The production of farmed Atlantic salmon has increased dramatically over recent decades and now dwarfs the catch of wild salmon in the North Atlantic. This increase in production has resulted in interactions between farmed and wild stocks, particularly in areas of intensive salmon farming activity, giving rise to concerns about the extent of the possible impacts.
ICES has been asked by NASCO (the North Atlantic Salmon Conservation Organization) to advise on the 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 advice will help inform a one-day, special theme session on this topic due to take place during the NASCO annual meeting in June. To develop the advice, ICES convened a workshop – Workshop on possible effects of salmonid aquaculture on wild Atlantic salmon populations in the North Atlantic (WKCULEF) - to review the latest available scientific evidence. The workshop brought together experts in aquaculture, parasites and diseases, genetics and wild Atlantic salmon to ensure appropriate representation. The ICES advice is published today.
A key concern associated with salmon farming relates to the effect of sea lice, an external parasite of salmonid fish. Sea lice can proliferate in cages of farmed salmon and affect the health and condition of farmed fish. There are various measures used to regulate sea lice numbers on salmon farms, but keeping levels low remains an ongoing challenge for the industry. Salmon farming has also been shown to increase the abundance of sea lice in the marine environment and to increase the risk of infection among wild salmon populations in affected areas, although there is considerable variability in the extent of such areas and from year to year.
Laboratory studies have indicated that sea lice can have detrimental impacts on salmon (and other salmonid species) and result in the mortality of fish above certain levels of infestation, giving rise to concerns about the effect this might have on wild populations. In practice, numerous factors and ecological processes will affect the survival of salmon during the marine phase of their life-cycle, and teasing apart the contribution of one specific stressor is problematic. Nonetheless, a number of investigations have explored the population-level impacts of sea lice infestation through studies involving the paired release of salmon smolts treated to provide short-term resistance to lice infestation and control groups. All these studies have reported an improved return rate for treated versus control salmon, although all showed significant spatial and temporal variability between individual trials.
One of the main concerns associated with salmon farming relates to the potential impacts of farmed salmon on wild populations is that posed by farm escapees. Farmed salmon are domesticated and display substantial differences from wild salmon in a wide range of fitness-related traits. Very large numbers of salmon escape from fish farms each year. The subsequent fate of many of these fish is unclear and the majority of escapees are never seen again. However, some enter rivers and farm escapees have been observed in rivers in all regions where farming occurs, contributing up to 50% of the spawning population in some rivers in some years.
The spawning success of escaped farmed salmon is much lower than in wild salmon, but despite this many wild salmon populations in salmon farming areas exhibit widespread introgression of genes from farmed salmon. Such introgression can reduce the viability of the populations in rivers through introducing maladaptive changes in life-history traits. The presence of farmed salmon and their offspring in a river can also result in decreased overall productivity of the wild population through competition for territory and food. In the long term, the introgression of farmed salmon is expected to lead to erosion of genetic diversity in wild populations and therefore to the decreased resilience of stocks.
ICES has also released advice today on Atlantic salmon at West Greenland, from North America, and the Northeast Atlantic.