Abstract:
A simulation model of the energy requirements over a breeding season for the Australian Gannet Sula bassana serrator Gray, suggests that two feeds per foraging trip are necessary for body maintenance and chick rearing. The calorific value of a single feed (based on regurgitation weights) is insufficient to meet these requirements. Australian Gannets take about 7% of fish production in the Hauraki Gulf and this is 1/3 to 1/4 of the total predation by seabirds.
The breeding biology of the Australian Gannet is very similar to that of the North Atlantic (sula bassana bassana) and Cape Gannets (Sula bassana capensis). The timing of breeding is less predictable and is probably related to sea temperatures which control the influx and spawning of pelagic fish in the Hauraki Gulf. Pilchards, sardinops neopilchardus are the most important prey species numerically and by weight. They appear to be the most abundant prey species during the Gannet's breeding season.
Behaviourally, the Cape and Australian Gannets are more similar to each other than they are to the North Atlantic Gannet. The former do not guard their chick continuously and the adults leave the gannetry when the chick has fledged. The North Atlantic Gannet chick is continuously guarded and an upsurge in breeding behaviour occurs once the chick has fledged.
Indirect evidence does not support the hypothesis that "the Australian Gannet is capable of raising two chicks and is not limited by food availability." The evidence was:- less time was spent by pairs together at the nest site (than in the North Atlantic Gannet); the Australian Gannet was less successful than the North Atlantic and Cape Gannet at rearing another chick from artificially created twins; the average diurnal incubation shift was considerably longer than the average for the North Atlantic Gannet; the Cape and Australian Gannets do not guard their chick continuously which allows both adults to forage for longer; it was impossible for adults to feed twins at the same rate as singletons, after the twins were two weeks old; twins older than two weeks, were fed less frequently and had slower growth and fledged later than single chicks; from the energy costs of metabolism, activity and rearing a chick, it is necessary for an Australian Gannet to feed twice on a foraging trip. For successful rearing of twins, more than two feeds per foraging trip would be required or more regular foraging trips.
