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Has removal of fish farms decreased sea lice?

It’s premature for either activists or industry to draw conclusions, marine parasite expert says
Mainstream Biological Consulting collects juvenile salmon as part of sea lice surveys. | Submitted

For several years now, as part of certification with the Aquaculture Stewardship Council (ASC), salmon farmers in B.C. have contracted independent researchers to conduct annual sea lice counts on wild juvenile salmon. 

The Salmon Coast Field Station has also conducted sea lice counts on wild juvenile salmon in an area of the Broughton Archipelago where some salmon farms have been removed.

As long as the annual counting continues for several years after salmon farms have been removed from areas like the Discovery Islands and Broughton Archipelago, it could provide a good control study that might help scientists determine the extent of the impact salmon farms may be having on wild salmon in terms of sea lice outbreaks.

But anti-fish farms lobbyists are already claiming success, saying the removal of salmon farms from the Broughton Archipelago and Discovery Islands have had an immediate positive impact in the form of low sea lice levels on juvenile salmon. 

Since 2018, 13 salmon farms have been removed from the Broughton Archipelago region (with six remaining), and 11 farms have been removed from the Discovery Islands region, which is now totally free of salmon farms.

“The results are in,” Wild First says on its website. “There was such an immediate reaction; when the farms are removed, the fish are clean right away. 

 “Closures to date have shown positive impacts on the survival of wild salmon.”

Mowi Canada says publicly available data on sea lice counts on wild juvenile salmon for the Broughton Archipelago and Discovery Islands doesn’t support that claim.

“We have collected long-term data … for juvenile sea lice prevalence in all regions – including Discovery and Broughton – which shows neither claim is supported by the data,” said Mowi Canada director of communications Ian Roberts.

He added that the independent Salmon Coast Field Station has reported similar findings.

Lance Stewardson, a biologist at Mainstream Biological Consulting, has been doing sea lice monitoring on wild salmon since 2004, mostly on contract with the salmon farm industry as part of their licensing and ASC certification.

He has conducted sea lice counts on juvenile wild salmon in the Discovery Islands region since 2017. He said researchers look for two things: prevalence and intensity (sea lice per fish). There can be cases where sea lice are highly prevalent, but low in intensity, and vice versa.

In the Discovery Islands, he said the prevalence was relatively low in 2017 and 2018 – 8% and 4% respectively, and then very high in 2019 and 2020 – 29% and 21% respectively. After fish farms were removed, prevalence was 11% in 2021 and 9% in 2022.

So the prevalence did drop after salmon farms were removed, but were higher than in 2017 and 2018. The intensities did not vary much.

“We have two years of monitoring with farms active in the area that have infestation lower than the two years with farms removed,” Stewardson said.

“The two years with farms removed aren’t the lowest and they are not the highest of the six years that we have monitored. They are the middle, as far as infestation, prevalence and intensity in the areas with farms removed on pinks and chum salmon.”

The Salmon Coast Field Station also has published annual data on sea lice counts on wild juvenile salmon in an area of the Broughton Archipelago where one salmon farm was removed in 2019 and another in 2020.

In 2018, the year before the Glacier Falls salmon farm was removed, sea lice numbers were low relative to the previous three years. In 2019, the year the Glacier Falls salmon farm had been removed, sea lice numbers jumped.

“Sea louse numbers in 2019 were at their second highest levels since 2005 and are similar to numbers found in 2015,” the organization reported.

In 2020, after a second farm had been removed, the numbers declined to levels similar to 2018 and the period between 2006 and 2014.

In 2021, two years after the first farm was removed, and one year after the second was removed, sea lice numbers “were comparable to levels over the last five years, and higher than the period 2008-2014.”

"The long-term data suggests that, regardless of farm status, sea lice infestation rates on wild juvenile salmon fluctuate annually,” Stewardson said.

Simon Jones, a finfish parasite expert at Fisheries and Oceans Canada’s Pacific Biological Station, said sea lice naturally fluctuate in prevalence and intensity due to things like water temperature and salinity, and natural abundance of wild fish.

He said there’s no question that salmon farms can amplify sea lice infestations, which is why salmon farmers are required to treat sea lice in salmon farms.

“There are quite a few studies from 2010, early teens, that did suggest that, when treatments are effective, that we see lower levels of lice on the farms and on wild migrating salmon,” he said.

“But then you get issues like, are the lice becoming resistant to the treatments? And that certainly is happening in different parts of the world. So it’s a constant need for evolution in terms for new treatments, new ways and strategies for applying those treatments.”

Jones said it would be premature for either side to draw conclusions yet on sea lice levels on wild salmon in areas where salmon farms have been removed.

From 2004 to 2011, DFO did annual surveys on sea lice on wild pink and chum salmon in the Broughton Archipelago, where salmon farms were heavily concentrated.

“One of the things that was really obvious, right from the get-go, was just how different the levels of lice were from year to year on the wild fish,” Jones said. “We know that the farms are part of the lice equation. They have to be. You know it’s part of the story, but there’s an awful lot going on that’s part of the story as well.”

To draw any firm conclusions about the degree of impact of salmon farms on wild salmon, in terms of sea lice, long-range control studies are needed in areas where salmon farms have been removed, Stewardson said.

“We need to do a control where there’s no farms, but we have to do it in the same time frame and same parameter and over the same period so we can compare data,” he said.

Jones agrees: “We need multiple years of data to really believe that we could see a signal, if there is going to be a signal. Just being able to make a conclusion after one year or a small number of years of observation is very premature.”

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