Not guilty?
The latest research from Canada suggests that salmon farms are probably not the main source of lice infestation for wild fish
Salmon farms are not responsible for spreading sea lice among the wild fish population, according to a new report from the Canadian government’s scientific advisors. British Columbia’s farmers have welcomed the findings. The peer-reviewed Science Response Report, Association between sea lice from Atlantic salmon farms and sea lice infestations on wild juvenile Pacific salmon in British Columbia, was published on the Department of Fisheries and Oceans Canada’s (DFO) website by the Canadian Science Advisory Secretariat (CSAS).
The aim of the study was:
1. to estimate the number of Lepeophtheirus salmonis copepodids (the infective sea lice later larval stage, following the earlier nauplii stage) produced by Atlantic salmon farms in British Columbia;
2. to summarise counts of sea lice numbers on wild juvenile Pacific Salmon; and
3. to determine the statistical link between sea lice infestations on Atlantic salmon farms and prevalence on wild juvenile Pacific salmon populations in British Columbia.
The CSAS study made use of data gathered from a number of sources, including the lice counts that salmon farmers are required to carry out, and surveys of juvenile wild salmon.
The study also arrived at an estimate for the number of copepodids generated by salmon farms, based on the number of observed adult females and a modelled estimate of the copepodids produced by those females, based on published, peer-reviewed modelling approaches.
The researchers focused on four regions of the British Columbia coast where wild and farmed salmon could be expected to interact: Clayoquot Sound, Quatsino Sound, the Discovery islands and the Broughton Archipelago.
Following analysis, the report says: “We saw a positive but statistically insignificant association in all four regions studied.”
The salmon farmers generally apply their most intensive lice management measures prior to the out-migration window, that is the season in which wild juveniles swim out from fresh water rivers into the sea, when they are considered most at risk from infestation by lice.
Perhaps not surprisingly, then, the
CSAS study also finds: “The number of Lepeophtheirus salmonis adult females on Atlantic salmon farms and the corresponding
” No statistically significant association was observed
number of copepodids released from
Atlantic salmon farms were lowest during the out-migration window compared to the rest of the year across all Fish Health Surveillance Zones.”
Crucially, the study concludes: “No statistically significant association was observed between infestation pressure attributable to Atlantic salmon farms and the probability of
L. salmonis infestations on wild juvenile chum and pink salmon in Clayoquot Sound, Quatsino Sound, Discovery Islands, and Broughton Archipelago. However, the data suggests a positive trend in all studied areas. The lack of statistical significance implies that the occurrence of L. salmonis infestation on wild migrating juvenile Pacific salmon cannot be explained solely by infestation pressure from farm-sourced copepodids.”
The report also concludes, however, that sea lice numbers are on the rise in all the areas studied.
The authors also say that further work is required to refine monitoring of sea lice activities and to improve modelling, for a more accurate risk assessment.
Where does this leave policy?
Brian Kingzett, Executive Director of the BC Salmon Farmers Association (BCSFA), comments: “As a sector, we continue to improve and innovate to ensure that our operations have minimal impact on the surrounding ecosystem. As farmers, and as British Columbians, we care about wild salmon and agree wholeheartedly with DFO [Department of Fisheries and Oceans] Minister Joyce Murray regarding ensuring the protection of wild Pacific salmon.”
In 2020, Joyce Murray’s predecessor as Fisheries Minister, Bernadette Jordan, ordered the closure of all net-pen fish farms in the Discovery Islands region of British Columbia, and Canada’s federal government is consulting on a “transition plan” to end all netpen farming by the end of 2024.
The argument that salmon farms endanger wild fish by allowing sea lice numbers to multiply is one of the principal reasons given by campaigners for shutting down the net-pen sector. It is also a key element in regulators’ plans aimed at minimising the impact of farming on wild fish, such as Norway’s “traffic light” system and the
spatial plan being mooted by the Scottish Environment Protection Agency.
While the argument for a precautionary principle is still strong, the latest study from Canada implies that the risk to wild juveniles from sea lice does not come primarily from fish farms acting as a “reservoir” of lice.
Lice life cycle
So where do the lice come from? Earlier last year, Jim Powell, Director with Fidelis Aquaculture Management, presented findings from another study at the International Sea Lice Conference, held in the Faroes.
His talk, titled “Are farmed fish the progenitors of lice on wild Pacific salmon?” explained the aims of the research and set out its findings.
The purpose was to test a widely accepted paradigm for sea lice infestation:
1. wild salmon adults return bearing lice;
2. these infect salmon in cages;
3. lice incubate and multiply over the winter; and
4. smolts passing by farms on their way out to sea are infected, and the cycle starts over again.
The research was supported by the Wei Wei Kum First Nations, Grieg Seafood and Cermaq, in terms of providing resources and data, but none of the supporting organisations had any editorial control.
Powell began by looking at data from “pristine” areas of the British Columbian coast, where there are wild salmon but no fish farms.
From this he was able to estimate a “baseline” level of infestation at around 0.2 adult lice per fish – in other words, this was the typical level for infestation for wild fish in the absence of farms.
He also looked at data from farm lice counts, at other potential contributory factors and at the sea lice life cycle (the stage of development at which the lice were observed).
In most regions studies, lice counts were higher among the farmed salmon than among the wild salmon., but there were very significant shifts in lice counts between one year and the next in some regions.
The lice counts for wild fish in a number of regions did not vary greatly, however, even in years when the farms in the region were fallow and there were no farmed fish in cages.
Powell said: “We found no clear pattern of association.”
Significantly, periods where there were higher levels of lice on farmed fish did not translate to higher levels of lice found on wild fish.
Also, the sea lice life cycle analysis showed few motile lice on the wild fish, pointing to a winter infection period for all the fish as opposed to a model in which the farms act as a reservoir over winter and then reinfect wild fish in spring.
Powell concluded that farms are an unlikely source of nauplii, the free-swimming larval stage. Also, his best estimate of the time juveniles took to reach the open sea suggested that each individual would be likely to spend only a matter of minutes in the close vicinity of any given farm.
His analysis of wild fish lice counts in the Discovery Islands region, before and after farms were stocked in 2019, and before and after they were destocked in 2020, found very little difference. For example, the average count in 2020 while the farms were active was 0.51; after the farms closed it was 0.54.
Powell argued that the source of infection must be external to the farms, because after treatment the adult lice were eliminated – but reinfection began with more adult lice.
The likely suspects are wild fish that do not pass in between salt and fresh water in the way salmon do, such as Pacific sand lance, white sturgeon or ling cod, all of which can act as hosts for sea lice.
He concluded: “When motile lice numbers on farmed salmon are low during outmigration, farms are improbable as a source of lice transfer, or as a source of a generation of nauplii.
“There is an external reservoir of adult lice that is not completely generated by farmed salmon.”
Powell noted, however, that sea lice management practices still need to be improved.
None of the findings absolve farmers from the responsibility to manage sea lice, but they do suggest that the threats to wild salmon cannot simply be dealt with by reducing, relocating or banning fish farms.
FURTHER INFORMATION
Association between sea lice from Atlantic salmon farms and sea lice infestations on wild juvenile Pacific salmon in British Columbia, Canadian Science Advisory Secretariat
(CSAS) can be downloaded as a PDF from bit.ly/CSAS-sealice2023
” There is an external reservoir of adult lice that is not completely generated by farmed salmon