Seabird breeding populations on the far northern Great Barrier Reef, Australia: Trends and influences

All available records (1977–96) of 12 species of breeding seabirds in the far northern Great Barrier Reef (between 9°S and 16°S) were examined to detect any changes, particularly in relation to effects of trawling on populations. The study area included the ‘Green Zone’ (approximately 11–12°S), closed to trawling since 1983, and the well-studied Lizard Island area (14–15°S), which is open to trawling. Seven species (Sterna bergii, S. bengalensis, S. anaethetus, S. sumatrana, Anous stolidus, A. minutus and Sula leuco- gaster) were common and widely distributed, and five species were less common or local (Sterna fuscata, S. dougallii, S. caspia, Fregata ariel and F. minor). Large annual fluctuations in numbers in particular areas are a feature of most species in this region. Sterna bergii, a trawl-discard feeder, showed a clear order of magnitude increase in numbers in the study areas that may be related to the additional food provided by the discarded trawl by-catch. Other discard-feeding species showed no definite population trends. The numbers of S. bengalensis and S. fuscata in the ‘Green Zone’ have declined since the mid-1980s, possibly due to the birds moving away from the area. Both species appear to have increased in the Lizard Island area. It is postulated that the large increases in S. bergii numbers may affect other species, by competing for food and nesting sites. The seabirds fell broadly into three breeding groups: mainly wet season (summer) nesters (Sterna bergii, S. bengalensis, S. anaethetus, S. sumatrana and Sula leucogaster); dry season (winter) nesters (Sterna caspia, Fregata minor and F. ariel), and species that may nest in almost any month of the year (S. fuscata, Anous stolidus and A. minutus). Despite pronounced seasonal peaks in breeding activity among the species, most also show some nesting activity at other times of year. This suggests that they have the potential to breed at any time and are sufficiently flexible to take advantage of the onset of suitable conditions. At present, there are no definite indications that trawl discarding directly influences breeding seasonality, but there is evidence that extra food in the form of trawl discards may influence breeding success and hence population size in some species.