Salmon and a lousy louse

Fish farms harbour parasites, threaten wild fish varieties

By Tiasa Adhya
Published: Saturday 15 October 2011

imageSALMON will grant knowledge to whoever eats it. So goes the Irish mythology about the fish intertwined with culture and livelihoods of almost all North Atlantic and Pacific countries since ancient times.

Demand for the fish, high in good fat Omega-3 and other rare nutrients, has only increased over decades. So has its overfishing and commercial harvesting. While overfishing is pushing the wild robust salmon varieties beyond the point at which they can recover, a recent study warns that salmon farms, often located in estuaries, also contribute to the threat in the form of tiny sea lice.

The louse (Lepeophtheirus salmonis) holds on to the fish skin and feeds on its mucous, affecting its growth, swimming ability, immune system and adaptation to saltwater during migration from freshwater where it is spawned.

Though studies in the past few years have exempted salmon farms from spreading infection to the wild varieties, researchers from the US, New Zealand and Canada have gathered unequivocal evidence to implicate aquaculture of the fish. They analysed decades of population data of two wild varieties, pink and coho salmons, across the Broughton archipelago in western Canada. They observed a decline in the stock of wild salmons in areas where the juveniles had to take a route past salmon farms for migrating from the river to the sea. Studies have shown that salmon farms, where fish are kept crowded in a confined space, harbour sea lice. As the salmon farms come in the way of juveniles migrating from freshwater to the sea, the young fish are exposed to a cloud of sea lice and get infected, note the scientists in a paper published online in PNAS on August 22. Since juveniles are more vulnerable to the parasite than adults, the stock collapses.

The scientists have offered two solutions to this problem. “One is to use effective chemicals to combat the lice, feed premix slice for example. As the chemical is fed to farmed salmon, it gets absorbed through the gut and circulated throughout the tissues, killing the sea lice feeding on its skin, fins and gills,” says Martin Krkosek, lead author of the study. However, in Europe and on the east coast of Canada and Chile sea lice have developed resistance to the chemicals. So the effective way is to move these farms away from rivers to closed containment or pens, completely sealed off from the surrounding environment, says co-author Alexandra Morton from Salmon Coast Field Station, Canada.

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