Aspects of the Breeding and Feeding Ecology of the Australasian Gannet Morus serrator in Port-Phillip Bay, Victoria, 1988-92

Abstract

Breeding periodicity and success of Australasian Gannets Morus serrator were studied between 1988 and 1992 at two small colonies (Pope's Eye and Wedge Light), on artificial sites in Port Phillip Bay, Victoria. Samples of foods taken by adults ,were collected for identification of major prey species. Banding of chicks and observations of adults previously banded as chicks (particularly at Wedge Light) provided information on the source of breeding adults and age of return to colonies. Wedge Light colony, established in 1966, has reached some 50 pairs: breeding there is now limited by available space. At Pope's Eye, where breeding began before 1985, space also limited breeding but the provision of an additional platform in 1989 led to a doubling of adults nesting (to c. 120 in 1991-92). Nest building at the study sites began in August-September, and eggs were laid mainly in September; second, or replacement, eggs from November-December. Most hatching took place October-November, with chicks fledging January-February; there was some variation in timing between years. Numbers of clutches laid, as a proportion of nest attempts, were consistently higher at Wedge Light; at Pope's Eye some nests were lost presumably as a result of bad weather. Hatching success was similar at both sites (64%), some 89% of chicks hatched left the platforms; mean breeding success (chicks fledgedleggs laid) has 57%. Although relatively few chicks died or were lost (9-14%), about 36% of eggs failed to hatch (mainly through loss). Nest success was influenced, to some extent, by exposure, particularly at Pope's Eye where seas may wash over breeding sites and drainage is poor. The extended breeding period may reflect local weather conditions and less reliable food availability. In this study, individual prey items were mainly Pilchards Sardinops sagax (58.4%); other taxa (20 fish, four cephalopods and a shrimp species) were generally represented by a few individuals. On average, prey species weighed 45 g, were 141 mm long, 15 mm wide and 26 mm deep; Pilchards were mostly (72%) < 140 mm long. Most regurgitations collected (mean mass 146 g) contained only one fish species (82%). As in other dietary studies of gannets, prey species were primarily inshore, schooling and pelagic. Little is known of factors influencing the distribution or abundance of the prey species; food deliveries at breeding colonies were lighter than those reported elsewhere, which may indicate local foraging. At both sites, colonies have grown as a consequence, in part at least, of the recruitment of chicks fledged at Wedge Light. Expansion at Wedge Light is now impossible, and at Pope's Eye there is little additional area for breeding. The provision of more artificial platforms might further increase the local population.