Population changes of the Australasian Gannet Morus serrator (Gray) at the Motu Karamarama Gannetry, Hauraki Guf, New Zealand

Abstract

Population changes are discussed for Motu Karamarama (or Bush Island within the Motu Kawau Group), which is an established gannetry with little room for expansion. A visit was made to Mahuki gannetry, an island to the west of Great Barrier Island which, in contrast, had unlimited nesting space and was expanding rapidly.

Diurnally, the maximum number of occupied nests occurred at 06:00 and it is possible to make a 'time-of-day' adjustment to counts made at other times.

The intra-seasonal population variation reflected the breeding cycle. It was found that the maximum number of occupied nests did not correspond to similar stages in the breeding cycle from one season to the next. Diurnal and intra-seasonal population changes noted from ground counts made on Motu Karamarama apply to similarly established gannetries only.

Inter-seasonal population variation was found to be highly variable. This was partly because the timing of breeding in different seasons was unpredictable. Censuses at regular intervals (10 years) reflect the rate of colony growth or decline. Each gannetry was distinct from any other and no method was found from the study on Motu Karamarama that could be applied to data from other gannetries to give the maximum number of occupied nests.

This study covered three seasons (1978-1980) but the population study concentrates on the last two seasons.