**5. Social behaviour**

Social behaviour is particularly developed in fishes, such as shoal [51], which is a part of the social life and is present in more than 25,000 species [52]. Shoal is important and ensures protection against a potential predator (a particular prey is undetectable in the group), but also it increases the foraging efficiency (the amount of food per individual is higher in groups than for solitary fishes whatever their diet). Shoal—defined as a group of individuals [51]—may be influenced by environmental factors, and domestication is one of these factors; reared conditions modify the fish environment. It limits the available space for fishes that could have for consequences a non-response of the fishes to environmental stimuli [53]; in reared conditions, food is distributed *ad libitum*, and such situation modifies the foraging behaviour limiting the exploration of the environment [54] and the predator avoidance [12, 32, 55]. In domesticated fishes, there is less variability of the age and size of the individuals, and so, the relations between fishes are modified and the results are counterbalanced; in some studies, they show that there is an increase of the aggressiveness between individuals [56, 57], and in other studies, they find that the aggressiveness is higher in domesticated populations [55, 58]. Growth in rearing situations is influenced by intra-specific competition [59, 60].

One of the most important components of the social relations between individuals is the agonistic behaviour. Comparisons between wild and reared fishes show that new agonistic behaviours do not appear due to domestication [61]; agonistic behaviours are the same for both wild-reared individuals. In general, agonistic behaviours appear for the competition for resources: prediction is that agonistic behaviours must be less numerous when the quantity of resources increases. Domestication introduces the selection of individuals with a rapid growth; the consequences on the level of agonistic behaviours between individuals inside the groups are very dependent of the situation. Globally, it has been demonstrated that an effect on agonistic behaviours exists [62]. Agonistic behaviour can increase for domesticated fishes [58, 63, 64] or decrease [56] or be stable [57]. For example, the brown trout sea-ranched individuals have a higher growth rate and have no difference of activity with wild animals, but intensity of agonistic behaviours was higher in wild individuals [65]. These results could be interpreted as a consequence of the rearing conditions; in wild populations, agonistic behaviour has a function for space sharing, food accessibility [66], foraging efficiency and predator avoidance [67, 68]. So selection in rearing conditions leads to the individuals that have the most rapid growth but with particular behavioural traits (i.e. the most aggressive fishes); it is a known phenomenon, analysed as phenotypic selection (or economic selection by culturists) [69]. This implies that fishes are selected on their size and growth rate, and the dominance effect, which could be the result of competitive relationships, disappears if we introduce the size as variable [23]. But the dominance depends on the environment; this could be linked to the residence effect, which exists in wild fishes and not in reared ones [70]. In any case, competitive behaviours are the same; they vary in quality and intensity between wild and reared fishes [71]; for example, the high density for reared fishes in tanks could induce less territoriality and so a lower aggressiveness during dyadic confrontations [70, 72]. Competition and dominance have been tested in the salmon (*Oncorhynchus tshawytscha*) and the results showed that wild fishes were more aggressive than fishes from the first generation (F1) reared in aquaculture [73]. In general, the consequence of dominance is better growth rates for the dominant individuals whatever their origin (wild or reared). More recently, a relationship was found on the influence of domestication on brain size and aggressive behavioural changes. A study on rainbow trout lines highlighted that some behaviours such as 'freeze' and 'escape' are associated with a high level of domestication instead of 'display' and 'yawn' behaviours, which are linked to wild lines [74]. Moreover, these authors found that the total brain size and olfactory volume were associated with domestication.

An important consequence of the level of aggressiveness between individuals is the existence of cannibalism [75]. It could appear either within the same cohort or between different cohorts. Cannibalism is a natural phenomenon, which is for regulating natural populations in many fish species. In cultured fishes, cannibalism has a negative effect on the populations; some individuals switch from food given by humans to the attacks and consumption of conspecifics.
