The population status, breeding success and foraging ecology of Phoebetria albatrosses on Marion Island

Master Thesis


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University of Cape Town

Seabird populations breeding within the Southern Ocean have experienced various threats over the past few decades. Albatrosses are particularly at risk due to several factors, inter alia, accidental bycatch on fishing gear, ingestion of polluted marine debris, invasive predatory species at breeding sites, as well as climate change-induced alterations to ocean circulation patterns. The recent decline in many albatross populations is mainly attributed to incidental fishing mortality, which decreases adult as well as juvenile survival rates and is thus detrimental to these long-lived, low fecundity birds. Recently, changes in fishing regulations to require the use of various mitigation measures have reduced the number of seabirds killed by fishing vessels. However fisheries may still impact seabirds either by direct competition for the same prey, or through ecosystem cascades arising from the removal of predatory fish and squid. Sub-Antarctic islands are important breeding grounds for many seabirds, including albatrosses. Monitoring of seabirds breeding on sub-Antarctic islands is important to detect changes in population dynamics to be able to implement timely conservation measures. Marion Island, the larger of the two Prince Edward Islands, some 2000 km southeast of South Africa, is a breeding site for four albatross species including the sooty (Phoebetria fusca) and light-mantled albatrosses (P. palpebrata). The Prince Edward and Crozet Islands are the only places where both Phoebetria albatrosses breed sympatrically in substantial numbers. Both archipelagos are 46°-47°S, at the southern and northern limit for sooty and light-mantled albatrosses, respectively. At-sea observations and diet studies suggest that sooty albatrosses forage mainly in sub-tropical waters to the north and light-mantled albatross in Antarctic or sub-Antarctic waters to the south. The sooty albatross is listed as Endangered due to recent global population declines whereas the light-mantled albatross is Near-threatened. The only comprehensive study of these species at the Prince Edward Islands was conducted during the late 1970s but annual estimates of breeding populations have been made from 1996 onwards. The previous analysis of these counts, up to 2008, suggested that the sooty albatross population on Marion Island decreased from 1996 to 2008, whereas numbers of light-mantled albatrosses increased over this period. Extending the count series to 2014, trends for both species were reversed, with sooty albatrosses recently increasing and light-mantled albatrosses decreasing. However, the timing of sooty albatross counts is in question as these were done towards the end of the incubation period when many nest failures have already occurred. Breeding success of both Phoebetria albatrosses was estimated during 2013/14 and 2014/15. The success of sooty albatrosses (51% overall) was higher than estimated at Marion Island in the 1970s (19%), but it was still lower than that of a neighbouring colony on Possession Island, Crozet archipelago (65%). The sooty albatross success was however skewed by a sub-colony with a very low 5 breeding success; excluding this sub-colony the breeding success is similar to that of the Crozets. Light-mantled albatross breeding success was the same as past estimates and lower than colonies at Macquarie and Possession Islands. Additional monitoring of a sub-sample of nests within the monitoring colonies was done to determine incubation and brood guard (light-mantled albatross only) shift lengths for both species. The shift lengths and distributions were not significantly different from previous data on Marion Island or other breeding sites.