Wednesday, October 11, 2006
Salmon farming practices have come under fire for polluting oceans and damaging marine ecosystems. Now a new study heaps more criticism on the farms, suggesting that parasitic sea lice that flourish in salmon farms can kill as many as 95% of migrating wild Pacific salmon. The study, the first that attempts to quantify the effect of sea lice infection on wild Pacific salmon populations, further fuels an ongoing controversy over salmon farming practices in the United States and Canada.
Pacific salmon spawn in fresh waters, and the juveniles migrate seaward when they are still small and vulnerable to infection. Last year, John Volpe, a marine ecologist at the University of Victoria in British Columbia, Canada, and his colleagues showed that two common species of salmon--juvenile pink salmon and chum salmon--became heavily infected with the sea lice Leophtheirus salmonis when they passed a salmon farm off the coast of British Columbia Normally, sea lice thrive in the open sea because of many adult salmon there. But salmon farms, with thousands of salmon growing in net-enclosures, boast unnaturally high concentrations of sea lice. Studies have shown that young salmon that get infected can die when many lice feed off of them.
In the new study, Volpe and his colleagues set out to understand how these lice infections affect salmon populations, not just individual fish. He and his colleagues began by counting the lice on 14,255 salmon in various migratory routes that all run past salmon farms. After sampling, the fish were released back into the water. The numbers of lice, along with experimentally estimated mortality rates from lice infection, allowed Volpe's group to estimate mortality for the population sampled. Depending on environmental factors and the distance from the migratory route of the salmon to the fish farm, the lice could kill between 9% and 95% of the migrating salmon.
Until this study, "there was pretty compelling evidence that farms are the main source of lice, but there was no data on mortality," said Volpe at a press conference organized by a nonprofit organization called Pure Salmon, which lobbies for improving salmon farming practices.