A review of ecological and behavioural interactions between cultured and wild Atlantic salmon

Cultured Atlantic salmon (Salmo salar L.) may be introduced into natural systems intentionally or accidentally. As smolts or post-smolts, they move to the feeding areas of wild salmon in the North Atlantic Ocean. As maturing fish, they return to the area of release and enter rivers to spawn. Lack of juvenile river experience is the prime reason why cultured salmon often enter fresh water later in the season than wild fish. During spawning, cultured female salmon from fish farms make fewer nests, tend to breed for a shorter period of time, are poorer at nest covering, and retain greater amounts of unspawned eggs than wild females. Cultured male salmon from fish farms exhibit less combat and display behaviour, have greater difficulty in acquiring access to mates, show less quivering and courting behaviour, and have lower reproductive success than wild males. However, cultured male salmon are more involved in prolonged, reciprocal fights than wild males and are, therefore, more often wounded. The reproductive success of cultured salmon increases with the time the fish have lived in nature before maturing sexually; for cultured females released in nature at the smolt stage, reproductive success is similar to that of wild females. The relative reproductive success of cultured males is smaller than that of corresponding females. Within both sexes of cultured and wild salmon, competitive spawning ability increases with body size. As a phenotypic response to increased growth rate during the first year of life, cultured salmon tend to have smaller sized but more numerous eggs than wild fish of the same size. Offspring of cultured salmon are more generally aggressive, more risk prone, and have a higher growth rate than wild offspring. Consequently, their survival rate in nature may be lower.