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BC salmon farms increase sea lice numbers: Report

A new report says fish farms in the Broughton Archipelago encourage "exponential" growth of sea lice populations and pose a major challenge to the industry and to regulators.

The report was published online on December 21 in Aquaculture Environment Interactions. The abstract, re-paragraphed:

Outbreaks of parasitic salmon lice Lepeoptheirus salmonis in sea-cage salmon farming regions of coastal seas have challenged the productivity of salmon farming industries and the conservation of wild salmon. We used a simple mathematical model to evaluate the population ecology of louse outbreaks, parasiticide treatment, and louse population decline for 2 farms in the Broughton Archipelago region of British Columbia, Canada.

Results suggest that exponential population growth of lice within a farm, rather than sustained louse immigration from wild sources, drive outbreaks on farms. Model analysis indicates that louse infection pressure from farms to wild juvenile salmon may be minimized by parasiticide application 2 to 3 mo preceding the juvenile salmon outmigration. The observed timing of parasiticide use and population decline of lice on farms is consistent with reported declines of lice on wild juvenile salmon.

If parasiticides do not have adverse environmental effects and lice do not evolve resistance, optimized parasiticide use on salmon farms may help reduce the spread of lice to wild salmon populations.

In the discussion section of the report, the authors write:

Our results indicate that sufficient progeny of lice on farmed fish are retained in the farm environment to lead to re-infection of farmed fish and exponential growth of louse populations. This suggests that farms can be a source of lice in the local marine environment, where wild salmon migrate, as would be expected because farmed salmon greatly outnumber wild salmon, particularly during winter months

Thus, the exposure of wild juvenile salmon to lice during spring, and the productivity of local wild salmon populations, is likely dependent on the dynamics of outbreaks and control of louse populations on farms. ...

While our results suggest parasiticides may assist managers in reducing lice and infection pressure on wild salmon, there are at least 2 important caveats. Copepod parasiticides may have unintended consequences for non-target crustaceans in marine ecosystems and lice may evolve resistance. Indeed, recent data imply resistance has developed in New Brunswick, Norway, and Scotland, suggesting that resistance of lice to parasiticides in British Columbia is a likely outcome.

Thus, other management options, such as reducing farmed salmon density, harvesting early, moving farms off wild salmon migration routes, or switching to closed containment technology must also be considered as strategies for conserving and restoring wild Pacific salmon populations that migrate past salmon farms.

Crawford Kilian is a contributing editor of The Tyee.

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