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Smolt Treatment & Returns Study

Understanding the risk posed to wild salmon from sea lice infestation is dependent on understanding the effects that different numbers of attached lice have on salmon, at the individual level and at the population level.

Salmon Smolt Treatment-Returns Study

Scottish Aquaculture Research Forum

SARF SP010: Piloting a network for determining spatial and temporal variation in marine survival of Atlantic salmon and effects of anti-sea lice agents

This three year pilot project (2015-2018) was commissioned by the Scottish Aquaculture Research Forum (SARF) and the experimental work and analyses were undertaken by Marine Scotland Science. The work was funded by SARF (50%) and by the Scottish Salmon Producers Organisation (SSPO) (50%). 

This project is now in its final stages and an interim final report has been published on the SARF website.


Salmon aquaculture can result in elevated numbers of sea lice in open water and hence is likely to increase the infestation potential on wild salmonids. This in turn could have an adverse effect on populations of wild salmonids in some circumstances. The magnitude of any such impact in relation to overall mortality levels is not known for Scotland.

Information on how small or large this impact might be is important, so that the risk to wild fish can be assessed, and considered as to support the sustainable growth of the salmon aquaculture industry.

Quantifying the actual level of impact from sea lice on wild fish populations is difficult.

A number of data sets exist, such as salmon rod catches and counter data, which can be examined to look for associations between returning adult salmon numbers to rivers, and the presence of  fish farms. Such data can reveal correlative trends that may be suggestive of relationships between salmon farming and wild salmon stocks. However, this type of data cannot specifically quantify an effect to sea lice given that there are many other variables which affect wild salmon stocks. Similarly if changes occur in these additional variables that affect the overall returning numbers of salmon, then the real impact of sea lice may be disguised. Ideally each variable that may influence marine mortality of salmon should be controlled and examined individually to identify its effect. For most variables this is not possible.

To examine the effect of sea lice present in the coastal environment on wild salmon returns, experimental treatments have been carried out in Norway and Ireland where migrating salmon smolts, half of which have been treated with an anti-sea lice medicine, are released and returning numbers of adults in each group are compared. Therefore, the only difference in the variables that the fish experience will be that half are exposed to a treatment to protect from sea lice infection on their outward migration.

The treatment recapture approach needs to be carried out over a number of years to account for variability to obtain a robust estimate of impact. This can be relatively expensive in terms of  the logistics, equipment, and labour involved in catching, tagging and treating smolts, and then the identification of returning adults.

Objective of Project

The objective of the SARF SP010 project was to investigate the feasibility of using the aforementioned treatment release methodology in Scotland, and to establish some initial experimental sites which would be incorporated into a wider network, should the approach be adopted. A network of sites is necessary to relate spatial variations in estimated mortality due to sea lice to distributions of salmon farms.

Nearly all other treatment release studies in Norway and Ireland have used ranched or hatchery reared fish. Expert comment on these studies has highlighted that such fish may not be representative of wild salmon in terms of sea lice impact. In addition, use of ranched or hatchery-reared fish may risk genetic introgression with native wild populations in some situations, in an expanded network. Therefore the SARF SP010 project exclusively used wild salmon smolt populations for tagging and treatment. The project established a site on the west coast of Scotland, within the aquaculture zone, and a site on the east coast of Scotland away from aquaculture sites.

Outcome of Project

The project has established methodology, including refinement of trapping methods for application to Scottish west coast rivers, and has made estimates of marine survival of wild salmon on selected sites on east and west coasts.

This pilot project has suggested a very low level of marine survival of wild salmon on two tributaries of the River Lochy (the west coast study sites) during 2016 and 2017. These results, together with recent trends in rod catches, leads to a conclusion that the approach (smolt capture, tagging, anti-sea lice treatment of 50% and identification of adult returners), should not be pursued for assessing impacts of sea lice from aquaculture in Scotland at present. This is because adult salmon return rates appear now to be too low to deliver conclusive results. An alternative approach, similar to that used in Norway and Ireland, of using reared salmon would be difficult to justify on the basis of high cost, lower experimental rigour, and in some circumstances a risk of genetic introgression. There may be potential to review options and reconsider an experimental approach to assessment of impacts of sea lice if overall levels of marine survival of wild salmon increase.

Once established, the Aquaculture and Wild Fisheries Interactions Group will consider next steps.