**3. Foraging behaviour**

Foraging is not only the activity, which consists to take off resources in the environment, that is, prey, but also the choice of the best site or the most favourable period where and when to forage. The animal must be at the good place at the best moment. This aim seems easy for animals in controlled environments where the food is abundant and regular; but this fact could be a disadvantage when aquaculture-reared fish are released in natural environment in order to supply the low level of the wild stocks.

Fishes change their foraging habits with domestication. Zebra fish (*Danio rerio*) and coho salmon (*Oncorhynchus kisutch*) change the place where they forage after domestication after just one generation. Domestic fishes swim at the surface of the water column instead of the lower part for wild animals [19, 20]. One of the consequences is that farmed animals had a higher rate of prey capture than their wild congeners [21, 22]. These changes in foraging behaviour could be the result of changes in the relation of the fishes with its environment: as the predation rate was lower for farmed fishes, they adopt a more risky behaviour near the surface; the farmed conditions modified also the social relationships between individuals and could result in a lower influence of dominance in the foraging behaviour [23].

Perhaps, the main difference is that the natural environment provides a lot of different situations to which fishes have to adapt. It seems that the environmental complexity of natural environments may facilitate training to different situations [24], with a more important prey variability [25–27] or opportunity of social learning [28]. Consequences could be measured when farmed fishes were realised into natural environment: they use less of natural objects such as stones or leaves for digestion than wild animals [25] or they make no difference between prey of different profitability [26] and they do not choice an unknown prey [27].

The conditions of foraging allow the fish to get a certain amount of resources from the environment and could explain important differences between hatchery-reared and wild individuals in terms of survival and growth. If we compare the survival rate of aquaculturereared or wild Chinook salmon fry (*Oncorhynchus tshawytscha*) facing predation by rainbow trout (*Oncorhynchus mykiss*) or sculpin (*Cottus rhotheus*) under experimental conditions, wild fry had a survival advantage within the two next years of experiment [29]. So it is possible that the domestication can affect the vulnerability of juveniles of salmon after only one generation in a culture system. But it is not always the case. For example, the survival of Atlantic salmon (*Salmo salar)* in the Baltic Sea was examined in relation to the origin, and prey fish abundance (here herring *Clupea harengus* and sprat *Sprattus sprattus*). The study was based on recapture data for tagged hatchery-reared, and wild smolts demonstrated a combined influence of origin and environmental factors on survival; prey fish abundance had no influence on the survival of reared or wild smolt groups [30]. The results suggest that some larger smolt of the reared groups compared with the wild groups compensated for their lower ability to live in the wild.
