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Royal Society Eyes Danger in Fish Farm Clusters

UK - Salmon farms clustered on the migratory route of wild salmon could be killing large numbers of fish in the wild, according to scientists writing today in the journal Proceedings of the Royal Society B.

Sea lice are parasites which commonly infect adults of both wild and farmed salmon populations, causing ill health and even death.

Unlike adults, juvenile salmon are rarely affected by sea lice in the wild, except when they migrate past salmon farms, according to field studies. But scientists have been confused to find that under experimental conditions, sea lice seem to have little effect on the juvenile fish. Getting to the bottom of this disagreement between field and experimental studies has important implications for policy, as well as scientific understanding, according to the authors.

Now, Martin Krkosek from the University of Alberta, and colleagues, have found the solution. They created a mathematical model of salmon-louse population dynamics and tested it by monitoring almost ten thousand juvenile salmon which had been naturally infected with sea lice in the salmon farming region of British Colombia. Having caught the fish they then reared them in ocean enclosures and monitored numbers of lice and the survival of the salmon.

From the model, Krkosek realised that the key was in the length of time the wild fish were exposed to farms, and hence to sea lice. The exposure time in experimental studies is only a few hours, much less than the two or three month migration of juvenile salmon past multiple salmon farms, say the authors. Their study showed that after brief exposure juvenile salmon could shake off the parasite, but over a prolonged period of up to 80 days lice accumulated enough to kill substantial numbers of the fish.

Salmon farms are built in regional clusters for good business reasons, but the result is that wild salmon must migrate past a number of farms before they reach the open ocean. And it's this prolonged exposure that needs to be addressed, say the authors. "For policy this means that the coastal planning and management should consider minimizing the exposure time of juvenile salmon to sea lice from multiple salmon farms sited sequentially on migration routes".

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