The production of salmon in the north Atlantic reached 700.000 tonnes last year. These fish are kept in cages and despite numerous precautions remain very vulnerable. Some fish inevitably escape and mix with wild salmon populations. What are the consequences of this?

The production of salmon in aquaculture reaches 700.000 tonnes per year in the north Atlantic alone. Nearly all of these fish are farmed in cages and despite surveillance it is inevitable that some fish will escape. This happens after storms or attacks from predators or following damage to nets and installations. The most serious incident happened in the spring of 2002 in the Faroe islands when 600.000 fish were accidentally released as a result of a storm. It is estimated that every year approximately 2 million salmon escape from fish farms. This figure represents 50% of wild salmon stocks in the Atlantic. These farmed fish carry on their life in the sea and when they reach maturity they turn up in estuaries with the wild salmon which presents possible genetic problems. This possibility constitutes yet another threat to salmon populations which are already in difficulty.

During a 10-year study the Marine Institute and the University of Belfast have monitored first and second generation hybrids between wild fish and farmed fish during both freshwater and marine phases. The percentage of fertilized eggs (eye egg stage) has been determined together with the survival rate of young parrs and smolts. The eggs from each family have been put in incubators under the same conditions and then released. Genetic profiling has permitted the study of the returning adult fish caught in nets, with a line or in traps on dams.


This experiment reveals that farmed salmon have a genetic influence and compete with wild populations. After several generations in fish farms survival capacity in the marine environment decreases considerably. Overall, fish from farmed stock do not exceed 2% of survivals of natural salmon (study of 3 groups). Hybrids give intermediate results : from 27 to 89% compared with wild fish. It is interesting to know what becomes of second generation hybrids : 70% of the eggs die during the first weeks of incubation! Is there a genetic problem which leads to such heavy losses? Scientists think there is.
Young salmon from farmed stock present faster growth than hybrids. This seems logical as selection took place on growth criteria. They also displace the wild fish forcing them to go off to less favourable zones. Over several generations the cumulative effect is a scarcity of wild fish.

Voluntary releases of smolts produced in too large numbers (in Scotland in particular) to strengthen the natural populations end up having the opposite effect, that is to say, the deterioration of living conditions for juveniles and scarcity of wild fish. The situation is the same for common trout bred in the sea although there is a tendency towards the production of sterile fish. Philip McGinnity and Andy Ferguson conclude that it is high time that specific regulations on salmon and sea-trout farming were introduced.