Justification of Red List Category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
Brooke (2004) estimated the global population to number >5,200,000 individuals, while national population sizes have been estimated at c.50-10,000 individuals on migration in Taiwan and c.10,000-100,000 breeding pairs and c.1,000-10,000 individuals on migration in Japan (Brazil 2009).
The population is suspected to be in decline owing to unsustainable levels of exploitation, persecution, predation by invasive species and the over-exploitation of tuna fisheries (Brooke 2004).
The Wedge-tailed Shearwater ranges throughout the tropical Pacific and Indian Oceans roughly between latitudes 35°N and 35°S, breeding on a large number of oceanic islands and on the east and west coasts of Australia (del Hoyo et al. 1992).
This marine species can nearly always be found over pelagic waters except when at colonies. It feeds mostly on fish, with some cephaolopods, crustaceans and insects. It catches prey mainly on the wing by dipping but also by surface-seizing or pursuit-plunging. It will congregate with other seabirds and dolphins when around schooling fish, and will often attend trawlers and smaller fishing boats. Its breeding season is very variable nesting in burrows in colonies on offshore islands or atolls (del Hoyo et al. 1992).
Competition with commercial fisheries likely poses the greatest threat to this species. Over-exploitation of tuna fisheries is greatly reducing prey availability (Brooke 2004) as the shearwaters rely on tuna to herd shoals of small fish to the surface where they become available for surface-feeding (Ratcliffe 1999). Human disturbance by visitors to islands where the species breeds have been found to negatively affect breeding success (Benoit and Bretagnolle 2002). Brown Rats Rattus norvegicus were previously present on many of the offshore islands where this species bred, and in some cases caused local extinction of populations. Following the eradication of rats from several islands, the species has reclaimed past territories, and is again successfully breeding in these areas (Smith et al. 2006). The presence of feral cats Felis catus is encouraged through feeding by humans, and are causing a much lower fledging rate for chicks on Malaekahana (Smith et al. 2002). Small amounts of plastic have been found in Wedge-tailed Shearwater chicks (in majority smaller than 1cm2), but none in adults as of yet. If plastic pollution in the foraging areas increases, this could potentially impact fledging success and eventually adult mortality (Verlis et al. 2013). Increased sea surface temperature has been demonstrated to reduce the provisioning rate, and above a particular temperature threshold reproductive success is nil (Chambers et al. 2011). At present this impact appears restricted to a minority of the breeding population.
Text account compilers
Calvert, R., Ekstrom, J., Butchart, S., Hermes, C., Martin, R., Newton, P., Stuart, A., Bennett, S.
BirdLife International (2019) Species factsheet: Ardenna pacifica. Downloaded from http://www.birdlife.org on 26/03/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 26/03/2019.