Young sockeye salmon from B.C.'s Fraser watershed are infected with higher levels of sea lice after swimming past salmon farms, a new study has found.
And those salmon carry an "order of magnitude more" of the parasites than salmon that don't swim past salmon farms, said a study published in PloS One this week.
It's not clear what problems could result from the infections, said the lead author of the study, Michael Price, a biologist with the Raincoast Conservation Foundation.
However, he suggested the parasites may affect the health and behaviour of the fish. It is also possible that the sea lice may transmit diseases or may be a warning sign of other pathogens from fish farms, he added in a statement.
In recent years, some scientists have suggested that sea lice from fish farms may have been responsible for declines in the numbers of wild salmon off the Pacific coast. Recent studies have not found a clear relationship between sea lice and wild salmon deaths.
In the new study, Price and his collaborators captured hundreds of young salmon in the Discovery Islands, between Vancouver Island and the B.C. mainland, where there were 18 active salmon farms, and the north coast of B.C., where there are no salmon farms, in 2007 and 2008.
They recorded whether the fish were "upstream" or "downstream" from salmon farms, recorded the number and type of sea lice on each fish, and used genetic analyses to figure out whether the salmon originally came from the Fraser or Skeena watersheds. Fish from the Fraser watershed migrate past salmon farms, while those from the Skeena do not.
Few sea lice off north coast
Downstream from salmon farms, the study found an average of 4.83 sea lice per fish in 2007 and 1.61 sea lice per fish in 2008. Upstream from the farms, they found 1.10 and 0.95 sea lice in 2007 and 2008 respectively.
In 2007, there were just 0.17 sea lice per fish off the north coast, where there were no salmon farms. The paper acknowledged that the use of different sampling gear or environmental conditions may have been partly responsible for the difference, but concluded that neither temperature or salinity differences could explain the much lower incidence of sea lice infections.
The study was co-authored by researchers at the Raincoast Conservation Foundation, Wastershed Watch Salmon Society, Simon Fraser University and the Skeena Fisheries Commission. It was funded by the Coastal Allliance for Aquaculture Reform, several family and environmental charitable foundations and the Natural Sciences and Engineering Research Council.
In December, University of California biologist Gary Marty and his colleagues published a paper that found the survival of wild pink salmon appeared unrelated to the number of lice found on farmed fish or to farm fish production.
In January, a study led by Fisheries and Oceans Canada scientist Kristina Miller and published in Science found a link between signs of a possible viral infection in sockeye salmon and mortality of the fish before spawning.