Long-term sea lice monitoring shows no change after salmon farm closures

Long-term scientific study shows the closure of salmon farms located in the Discovery Islands area of British Columbia (B.C.) did not result in a change of sea lice levels on juvenile wild salmon, leaving broader environmental influences as the key drivers of sea lice trends.

Recently published 2022 research data based on long-term monitoring of wild juvenile salmon along the coast of B.C. is providing researchers important long-term information on water quality, fish sample composition, fish size, and sea lice abundance from which to draw insights.

Professional biologists along B.C.’s coast have monitored juvenile Pacific salmon near to and far from salmon farm locations during the spring months when most species of salmon are out-migrating to sea from their natal rivers.

Mainstream Biological Consulting, Pacificus Biological Services and the Kitasoo/Xai’xais First Nation have completed this spring’s (2022) annual wild fish monitoring at 119 locations in five regions where Mowi Canada West farms salmon, using capture and sampling protocols adapted from Fisheries and Oceans Canada (DFO). All monitoring data is analysed by the Centre for Aquatic Health Sciences.

Key findings in the data include:

• Between 70% and 95% of wild juvenile salmon host two or less sea lice of any life stage, irrespective of the research team or whether sampled near to or far from a salmon farm.
• No location (whether near to or far from a salmon farm) has demonstrated consistent levels of sea lice prevalence or abundance over eight years of annual sampling.
• While highly variable, general increase in juvenile wild salmon size and weight is consistent with time spent at sea and rising sea temperatures in spring.
• Variability in temperature and fish condition is localized and not showing obvious trends year on year.

“The long-term data suggests that regardless of farm status, sea lice infestation rates on wild juvenile salmon fluctuate annually,” says Lance Stewardson, R.P.Bio., at Mainstream Biological Consulting. “In the Discovery Islands area, for example, in years 2021 and 2022 – when salmon farms were closed – the infestation on juvenile pink and chum salmon was within the range of the infestation witnessed between 2017 and 2020 when farms were operational. In the Broughton Archipelago, where some salmon farms have been closed in 2021-2022, the spring of 2022 was witness to the highest sea lice infestation we’ve encountered since 2016.”

Jennifer Russell, R.P.Bio., at Pacificus Biological Services Ltd. adds: “Committing to long-term study is the most effective way to achieve reliable results. Collecting data from before, during, and after operations is the best way to achieve answers. Examining a particular issue in this way is more reliable than opportunistic data collection.”

Sea lice are a tiny crustacean that live naturally on the skin of many marine fish. Sea lice numbers are monitored closely on salmon raised at B.C. farms, and includes veterinarian oversight.

Mowi acknowledges that effective sea lice management is important for fish welfare and to ensure sea lice on salmon farms do not negatively impact wild salmonids. Canada’s regulatory framework and local knowledge help define a management program that applies strategic, preventive, biological and non-medicinal measures to effectively control sea lice on farm-raised salmon. Mowi Canada West’s sea lice management results are published online monthly.