Sea lice infection levels on juvenile salmon during early seawater residency and migration out of the Strait of Georgia

Description

There have been hypotheses that suggest the poor returns of Fraser River sockeye salmon (FRSS) could be caused by infections with sea lice acquired from salmon farms during their northern migration from the Strait of Georgia. To determine the potential impact from salmon farms, background information was needed about the species of sea lice that were present and their numbers on juvenile salmon and non-salmonid hosts.

This multi-year project involved sampling of juvenile salmonids and non-salmonids for sea lice during the out migration period. Samples were collected from  numerous sites throughout the Strait of Georgia and the Johnstone Strait to examine the role that salmon farms may play as a source of sea lice infections in wild fish. Prevalence and abundance of the different sea lice species and their developmental stages were determined. Samples of juvenile FRSS were also examined for the presence of other pathogens and disease.

This project  provides valuable information on:

1. the species composition and abundance of sea lice in these areas,
2. when and where fish become infected with sea lice,
3. whether patterns of sea lice infection vary among years, and
4. the overall health status of juvenile Fraser River sockeye salmon.

Findings

Two species of sea lice are commonly found on farmed and wild salmon in British Columbia: Caligus clemensi and Lepeophtheirus salmonis. On the wild juvenile FRSS examined, C. clemensi was the dominant species. The overall abundance of C. clemensi among areas and for all three years, was between 6.4- and 167-fold greater than the overall abundance of L. salmonis.

The prevalence and abundance of C. clemensi and L. salmonis was generally high on juvenile FRSS that migrated south-easterly from the mouth of the Fraser River into the Gulf Islands relative to other areas. On juvenile FRSS caught to the north, the prevalence and abundance of C. clemensi and L. salmonis increased significantly with distance from the river mouth, a stand-in for residence time in seawater. Migration distance, migration direction and month were all significant predictors for sea lice prevalence and abundance.

Early developmental stages (copepodids and chalimus larvae) of C. clemensi were found in relatively equal proportions throughout the Strait of Georgia, the Gulf Islands and the Discovery Islands. This finding suggests that abundant and widespread sources of the infectious stages of C. clemensi occur outside of the Discovery Islands. Non-salmonid hosts infected with C. clemensi were abundant in our catches throughout the Strait of Georgia.

Publications

Laurin, E., Bradshaw, J., Hawley, L., Gardner, I.,  Garver, K., Johnson, S.C. and Thakur, K.K. 2020. The importance of sample size for appropriate interpretation of prevalence estimates in surveillance studies in mixed-stock fisheries: A case example of Infectious Haematopoietic Necrosis Virus detected in Fraser River sockeye salmon (Oncorhynchus nerka) in marine and freshwater environments from 2010 to 2016 in British Columbia, Canada. Can. J. Fish Aquat. Sci. 19 December 2020 • https://doi.org/10.1139/cjfas-2020-0279

Program Name

Program for Aquaculture Regulatory Research (PARR)

Year(s)

2010 to 2014

Principal Investigator(s)

Stewart Johnson, Research Scientist, Fisheries and Oceans Canada, Pacific Biological Station, Pacific Region
Email: Stewart.Johnson@dfo-mpo.gc.ca

Richard Beamish, Research Scientist, Fisheries and Oceans Canada, Pacific Biological Station, Pacific Region
Email: richard.beamish@dfo-mpo.gc.ca

Marc Trudel, Research Scientist, Fisheries and Oceans Canada, Pacific Biological Station, Pacific Region
Email: marc.trudel@dfo-mpo.gc.ca

Team Member(s)

Chrys Neville, Biologist, Fisheries and Oceans Canada,  Pacific Biological Station, Pacific Region

Kyle Garver, Research Scientist, Fisheries and Oceans Canada, Pacific Biological Station, Pacific Region

Simon Jones, Research Scientist, Fisheries and Oceans Canada, Pacific Biological Station, Pacific Region