Recent changes in the status and distribution of moulting Common Eiders Somateria mollissima in Shetland

Abstract

Common Eiders Somateria mollissima in Shetland are believed to be essentially resident within the archipelago, and may be closer to S. m. faeroensis than to S. m. mollissima, the subspecies found in mainland Scotland. Surveys during the late summer moulting period indicated the population in Shetland declined from an estimated 15,500 birds in 1977 (subsequently revised to 17,000) to c. 6,000 by 1997. Further surveys in August 2009 and August 2012 located 5,782 and 4,627 birds, respectively. The 2009 count suggested little change in numbers since the late 1990s, whilst the 20% difference between 2009 and 2012 is believed to represent a genuine decrease rather than any artefact of survey coverage or accuracy. The distribution of moulting Eiders changed fundamentally during the 2000s, from traditional sites on the exposed, outer coastline to the vicinity of shellfish and finfish aquaculture sites on the sheltered, inner coastline; by 2009-12 approximately two-thirds of the moulting population was associating with aquaculture sites. The reason for the recent decrease in the population is unknown, but possible contributory factors discussed include mortality from oil pollution, deterrence measures taken at aquaculture sites, and predation by marine mammals, especially by Killer Whales Orcinus orca.