THE EFFECT OF GROUP-SIZE ON SPACE USE AND AGGRESSION AT A CONCENTRATED FOOD SOURCE IN BLUE GOURAMIS, TRICHOGASTER-TRICHOPTERUS (PISCES, BELONTIIDAE)

We examined how spatial distribution and the use of aggressive behavio r by blue gouramis, Trichogaster trichopterus (Belontiidae). in the pr esence of a concentrated food soul:ce were affected by group size and by the short-term presence and absence of food. Gouramis aggregated in the area of the food source, and the frequency of aggressive acts per fish was higher for fish near the food source than for fish away from it. The frequency of aggressive acts per fish near the food source de creased with group size and was about 50 times higher in groups of 2 a nd 4 than it was in groups of 16 and 32. In group sizes 2 and 4? the f requency of aggression was unaffected by the presence and absence of f ood, but in larger group sizes aggression increased during short inter vals without food. The mean proportion of time spent near the food sou rce increased from group size 2 to group size 8 and decreased from gro up size 8 to group size 32 and was greater during intervals of food av ailability than during intervals in which food was not available. We s uggest that gouramis adjust their relative use of contest and scramble competition according to the costs and benefits of aggression as dete rmined by the number of competitors and by the potential for missed fe eding opportunities. The average proportion of time spent in the vicin ity of the food source appears to be influenced by both the rate of ag gression and by the net rate of gain available.