The population of eider ducks in the isles has declined dramatically over the last 30 years, with a marked decrease between 2009 and 2012, when the latest survey was carried out.
Eider ducks are vulnerable to inshore oil pollution, and the Shetland Oil Terminal Environmental Advisory Group (SOTEAG) has monitored their numbers since the oil terminal was built.
Surveys in the 1970s found flocks used the same headlands or offshore skerries each year, where they could feed, roost ashore and find shelter in varying wind directions.
The population was estimated at 17,000 birds in 1977, but the Esso Bernicia oil spill in winter 1978/79 and an unexplained die-off in the main Bluemull Sound wintering flock in 1979/80 reduced this to 12,000 by the early 1980s, with a further decline to 6,000 by 1997.
The Braer oil spill in January 1993 and oiling incidents around Bressay between 1992-96 from eastern European fish factory ships contributed to this decrease to some extent, but could not account for this halving of the population. Predation of ducklings by gulls and skuas is another probable contributory factor.
Eider ducks are common in northern Britain but genetic studies suggest those in Shetland are closer to the Faroese race than those breeding in mainland Scotland, which makes them of special conservation concern.
They are resident in Shetland but make seasonal movements around the isles and shift their feeding locations in response to short-term weather conditions. However, in late summer adults become flightless during their annual moult, and by August flocks are restricted to a swimming distance of about three kilometres, and are therefore relatively easy to locate and count.
The situation became complicated in the early 2000s as increasing numbers of eiders began spending the summer near aquaculture sites. A survey in 2009 counted 5,800 birds, little changed since 1997, but they had changed their location, with 70 per cent near aquaculture sites in the inner voes.
For example, only 164 birds were counted in 2009 at “traditional” sites in the South Mainland compared to 1,580 in 1997, whereas numbers between Burra and Vaila Sound had increased from 370 to 2,650.
Eiders have generally been tolerated at salmon farms, where they feed on mussels growing on the infrastructure, and find shelter. However, there has certainly been conflict with eiders feeding at mussel lines, and while various anti-predation measures have been tried out, it is difficult to see how these could have been responsible for the loss of 1,000 birds in just three years.
One contributory factor could be increased predation by killer whales. Although few attacks on eider flocks have been witnessed, they have resulted in the death of up to 50 birds in a matter of minutes – this has happened in Faroe.
The 2012 survey was funded by the Sullom Voe Association Ltd, and David Parnaby (FIBO), Penny and Sheila Gear (Foula), Rory Tallack and Howard Towll (Shetland Amenity Trust) and Newton Harper (RSPB) participated. Thanks also to boatmen Jim Dickson, Victor Gray, Jerry Ramsay, George Williamson and Jonathan Wills.
The next census is planned for August 2015 and hopefully there will be no further decline in numbers to report then.
Martin Heubeck and Mick Mellor