Are Kelp Gulls Larus dominicanus replacing Pacific Gulls L. pacificus in Tasmania?

Abstract

The nominate subspecies of the Pacific Gull Larus pacificus, widespread along the coast of southern Australia, may be under threat from the slightly smaller, but opportunistically competitive, self-introduced Kelp Gull L. dominicanus. To assess this threat to the Pacific Gull in Tasmania, we documented colony size of large gulls across many Tasmanian islands over a period of 24 breeding seasons (1985–2009). The most northerly Kelp Gull nests on the Tasmanian mainland were located at Paddys Island, St Helens. There were no reports of Kelp Gulls along any part of the northern coast of Tasmania abutting Bass Strait, although there were sporadic sightings on islands of the Furneaux Group. The stronghold of the Kelp Gull in Tasmania is the Estuary of the Derwent River and its surrounding bays and channels, where this species is present in much larger numbers than the Pacific Gull, but nevertheless co-exists with that species. We found no evidence for dramatic changes in numbers since 1985. All Pacific Gull nests were on small islands, and there were none at Orielton Lagoon, which became the third biggest Kelp Gull colony studied in the south-east of Tasmania. At the largest Kelp Gull colony (Green Island), the number of Pacific Gull nests declined while numbers of Kelp Gull nests increased, but no such inverse correlation was observed elsewhere, including Curlew and Hog Islands, where numbers of Kelp Gull nests increased. We conclude that numbers of Pacific Gulls have remained reasonably stable, despite increases in Kelp Gull nests at certain sites.