Workshop to address the NASCO request for advice on possible effects of salmonid aquaculture on wild Atlantic salmon populations in the North Atlantic [WKCULEF], Copenhagen, Denmark, 1–3 March 2016.
Chairs: Ian Russell (UK) and Ole Torrissen (Norway).
Number of meeting participants: 25 representing six countries: Norway (ten), Ireland (four), UK (Scotland) (four), Canada (three), UK (England & Wales) (two) and USA (one). Additional participants also attended from the ICES Secretariat.
WKCULEF met to consider a question posed to ICES by the North Atlantic Salmon Conservation Organisation (NASCO): Advise on possible effects of salmonid aquaculture on wild Atlantic salmon populations focusing on the effects of sea lice, genetic interactions and the impact on wild salmon production.
This question was originally included among a suite of questions developed by NASCO, and due to be addressed by the annual meeting of the Working Group on North Atlantic Salmon (WGNAS). However, given that the question was pertinent to other Expert Groups at ICES, particularly the Working Group on Aquaculture (WGAQUA), the Working Group on Pathology and Diseases of Marine Organisms (WGPDMO) and the Working Group on the Application of Genetics in Fisheries and Mariculture (WGAGFM), it was recommended that the question would be best addressed by means of a Workshop, independent of the Working Groups. WKCULEF enabled experts in aquaculture effects, wild Atlantic salmon, disease transmission and genetic interaction to share and discuss relevant information and recent findings, in order to meet the objectives and timeline of the request.
The terms of reference were addressed though a comprehensive review of the recent peer-reviewed literature. This was facilitated by a range of presentations from participants, by reviewing working documents prepared ahead of the meeting as well as the development of documents and text for the report during the meeting. The report is structured in two main sections, one focusing on the effects of sea lice and the other on genetic interactions. The third issue specified in the question from NASCO, namely the impact of salmon farming on wild salmon production, has been relatively poorly researched and most information derives from attempts to evaluate population level effects related to sea lice infestation and genetic introgression. This information has therefore been reported in the sea lice and genetics sections of the report, respectively.
WKCULEF briefly discussed microbial diseases in aquaculture and the potential impact on wild salmon. However, it was not possible to review this issue in detail and it has not been included in this report.
The key findings of the Workshop were:
• The sea louse (Lepeophtheirus salmonis) has widespread geographic distribution, is an important parasite of salmonids and has been a serious problem for the Atlantic salmon farming industry since the 1970s. Sea lice have a greater economic impact on the industry than any other parasite and control of lice levels on farms is of key importance.
• Salmon farming has been shown to increase the abundance of lice in the marine environment and the risk of infection among wild salmonid populations. However, there is considerable uncertainty, and spatial and temporal variability, about the extent of the zones of elevated risk.
• It has been shown in laboratory studies that 0.04–0.15 lice per gramme fish weight can increase stress levels. Laboratory studies have also demonstrated that infections of 0.75 lice per gramme fish weight, or approximately eleven sea lice per fish, can kill a recently emigrated wild salmon smolt of about 15 g if all the sea lice develop into pre-adult and adult stages.
• A number of studies in Norway and Ireland have estimated the relative marine survival of smolts treated to provide lice resistance and control groups. All studies have reported an overall improved return rate for treated salmon, but all showed significant spatial and temporal variability in the magnitude of the treatment effect.
• The survival of Atlantic salmon during their marine phase has fallen in recent decades. This downturn in survival is evident over a broad geographical area and is associated with large-scale oceanographic changes. Viewed against current marine mortality rates commonly at or above 95%, the ‘additional’ mortality attributable to sea lice has been estimated at around 1%.
• In some studies, the impact of sea lice has also been estimated as losses of returning adult salmon to rivers. These estimates indicate marked variability, with losses in individual experiments ranging from 0.6% to 39%. These results suggest that sea lice induced mortality has an impact on Atlantic salmon returns, which may influence the achievement of conservation requirements for affected stocks.
• Much of the heterogeneity among trials comparing the survival to adulthood of juvenile salmon administered sea lice medicines and control groups could be explained by the release location, time period and baseline (i.e. marine) survival. In a recent meta-analysis of Norwegian data, baseline survival was reported to be the most important predictor variable. When this was low, the effect of treatment was high. In contrast, when baseline survival was high, the effect of treatment was undetectable. However, it is unclear whether baseline survival is affected by sea lice exposure.
• Each year, large numbers of domesticated salmon escape from commercial fish farms. While many of these are reported, the true number of escapees is likely to be significantly higher. Escapees are observed in rivers in all regions where farming occurs, although the numbers of escapees vary both spatially and temporally. It has been noted that in some rivers in some years, the numbers of escapees have approached 50% or more of the spawning population.
• The spawning success of escaped farmed salmon is much lower than wild salmon. Despite this, genetic studies have demonstrated that farmed salmon have displayed widespread introgression in a large number of Norwegian populations where this has been investigated. Introgression has also been shown in other countries, but the full extent of introgression remains to be investigated.
• Farmed salmon are domesticated and display significant genetic differences to wild salmon in a wide range of fitness-related traits. Whole-river experimental studies have demonstrated that the offspring of farmed and cultured salmon in general, display lower fitness than their wild counterparts in the wild.
• Juvenile escapees and the offspring of farmed salmon compete with wild salmon for territory and food. Therefore, their presence in the natural habitat will reduce the total production of wild fish. Studies have also shown this can result in a decreased overall productivity of the population.
• Where farmed salmon have successfully interbred with natural populations, it is likely that recipient populations will display changes in life-history traits. These changes are likely to be maladaptive for the wild population.
• The long-term consequences of introgression across river stocks can be expected to lead to reduced productivity and decreased resilience to future impacts such as climate change (i.e. less fish and more fragile stocks).
• The evidence from studies in the wild, and the extensive literature relating to salmonids in general, demonstrates that the offspring of farmed salmon display reduced fitness in the wild. However, the results of these studies suggest that the relative success of farmed salmon and, likewise, the relative potential negative effect on a native population, is likely to vary in time and space. Wild populations that are already under evolutionary strain from other challenges such as disease pressure, sea lice infection, over exploitation, habitat destruction and poor water quality are more likely to be sensitive to the potential negative effects of genetic introgression and loss of fitness. Therefore, such effects have to be seen in the context of other challenges.
• While recognising that there were still uncertainties, WKCULEF considered that the evidence relating to the impacts of escapees / genetic introgression provided a clear indication of impacts on wild salmon populations. A substantial reduction of escaped farmed salmon in the wild, or sterilization of farmed salmon, would be required in order to minimize effects on native populations.
In reviewing the latest evidence pertaining to sea lice and genetic interactions, WKCULEF considered where there were gaps in current knowledge and identified areas for further investigation.