The North Atlantic Salmon Conservation Organization (NASCO) has asked ICES to the identify the possible effects of salmonid aquaculture on wild Atlantic salmon populations. NASCO has specifially asked ICES to focus on three main areas: the effects of sea lice, genetic interactions, and the impact on wild salmon production.
This week, a special workshop (WKCULEF) to discuss these issues has been taking place in Charlottelund, Denmark.
Salmon aquaculture in the North Atlantic currently produces over 1.5 million tonnes and dwarfs the catch of wild fish. Aquaculture technology and fish-farm safety have significantly increased over the past decade or more, however salmon still escape into the wild. Historically escapes have ranged from large obvious events to the small and unnoticed, with the great majority of escapees disappearing, never to be seen again. Their probability of surviving to adulthood and maturity is slim, but this varies widely and genetics has shown some escapees do enter rivers and successfully spawn with native populations. The effects of this genetic introgression are not entirely clear. Natural selection is likely to enable some recovery back to "original" wild genotype over repeated generations, however, full return to original genotype does not appear to be probable and this may be compounded by repeated escape events.
Aquaculture production has also been affected by sea-lice ectoparasites (Lepeophtheirus and Caligus species) and this has had implications for wild populations. Management and technology developments have gone some way in reducing outbreaks, but uncertainties remain as to the extent to which wild stocks are impacted. Wider marine mortality is still seen as the largest loss to returning wild salmon populations, however parasitisation is part of this and estimates of impacts vary widely. WKCULEF have managed to review current literature with the aim of reporting objectively and removing some of the ambiguity arising from differences in reporting and analyses.
The further question of
effects on wild productivity is equally difficult to elucidate. As with introgression and lice-induced
mortality, estimates and associated variability vary across the natal range of
Atlantic salmon. Generally, returns of
salmon to home waters are declining and to separate out the causes of this is
no simple task. Clearly further work is necessary
on these issues, though as the workshop concludes, all contributors feel the
workshop has successfully framed the present situation and knowledge.
ICES releases advice on possible effects of salmonid aquaculture to NASCO on 6 May.
WKCULEF participants at Charlottelund Castle.