Catch per unit effort, foraging efficiency, and parental investment in breeding great cormorants (Phalacrocorax carbo carbo)

Abstract

The feeding ecology of breeding great cormorants (Phalacrocorax carbo carbo) was studied in 8 males and 6 females using automatic nest-balances and a radio-tracking system. Birds were shown to be extremely efficient predators with median catch per unit effort of 15.2 g of fish taken per minute spent underwater (n=89; gross foraging efficiency 3.25) in males and 9.0 g min⁻¹ in females (n=91; gross foraging efficiency 3.46). Mean daily food intake was also high (828±166 g, n=7 in males and 829±271 g, n=6 in females) and was positively related to brood biomass. Catch per unit effort was not related to foraging range, showing that birds do not tend to visit distant foraging areas to compensate for prey stock depletion around the breeding site. Furthermore, breeding adults responded to increasing food requirements of the brood by increasing the number of foraging trips per day, but maintained constant foraging effort, load size and catch per unit effort. Parents thus increased their overall foraging effort and food intake but kept their body mass constant.