The first link between salmon farms on the British Columbia coast and elevated levels of sea lice on juvenile Fraser River sockeye salmon has been demonstrated by new research published today.
While there has been speculation that lice from captive salmon has been transferred to wild salmon, the new study is the first to show a potential role of salmon farms in sea lice transmission to juvenile sockeye salmon during their critical early migration to the sea.
The research by scientists from Raincoast Conservation Foundation, Watershed Watch Salmon Society, and the Universities of Victoria and Simon Fraser is published in the journal "Public Library of Science ONE."
The authors conclude that their work "demonstrates a major migration corridor past farms for sockeye that originated in the Fraser River, a complex of populations that are the subject of conservation concern."
The rapid growth of marine salmon farms over the past two decades has increased host abundance for pathogenic sea lice in coastal waters, and wild juvenile salmon swimming past farms are frequently infected with lice, the authors say.
"Given the high intensities of lice observed on some juveniles in this study - up to 28 lice on a single fish - there's an urgent need to understand the extent of threat posed by sea lice to juvenile Fraser River sockeye," said co-author Dr. Craig Orr of the Watershed Watch Salmon Society.
Sea lice on a juvenile salmon caught in the Georgia Strait (Photo by Daniel Beltra courtesy Greenpeace)
The scientists examined sea lice on migrating sockeye in an area of Canada's west coast between Vancouver Island and the mainland known as the Discovery Islands, taking samples in 2007. This region hosts the northeast Pacific's largest salmon farm industry, 18 active salmon farms, and also hosts one of the largest migrations of salmon in the world, primarily to and from the Fraser River.
The scientists genetically identified 30 distinct stocks of infected Fraser sockeye that pass by open net-pen salmon farms in the Strait of Georgia, including the endangered Cultus Lake stock.
The study found that "parasitism of Fraser sockeye increased significantly after the juvenile fish passed by fish farms."
These same species of lice were found in substantial numbers on the salmon farms.
Not only did juvenile Fraser sockeye host higher lice levels in the Georgia Strait after they passed salmon farms, the researchers found that these fish hosted "an order of magnitude more sea lice" than Skeena and Nass River sockeye that migrated along the north coast where there are no farms.
The new study contradicts the Canadian fisheries agency's statement that, "Juvenile sockeye that migrate past salmon farms in the Discovery Islands are significantly larger than pink salmon ... when they migrate into the ocean, well beyond the threshold for susceptibility to sea lice."
Sea lice can compromise regulation of fluid within the bodies of the host fish, induce behavioral changes that increase predation risk, reduce growth rates and, in sufficient numbers, result in host death. Sea lice also have been shown to serve as vectors for the spread of fish diseases.