Farms unfairly blamed for lice levels

Authors of a paper published in the Canadian Journal of Fisheries and Aquatic Sciences have reported that mismanagement of farms contributed to the abnormally high prevalence of Lepeophtheirus salmonis on wild juvenile salmon in BC last year.

Doctors Andrew Bateman and Stephanie Peacock led the study, looking at the level of salmon lice on wild salmon that were sampled in the Broughton Archipelago in the spring of 2015.

According to their data, more than 70% of salmon sampled were infected by the ubiquitous parasite, which were at the highest levels since 2005.

The authors suggest that it was a combination of both environmental conditions and poor farming practices that resulted in higher lice levels.

“It was sort of a perfect storm of environmental conditions and mismanagement of treatment,” said Peacock.

“A lot of people talk about how sea lice are natural, but in farms, you have these parasites in larger numbers. Juvenile wild salmon are then exposed as they migrate past these areas.”

However, in 2013, a brilliant collaborative study published in the journal PLoS One by some of the same authors (Peacock and Krkosek), suggested that treating farmed salmon with SLICE in January or February minimizes the risks parasites on farms may pose to out-migrating juvenile wild salmon.

“Adapting the management of parasites on farmed salmon according to migration of wild salmon may therefore provide a precautionary approach to conserving wild salmon populations in salmon farming regions,” the study concluded.

In fact, the approach was so convincing that the entire industry adopted it, as suggested.

Now, every spring, before wild juveniles are about to migrate to the ocean, all salmon farms treats their fish with SLICE – in a coordinated fashion – to prevent potential transfer of the louse.

However, the same authors suggest that salmon farmers are at fault for mismanaging treatments, and instead propose a treatment in the preceding fall.

“Even though parasite treatments on farms were effective, we saw that treatments failed to protect wild salmon, and this happened at a time of unexpected warming in ocean water in these regions,” said Dr Martin Krkosek, study co-author.

The study shows that coordinated treatments of farms with SLICE in the winter was effective at reducing lice levels on farms. So what’s the problem?

Wild salmon caught by the researchers in the spring had high a prevalence of sea lice, and these were attributed to the farms. Although the authors show no proof of burden that these lice on wild fish were from farms, they make the assumption that they were derived from farms.

Some studies have shown that there are other species of fish that can act as reservoirs for L. salmonis – for example, the 3-spined stickleback. In other studies, researchers have found high levels of L. salmonis in areas lacking farms or wild populations.

The take home message? Researchers don’t fully understand the host-parasite dynamics or relationships between sea lice and salmon.

Why so many lice?

The team looked at four contributing factors to determine the cause for the high lice levels, including numbers of migrating adult salmon, water temperature, resistance of sea lice to treatments, and timing of said treatments.

The fall of 2014 saw a very healthy return of adult pink salmon. Despite their resistance to L. salmonis as juveniles, adult pink salmon are known to support very high levels of salmon lice. Therefore, upon return to near-shore waters, these lice transfer to farmed Atlantic salmon in the area.

This was coupled with abnormally dry and warm conditions throughout the winter of 2014 and into the spring of 2015.

Even thought there was heavy parasite pressure from wild stocks and quicker parasite growth rates associated with higher temperatures, lice collected in their study continued to show sensitivity to SLICE (emamectin benzoate, EMB).

The authors found that “Sea lice collected from wild juvenile salmon were sensitive to EMB, with no evidence of reduced sensitivity in 2015 relative to 2012”.

Which is good news for BC farmers – as the application of pesticides can be associated with many problems, one being the development of resistance the parasite to the drug, such has been observed in Norway, Scotland, Chile and Atlantic Canada.

Among the many factors that contribute to the continued efficacy of EMB against L. salmonis, one of the most important is the minimal usage of the drug on farms. Carefully planned and applied treatments when absolutely necessary ensures the lasting effectiveness of EMB, and follows the models of responsible integrated pest management.

Currently, BC is the only salmon farming location in the world where the treatment remains 100% effective, and to keep it that way the application of EMB has been carefully calculated.

“Although the proportion of treatments that occurred during the 2015 wild salmon out-migration was higher than in non-outbreak years, treatment timing did not contravene license conditions more than in non-outbreak years,” said the authors.

“This points to factors other than changes in compliance with license conditions driving the 2015 outbreak,” the authors concluded.

Although treatments on the farms worked when they should have, the authors concluded that a treatment in the late fall would be better for lowering lice levels on farms.

However, treating their fish for sea lice prior to an influx of migrating adult wild salmon bringing in millions of sea lice into near-shore waters would be illogical and redundant – as second treatment, again in the winter, would still be required prior to out-migration in the spring.

Open to discussion

“This latest study confirms what researchers reported last year when monitoring sea lice levels on wild fish in many areas of the coast near and far from salmon farms. That is, rising water temperatures in 2015 and a higher-than-normal return of adult wild salmon the previous fall were excellent conditions for the proliferation of the common sea louse,” said Jeremy Dunn, Executive Director of the BCSFA.

“We are pleased that the research team has confirmed that management of sea lice abundance on our farms is generally effective. We will review this study’s recommendations to see if we can continue to improve our management strategies that can effectively manage our farms during the years we see high levels of sea lice naturally occur in the ocean,” he added.

“We have also invited Dr Krkosek to speak at our next Collaborations on the Coast workshop coming up this February,” Dunn concluded.