Effects of the fishing strategy, the relative position and the size of fishing groups on the foraging success of Cormorants Phalacrocorax carbo during winter.

During five consecutive winters (october-march, 1991-1996), the group fishing behaviour of Cormorants was studied in northern Spain and south-western France. The number of feeding attempts, the foraging success, the number and the size of fishes caught were recorded forthnightly during a whole-day observation period. The influence of the foraging strategy and the relative position of individuals within the fishing group (front, middle and back) on some parameters related to foraging success and aggression rates were studied by direct field observations (Table 1). Cormorants used three foraging strategies during group fishing: line fishing, zig-zag fishing and dispersion of fish schools towards the banks of the main feeding areas (Fig. 1 and 2). The length of fishes caught by group fishing Cormorants varied among different foraging strategies and with the relative position of birds within the group. Larger fishes were caught both with the zig-zag fishing strategy and in the middle position of the foraging groups. Kleptoparasitic attacks and aggressions were more frequently observed in the middle position of groups and during the zig-zag movements (Table 2). Individuals in larger groups had a higher foraging success (Fig. 3) and made shorter dives than individuals in small groups. Direct correlations between group size and rates of aggressions and kleptoparasitic attacks were found, indicating that these aggressive behaviours are density dependent.

Lekuona, J. M, Ardeola 46(1), 1999, 13-21