Hakai Magazine

juvenile salmon with sea lice
Sea lice, the tiny brown parasites attached here to a juvenile salmon, are a concern for both farmed and wild salmon. Photo by Paulo Oliveira

Overlooked Sea Louse May Be a Big Problem for Salmon

Most scientific research has focused on one species of sea louse—failing to account for the other, far more common, species.

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by Brian Owens

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Sea lice attach to the skin of fish, and feed on their mucus, tissues, and blood. These parasites are one of the major threats to both wild and farmed salmon. To date, however, most research on sea lice has focused on just one species, Lepeophtheirus salmonis.

L. salmonis is a salmon-infesting specialist that plagues aquaculture operations, which explains why it’s drawn the most attention. But it is not the only louse that hurts salmon—Caligus clemensi is a generalist that attacks salmon as well as other fish. “One of the things that is still unclear in the world of salmon lice is how these two species co-infect different Pacific salmon species,” says Cole Brookson, a biologist at the University of Alberta.

To answer that question, Brookson and his colleagues analyzed a huge data set, compiled from observations made between 2015 and 2019, that showed how many lice were infecting juvenile wild salmon of different species off the coast of British Columbia. Using this data, they determined how each kind of louse affects the different species of salmon. They focused on juvenile salmon because the period when a fish is leaving its natal stream and is heading out to sea is the most crucial part of its life cycle. This is when it faces the most danger from predators, and when lice infestations can do the most damage.

In wild salmon populations, the scientists found that the generalist C. clemensi is actually far more abundant than the specialist L. salmonis: there were five, seven, and 39 times more of them on pink, chum, and sockeye salmon, respectively.

Brookson says it is not yet clear what drives the differences between how the lice affect the three species of salmon. “There are likely differences between the salmon species with regard to their ability to resist or mediate infection, but it’s not clear how that interacts with other factors like ocean temperature or salinity,” he says.

Unlike L. salmonis, less is known about how C. clemensi affects both farmed and wild fish, as it was only recently that researchers found proof that it actually harms its host. Aquaculture regulations in Canada only require operators to treat their fish for lice once they are above a certain threshold of L. salmonisC. clemensi is not considered at all. “In theory, you could have a pen full of fish that are completely infested with C. clemensi and there is no regulatory requirement for you to treat them,” Brookson says.

Inka Milewski, a biologist at Dalhousie University in Nova Scotia, says the research should be a wake-up call for aquaculture and fisheries managers to change how they think about the health of their fish. “The singular aquaculture management focus on L. salmonis has failed to identify and address management issues relating to other co-occurring sea louse species,” she says. “It highlights and reiterates the need for a shift from single-species [management] to broader ecosystem-based management, whether it’s for fish, sea lice, or even viruses.”

Brookson and his colleagues’ analysis also shows something else important: while most L. salmonis infecting wild salmon was probably picked up from farmed fish, the C. clemensi is coming from elsewhere—likely wild Pacific herring, which the generalist louse also infects. This suggests that even if a L. salmonis outbreak stemming from a salmon farm can be treated and contained, C. clemensi will always be able to bounce back.

By better understanding C. clemensi, scientists will be better able to manage its effects on threatened wild salmon, Brookson says. “We should focus more attention on this species, which dominates the infestation of these fish.”