Article

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

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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.

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... Eiders were counted in up to 30 standard survey areas around Shetland from 1996 to 2019 (see Figure 1 and online Table S1 for locations). Five survey areas were added during this period in response to increasing sightings of birds in more sheltered, inshore locations with aquaculture sites (survey areas 5, 9, 17, 21 and 26;Heubeck & Mellor 2013). The boundaries of the survey areas were defined to include the locations of all records of Eiders recorded during surveys from 1973 to 1993, and for the five areas mentioned above, also the locations of casual records of birds up to 2004/2005 and locations of aquaculture sites in these areas (Shetland Biological Records Centre 1970, Jones & Kinnear 1979, Heubeck 1987, 1993a, 1993b, Heubeck & Mellor 2013. ...
... Five survey areas were added during this period in response to increasing sightings of birds in more sheltered, inshore locations with aquaculture sites (survey areas 5, 9, 17, 21 and 26;Heubeck & Mellor 2013). The boundaries of the survey areas were defined to include the locations of all records of Eiders recorded during surveys from 1973 to 1993, and for the five areas mentioned above, also the locations of casual records of birds up to 2004/2005 and locations of aquaculture sites in these areas (Shetland Biological Records Centre 1970, Jones & Kinnear 1979, Heubeck 1987, 1993a, 1993b, Heubeck & Mellor 2013. ...
... Complete coverage of areas 5 and 21 only began in 2005, and areas 9, 17 and 26 in 2006 (Heubeck & Mellor 2013). However, complete coverage in all survey years also did not occur in certain other survey areas because periods of poor weather and sea conditions occasionally prevented surveys. ...
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