Abstract:
Globally albatrosses (Diomedeidae) are under pressure as a result of, interalia, climate change and fisheries in the Southern Ocean. Sooty albatrosses (Phoebetria fusca) are listed as Endangered due to global population declines up to 2008, largely due to incidental mortality on long-lining fishing gear. However, exact population sizes, trends and pelagic distributions of sooty albatrosses from major breeding sites are not well known. Gough Island, the Tristan da Cunha archipelago and the Prince Edward Islands provide breeding grounds for ~80% of the global sooty albatross population. Annual counts of breeding sooty albatrosses on Marion Island have been conducted since 1996 and a recent study shows that the population has increased over the past decade, with the 2015 incubator count the highest ever recorded. This is a reversal of previous trends and together with the recent assessment of the Gough population as stable (not decreasing), suggests that the species global threat status should perhaps be revised. By comparison, light-mantled albatross (P. palpebrata) numbers on Marion Island have decreased over the last decade. These trends are consistent with southward movement of frontal systems favouring sooty albatrosses at the Prince Edward Islands.
Breeding sooty albatrosses from Gough, Tristan and Marion Island were tracked with GPS loggers between 2013 and 2015. Birds from all three sites had similar latitudinal foraging ranges, concentrating around the Sub-Antarctic (SAF) and Sub-Tropical Fronts (STF). The position of the respective breeding sites, in relation to the fronts, resulted in Gough and Tristan birds travelling further than Marion birds. Gough and Tristan birds compensated by flying faster and spent the same amount of time foraging as Marion birds during the incubation period. However, while brooding, Gough and Tristan sooty albatrosses spent less time foraging. The position of Marion Island in relation to the major fronts might be favourable for sooty albatrosses as these fronts are moving polewards. Sooty albatrosses from all three sites overlapped with long-line fishing effort in the southern Atlantic and Indian Oceans. However, Marion Island birds showed a greater overlap than Gough and Tristan birds. This is contrary to previous estimates of fisheries overlap and a conservation concern as the southern Indian Ocean is the most data deficient and the area with the highest rate of fisheries related seabird mortality.
