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The regional population of Subantarctic Skua Stercorarius antarcticus has undergone a population decline of greater than 50% of the past three generations satisfying criterion A2. In addition, the species is restricted to two locations in the region satisfying criterion D2.
Subantarctic Skuas breed on the Antarctic Peninsula as well as several Subantarctic islands including the Prince Edward Islands (Hemmings 1994, Barnes and Huyser 1998, Wanless and Ryan 2005). Birds are distributed along the coastal areas of Marion Island with ‘clubs' containing non-breeding and immature birds, often in close proximity to penguin colonies (Cerfonteyn 2013). Non-breeding birds disperse throughout the Southern Ocean (Wanless and Ryan 2005). Birds are present throughout the year on the trawling grounds off the Western Cape although the species is more abundant during the winter months (Sinclair 1980, Ryan 1997). It is scarce in oceanic waters but often occurs close inshore (Ryan 1997).
The global population is estimated at between 6 000 and 15 000 mature individuals (BirdLife International 2014). Within South African waters, Crawford et al. (1991) estimated there to be c. 12 500 birds present in winter and c. 4 000 in summer. 450 pairs were reported on Marion Island in 2010/2011 by Cerfonteyn (2013) with a further 267 pairs on Prince Edward Island (Ryan et al. 2009) giving an estimated total of 717 pairs on the Prince Edward Islands. Confidence in the population breeding on the Prince Edward Islands is high.
The global population is suspected to be stable in the absence of evidence for any declines or substantial threats (BirdLife International 2014an). However, the regional population has undergone a drastic decline. Following the eradication of feral cats in 1991, the Marion Island population decreased from 878 pairs in 1987/1988 (Hunter 1990) to 282 pairs in 2008/09 2009 (Ryan et al. 2009). This trend appears to be counter-intuitive, but may suggest that predation by mice, which were in turn suppressed by cats, may have a more severe effect on burrowing petrels (an important food source for skuas) than initially thought. In 2010/2011 450 pairs were reported on Marion Island by Cerfonteyn (2013). Ryan et al. (2009) reported a decline of 5% per year on Marion Island although Cerfonteyn (2013) reports a less marked decline of 3% per year. The breeding population on Prince Edward Island appears to be stable (Ryan et al. 2009). The regional population (birds breeding on the Prince Edward Islands) is estimated to have declined by 62.2% over a three generation period (51.3 years). Confidence in this trend estimate is high.
Subantarctic Skuas do not face many threats. They have been recorded being caught in pelagic longline and demersal trawl fisheries off South Africa (Watkins et al. 2008, Petersen et al. 2009a,b, Maree et al. 2014), but not in significant numbers. In addition, this threat will probably be ameliorated by mitigation measures to protect albatrosses and other species. Predation by cats on the Marion Island burrowing petrel population negatively impacted the breeding population of Subantarctic Skua as burrowing petrels are a major food source for breeding skuas (Hunter 1990); however, skua numbers crashed after cats were eradicated from the island.
No access is allowed to Prince Edward Island except for research purposes every 4-5 years. The Prince Edward Islands are listed as a Ramsar Wetland of International Importance, as a Special Nature Reserve under the Environment Conservation Act (No 73 of 1989) and are managed under the Prince Edward Islands Management Plan 1996. The Prince Edward Islands Marine Protected Area was proclaimed in 2013.
A Population and Habitat Viability Assessment (PHVA) is recommended, as is a Biodiversity Management Plan, culminating in a National Species Recovery Plan. It has been suggested that should a mouse eradication programme be undertaken at Marion Island, it could cause significant mortality of skuas due to secondary poisoning (Wanless et al. 2010). Even in the extremely unlikely event of local extinction at Marion Island arising from mouse eradication work, birds from Prince Edward Island would recolonise Marion Island. Nevertheless, efforts to remove mouse carcasses, and to treat poisoned skuas should be considered to limit mortality as far as possible.
* The driver of the regional decrease is at Marion Island, because the Prince Edward Island population has ostensibly remained stable (Ryan et al. 2009). The cause for this (which contrasts with the fate of other seabirds common to the two islands) should be investigated.
* The potential effects of a mouse eradication effort should be investigated, including considering timing poison baiting when skuas are at lowest densities or absent from the island (Wanless et al. 2010).