Population size and trends of southern giant petrels (Macronectes giganteus) nesting at Signy Island, South Orkney Islands

Abstract

The southern giant petrel (Macronectes giganteus) has a circumpolar distribution and breeds on subantarctic islands and a few continental Antarctic sites. Although this species has recently been down-listed to “Least Concern” by the World Conservation Union (IUCN), there are strong fluctuations in abundance and variable long-term trends recorded at different sites. Systematic, long-term monitoring is essential to determine drivers underlying its population dynamics. Here, we examine long-term changes in population size and productivity of southern giant petrels at Signy Island, South Orkney Islands. Comparing estimated numbers of breeding pairs over the whole island in 2000/2001, 2005/2006, 2009/2010 and 2014/2015 with historical data revealed several phases of population change: a 64 % decline (6.2 % per annum) from 1968/1969 to 1984/1985, a 162 % increase (6.2 % per annum) to 2000/2001, stability until 2005/2006, a 56 % decline (18.3 % per annum) to 2009/2010 and stability until 2014/2015. This represents a 1.8 % decline per annum between 1968/1969 and 2014/2015. Annual counts within focal study areas suggested a more rapid increase from 1996/1997 to 2006/2007, but the same downward trend from 2006/2007 to present, underlining potential pitfalls in inferring trends from part-island counts. There was also a 20 % decline in breeding success from 1996/1997 to 2014/2015. Our results indicate substantial fluctuations in southern giant petrel abundance at Signy Island over 4–5 decades and a recent decline in breeding numbers and success. As the southern giant petrels breeding at the South Orkney Islands represents ~5–10 % of the global population, continuation of these declines would be of high conservation concern.