Genetic delousing

Photo: The Norwegian Seafood Export Council

As climate change and increasing aquaculture production fuel the sea lice problem, some researchers hope knowledge from the soon-to-be sequenced salmon genome will give fish farmers an edge in an intensifying battle against the tiny parasites.


“Sea lice is maybe the number one problem for salmon aquaculture,” says Sigbjørn Lien, professor at the Norwegian University of Life Sciences in Aas. He is the lead investigator on a new research project that’s trying to identify salmon genes associated with lice resistance.

Nina Santi, research director of Norway’s Aqua Gen breeding company, says the government has become more concerned about sea lice infestations in wild salmon over the last couple years and this has led to stricter rules for granting and extending farming licenses. In order to compete for new licenses, those in the industry are now required to show that their practices have low impacts on wild salmon.

“The authorities are more or less putting the growth of the industry on hold and they’re signalling very strongly that the industry needs to control sea lice levels,” she says.

Sea lice, which can grow to be up to a centimetre in length, attach themselves to the skin of salmon and drink the animals’ blood. Though a few lice won’t cause much harm to a large fish, the parasites are deadly when present in large numbers.

Frank Nilsen, head of the Sea Lice Research Centre at University of Bergen and member of the research team, says sea lice pose a greater threat to fish than ever before. This is because aquaculture production around the world is growing and as the number of fish increases, so too does the number of sea lice. This is especially true when large numbers of farmed fish are kept in close quarters because when sea lice reproduce, their larvae are almost guaranteed to find new hosts.

Because sea lice have the potential to devastate salmon farms and spread to wild fish, lice numbers are closely monitored and controlled. In Norway, fish farmers catch dozens of their fish every one or two weeks depending on the season and examine them for the parasites. If the average number of lice exceeds 0.5 per fish, the farm must be treated with cleaner fish or with drugs that are released into the water.

But no new drugs have appeared on the market in the last decade and sea lice appear to be developing a resistance to the old, much-used chemicals.

“The efficiency of these drugs has decreased quite a bit,” Nilsen says. “It is much more difficult now to actually control the lice.”

The reduced effectiveness of the drugs is particularly concerning these days because warming water temperatures are speeding up the growth of sea lice so they are able to reproduce sooner. Nilsen says this is especially worrying for northern fisheries, including those in Finnmark, where cold waters have traditionally kept sea lice numbers reasonably low.

If traditional drugs aren’t cutting it anymore, Nilsen says it’s time to look for other ways to control the parasites and one solution may be to breed fish more resistant to the lice.

Nilsen and Lien explain that some fish populations are better able to fight off sea lice than others and that this trait appears to have a genetic component. Over the next four years, the researchers will sequence the genomes of fish from families very sensitive to sea lice and compare these to genomes of lice-resistant fish – work that will be possible when the Atlantic salmon genome is fully sequenced this winter and can be used as a template.

By doing this, the researchers will get a sense of whether there are genes that code for resistance to sea lice and, if so, where these are located. If such genes are found, it may be possible to develop a genetic test that fish farmers can use on their brood stock in order to determine what fish are genetically predisposed to be resistant to the parasite. Such technology is already used by breeders to screen fish for susceptibility to the infectious pancreatic necrosis virus.

But Nilsen cautions that genetic screening won’t be a silver bullet against the sea lice problem – it just has the potential to be one step towards helping reduce the number of sea lice.

“It doesn’t seem we’ll be able to create a resistant salmon, I don’t think that’s possible,” he says. “But it has been claimed that if you have a contribution from breeding and you have a contribution from other non-medical action, that can reduce the dependency on medicine.”