Effect of the observer in the aggressiveness of small groups of juveniles white-seabream (diplodus sargus cadenati De la Paz, Bauchot y Daget, 1974)

The groups of juvenile white-seabream showed a linear hierarchical structure. However, the aggressiveness of the group increases in front of a co-specific observer, whereas it decreased significantly if the observed is a potential predator (i.e. Gobius paganellus). When the observer fish is an unknown species (i.e. Tilapia nilotica), juvenile increase the exhibition degree in front of the observer, but there are not aggressive interactions. Otherwise, the interactions are of different intensity depending on the length of the observer fish, increasing with the size of this one. Also, the aggressiveness was always significantly increased after the fish group has had access to food. On the other hand, the aggressive behaviour of the whole group (type of interactions and their frequency), in front of different observer fish, is strongly determined by the behaviour of the dominant fish.