Dusky damselfish (Stegastes fuscus) behaviour was investigated under natural and controlled
conditions to evaluate how territory and familiarity affect aggressive behaviour.
In the natural environment, fish occupies territories of 2.74 +/- 1.3 m(2)and the
most frequent behaviours exhibited were monitoring swimming, feeding, shelter occupation
and agonistic interactions. Larger-territory individuals spend more time in monitoring
swimming than smaller-territory ones, which results in a reduction of food intake.
Agonistic interactions were more frequent with heterospecifics than conspecifics.
Whenever agonistic interaction occurred, confrontations were not frequent. Territorial
behaviour in the lab was evaluated considering two variables: prior residence and
familiarity between conspecific opponents. Resident fish invested more against intruders
irrespective of the intruder's identity (familiar or unfamiliar). When no previous
territory was established,S. fuscusshowed lower aggressive behaviour, and it was even
lower when a familiar fish was the opponent. Data from the field and the lab suggest
thatS. fuscusterritoriality and aggression are related to the size of the defended
area and the competitor's identity. Thus, reduced reef areas may result in increased
interindividual aggression, while the intruders' recognition may be a key trait controlling
agonistic behaviour in the reef community whereS. fuscusis inserted.