Surveys reveal increasing and globally important populations of south polar skuas and Antarctic shags in Ryder Bay (Antarctic Peninsula)

Despite their importance in ecosystems, population sizes and trends are unknown for many seabirds, including in the Antarctic. Here we report on the first comprehensive survey of south polar skuas Stercorarius maccormicki and Antarctic shags Leucocarbo bransfieldensis in Ryder Bay, and collate previous count data. In austral summer 2017/18, totals of 259 skuas at club sites and 978 occupied skua territories were counted in 2.3 km 2 of suitable habitat at Rothera Point and adjacent islands. Based on the mean nearest neighbour distance (23.2 m), skua nest densities were comparable with colonies elsewhere. Long-term monitoring of skuas at Rothera Point indicated considerable annual variation and overall increases of 1.9 and 1.3% per annum, respectively, in breeding pairs from 1975/76 to 2017/18, and occupied territories from 1988/89 to 2017/18. In total, 405 pairs of Antarctic shags bred at two known and one newly discovered colony in 2017/18. Previous counts at the two known colonies indicated substantial annual variation and increases of 5.5 and 3.3% per annum, respectively, from 1985/86 to 2017/18 and 1989/90 to 2017/18. Factors leading to overall increases in both species, and the intermittent seasons of near-complete failure to breed, are unclear, but likely to reflect impacts of environmental change on their marine prey or sea ice. The breeding populations of south polar skuas and Antarctic shags in Ryder Bay represent 10.3 and 3.5%, respectively, of revised global estimates of 9500 and 11,684 breeding pairs. We recommend that the breeding colonies be included as important bird areas (IBAs) and within the Antarctic Specially Protected Area (ASPA) system, and provision made to conserve foraging areas at sea.