EXPERIMENTAL APPROACH TO THE PREDATION BY PISCIVOROUS BIRDS IN EXTENSIVE WATER AQUACULTURE

The predation by the Grey Heron Ardea cinerea and the Cormorant Phalac rocorax carbo has been studied in a dyked area intended for extensive brackish water aquaculture on the French atlantic coast. A pond was di vided in two parts, one of which (0,9 ha) was protected with nets and threads, the other one (0,3 ha) left without protection. Fish (Mugilid ae) were stocked in November, during the wintering period of the birds . Three weeks later, remaining fish in both parts of the ponds were co unted. During experimentation, number, presence and predation behaviou r of birds were observed. A spectacular but temporary gathering of bir ds occurred during the first days following the fish stocking. Cormora nts and Grey Herons respectively used deeper and shallower areas of th e pond. Intensity of predation (number of attacks/minute) was higher a mong cormorants. Dominant features of predation exerted by these birds are the rapid exhaustion of the fish population, the higher predation level by cormorants than by herons, although the latter obtains a ver y high success ratio (number of catches/number of attacks).