Epidemiology of net pen liver disease in British Columbia farmed salmon

Description

Current evidence suggests that net pen liver disease (NPLD) is caused by dietary exposure to microcystin (MC), which is a hepatotoxin produced by blue green algae. Microcystins from naturally occurring freshwater algal blooms are believed to be the major source of MC contamination in coastal waters. Historically, NPLD has been reported occasionally in wild and farmed salmonids in British Columbia (BC) and Washington State. In recent years, the incidence and severity of the NPLD has increased on BC salmon farms with millions of dollars of lost production in 2014 – 2016. The increased incidence of NPLD may reflect a change in the levels of MC in BC coastal waters and raises the possibility that MC may also be impacting the health and performance of wild aquatic animals.

This project generated knowledge that can be used to identify sites at risk of NPLD, and to improve or develop new disease management strategies. It can also be used to assess the risk of the toxin responsible for NPLD having effects on wild aquatic animals.

The objectives of this project were to:

1. Collect and organize various data pertaining to NPLD in BC
2. Refine methods of analysis for MC; to characterize the MC-isoforms and metabolites found in salmon and other marine animals
3. Track development of NPLD using histological methods; examine presence of liver lesions in other fish and determine the relationship with MC and/or its metabolites with disease
4. Identify the source of MC in coastal waters

The ability to predict whether NPLD is likely to develop would allow companies to make adjustments to their production plans that would reduce financial losses at impacted sites. This research program also supports the development of mitigation strategies, and the identification of the potential for MC to have broader ecosystem effects, on wild salmonids and other animals.

Findings

Algal toxins were commonly found by Solid Phase Adsorption Toxin Tracking (SPATT) and discrete water sampling at five fish farm sites from August 2017 to August 2019. The values obtained for microcystins (MC) were comparable to values from marine environments in California, where MC are a recognized environmental concern.

Acute exposure of Atlantic and Chinook Salmon to MC by gavage resulted in severe but reversible liver lesions, but did not result in Net Pen Liver Disease (NPLD). This suggests that the development of NPLD in wild and farmed fish may require exposure to higher MC concentrations during blooms and/or through the ingestion of contaminated natural feeds, or longer term chronic exposure to MC (possibly in conjunction with other toxins). Okadaic acid (OA), another algal toxin which co-occurs at much higher concentrations than MC in our samples, may contribute to the development of NPLD as OA is a hepatotoxin with a similar mode of toxicity.

Publications

• Shartau RB, Snyman H, Turcotte L, Bradshaw JC and Johnson SC (in draft). Acute microcystin exposure induces reversible histopathological changes in Chinook (Oncorhynchus tshawytscha) and Atlantic Salmon (Salmo salar).

Program name

Aquaculture Collaborative Research and Development Program (ACRDP)

Years

2017 – 2019

Principal investigator

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

Team member(s)

• Heindrich Snyman, Diagnostic Fish Pathologist, BC Animal Health Centre
• Pearse McCarron, Team Leader, Measurement Science and Standards, National Research Council, Canada
• Andrew Ross, Section Head, Fisheries and Oceans Canada, Institute of Ocean Sciences, Pacific Region

Collaborator(s)

• Barry Milligan, Fish Health Manager and Veterinarian, Cermaq Canada Ltd.
• Tim Hewison, Fish Health Coordinator, Grieg Seafood BC Limited