LAUSR

Abstract In intensive aquaculture, aggression among fish creates a disturbed social environment and leads to serious implications. Food is a resource of great importance for fish and its acquisition is an important enough reason for increased competition. Gilthead seabream Sparus aurata and European seabass Dicentrarchus labrax are the two main reared species of the Mediterranean aquaculture and known to exhibit aggressive behaviour. The present study aimed at investigating the effects of food quantity (low and high feeding ration, i.e. 1% and 3% of body mass) and method of food distribution (localised and dispersed) in the aggressive behaviour of these two species (mean initial body mass 39 g), before, during and after feeding. Analyses of data obtained showed that: a) Both species are most aggressive during feeding. b) In the case of gilthead seabream, aggression is intensified when fish are fed a low feeding ration, while the method of food distribution has no effect on aggression. However, a higher degree of social stress in feeding and non-feeding periods is indicated when the low feeding ration is distributed locally (i.e. appearance of dark vertical bands on the body). c) In the case of European seabass, aggression is intensified not only during feeding but also before and after feeding, especially at the low feeding ration under localised feeding. d) In both species aggressive acts are mainly biting and chasing, but the relative importance of each behavioural pattern is differently expressed in each species, especially before and after feeding. Present results add to a better understanding of gilthead seabream and European seabass aggression and could help towards an improved feeding practice in these species intensive rearing. Feeding an adequate amount of food and distributing it in a dispersed way can appease fish competition and spare energy for growth, thus providing for fish welfare and productivity.