Changes in the diet of the Australasian gannet (Morus serrator) in response to the 1998 mortality of pilchards (Sardinops sagax)

The diet of Australasian gannets (Morus serrator) breeding in Port Phillip Bay was investigated before and after a mass mortality of pilchards (Sardinops sagax) in southern Australian waters in 1998. Gannets usually feed on inshore pelagic schooling fish, such as pilchards and barracouta (Thyrsites atun), and to a lesser extent on other species. Pilchards represented similar to 60% of the gannet diet examined before the spread of pilchard deaths into Victorian waters, but this component declined to 5% following the mortality event. This reduction was compensated by a substantial increase in the amount of barracouta taken, supporting the view that the gannet is a flexible forager. However, the 1998 mortality of pilchards is likely to have wider implications since pilchards are an important prey for other piscivorous fish, seabirds and marine mammals.