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Moving Salmon Nets May Control Parasite Spread

GUELPH, CANADA - Researchers at the University of Guelph have used DNA barcoding techniques to shed new light on controlling the spread of the salmon louse, a parasite blamed for devastating wild Pacific salmon stocks and costing British Columbia's salmon-farm industry millions of dollars each year.

The research team, led by integrative biology professor Elizabeth Boulding, also confirmed that the Pacific salmon louse is a distinct “sister species” of the salmon louse that has plagued the East Coast and salmon fisheries and fish farms on both sides of the Atlantic Ocean.

Their findings in the current issue of the journal Aquaculture Research.

Boulding’s team discovered significant differences in gene frequencies between lice samples from salmon caught at different locations along the B.C. coast, as well as between samples from wild salmon and farmed fish taken from the same waters. This suggests a low level of migration of lice back and forth between farmed and wild fish, but Boulding is cautious about the conclusions that can be drawn from the data.

“Without question, we observed considerable population structure – differences in gene frequencies among the different populations,” she said, adding that the dispersal of lice appears to be limited during their free-swimming larval phase. This suggests the lice could not be transmitted between farmed and wild fish if the net pens are kept far enough away from the migration routes of the wild salmon.

“However, in order to understand when the salmon louse is transmitted between wild salmon and farmed fish, we would need to do a much larger study with hundreds more samples, all taken in the same year, from the five wild species of Pacific salmon – chinook, coho, sockeye, pink and chum – and from fish farms across the region.”

Salmon lice have always existed naturally in the region but generally do not cause problems for adult fish. But since the advent of salmon farming, the lice are found in higher densities in some areas due to the presence of large numbers of fish living year-round in net pens. Juvenile salmon (smolts) are vulnerable to potentially lethal infection. This is particularly true for pink salmon, which emerge from B.C.’s rivers at an earlier stage than other wild species on their journey to open water, where they spend their adult years. The current study analyzed 239 samples of lice from wild and farmed salmon hosts from British Columbia, Alaska and Japan, as well as 180 samples from areas on both sides of the Atlantic, including New Brunswick and Scotland. It included new samples obtained with the assistance of B.C. sports fishers, Stolt Sea Farm Canada and conservationist Alexandra Morton, as well as samples from previous studies.

Working with the Biodiversity Institute of Ontario (BIO), the research team extracted DNA from the lice, obtained the sequence of the mitochondrial “barcoding” gene and analyzed the frequencies of 45 different alleles.

In addition to confirming that the Pacific salmon louse is indeed a separate species, they found significant frequency differences in the sequences between lice taken from wild salmon caught in Barkley Sound and those from the Broughton Archipelago. There were also genetic differences between the lice found on wild hosts and those from farmed fish, and between lice taken from fish farms in different regions. The samples from wild hosts were collected in 2005, whereas the samples from fish farms were collected in 2006, which may account for some of the differences.

All farmed salmon on B.C.’s coast are Atlantic salmon because they are better suited to being raised in net pens. Like their wild cousins, they are vulnerable to infection from the Pacific salmon louse, costing the industry millions of dollars each year in losses and spending on chemical control measures.

Boulding said the study demonstrates the value of the genetic barcoding work being done by the BIO team in Guelph. Researchers based here are uniquely equipped to carry out a much larger study that would answer important questions about the salmon louse and possibly guide future public policy on salmon aquaculture, she said.

“This is a very important issue in British Columbia because it is the only place in the world where there are still wild stocks of salmon in areas where there are also fish farms.”

Funding for the study is from the Canada Foundation for Innovation, Genome Canada, the Ontario Innovation Trust, the Natural Sciences and Engineering Research Council, and the Gordon and Betty Moore Foundation.

Further Reading

- Read our recent report, The Global Spread of Infectious Salmon Anaemia, by clicking here.
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