The predation by the Grey Heron Ardea cinerea and the Cormorant Phalacrocorax carbo has been studied in a dyked area intended for extensive brackishwater aquaculture on the French Atlantic coast. A pond was divided in two parts, one of which (0.9 ha) wa protected with nets and threads, the other one (0.3 ha) left without protection. Fish (Mugilidae) were stocked in November, during the wintering period of the birds. Three weeks later, remaining fish in both parts of the ponds were counted. During experimentation, number, presence and predation behaviour of birds were observed. A spectacular but temporary gathering of birds occurred during the first days following the fish stocking. Cormorants and Grey Herons respectively used deeper and shallower areas of the pond. Intensity of predation (number of attacks/minute) was higher among cormorants. Dominant features of predation exerted by these birds are: the rapid exhaustion of the fish population; and the higher predation level by cormorants than by herons, although the latter obtains a very high success ratio (number of catch/number of attacks).