GROUP FORAGING BY THE HERBIVOROUS CICHLID FISH, PETROCHROMIS-FASCIOLATUS, IN LAKE-TANGANYIKA

Group foraging of the algae eating cichlid, Petrochromis fasciolatus, was studied along a rocky shore in Lake Tanganyika, in an area occupie d by the feeding territories of several herbivorous cichlid species. S olitary P. fasciolatus or schools comprising a few members only were n early always repelled from such territories, although larger schools ( 40-250 individuals) of P. fasciolatus were able to overwhelm the defen ses of territory owners and enable subsequent foraging. The frequency and duration of individual foraging sessions on the algal mats were po sitively correlated with group size, probably owing to individuals in large schools suffering fewer attacks from territory owners. Almost al l of the foraging sites of P. fasciolatus were inside the territories of other herbivorous cichlids, especially those of Neolamprologus moor ii (more than 80% of total), in which the algal density was more than 10 times that in territories of the other species. N. moorii territori es included only 4% rocky substrate, indicating that schools of P. fas ciolatus selectively chose territories containing rich resources.