Parasitic sea lice that jump from fish farms to wild salmon may be a much greater problem than suspected, according to a new report likely to inflame an ongoing battle between conservationists and aquaculture proponents. The authors say the lice could damage ocean ecosystems by infecting other species such as herring and stickleback--a staple of many marine animals including whales and seabirds.
Fish farms are generally large underwater cages housing hundreds of thousands of fish in cramped quarters, attracting disease and lice. In the wild, sea lice latch onto a fish's scales and live off its blood and outer coating of mucus, slowly eating it alive. That the farms act as reservoirs of infection for passing wild fish is generally accepted in Europe, but the premise has been hotly contested in the U.S and Canada. So researchers at the University of Victoria, Canada, conducted a field study in an attempt to resolve the debate.
Marine ecologist John Volpe and his colleagues examined more than 5,000 juvenile pink and chum salmon for lice infestations at short intervals along a narrow salmon migration route that runs next to a fish farm in British Columbia. The scientists plugged their lice counts, along with readings of water temperature (lice numbers fall as it gets colder), salinity, and distribution rates of lice larvae, into a mathematical model. The results, which appear this week in the Proceedings of the Royal Society, indicate the juveniles were generally free of lice as they began their ocean migration but became heavily infected after passing a farm.
"Infection levels near the farm were 73 times higher than normal," says Volpe. And the lice levels were above normal as far as 30 km from the farm. That's worrying, says Volpe, because the lice mature and reproduce along the way, making the "migrating school a moving cloud of infection" that can be passed to other fish species such as stickleback and herring.
"This is an excellent paper" that "for the first time nails down the spatial scale of sea lice impact," says Ransom Myers, a fisheries biologist with the Dalhousie University in Halifax, Nova Scotia. He adds that lice transfer from fish farms could partially explain the loss of salmon along the coast of Maine and in the Bay of Fundy, where several large farms are located. But others disagree. "The study is flawed due to lack of lice data from the farm itself," says Scott McKinley, an environmental physiologist at the University of British Columbia, Canada, who is studying the impact of sea lice himself. "This is just irresponsible and looks like fear mongering."