all salmon farms given the direction of prevailing oceanic flow and migratory direction, and Discovery Islands downstream represents all collection sites downstream of farms for 2007 (solid line) and 2008 (dotted line). Estimates were obtained by back-transforming least-squares means; error bars, by back-transforming the least-squares means ±1 standard error.

Figure 4. Sea louse abundance over time on Atlantic salmon on named salmon farms in the Discovery Islands.

Legend: Caligus clemensi at top, and Lepeophtheirus salmonis at bottom. Period of sockeye collection during 2007 and 2008 in shaded grey.