MYSTERY surrounds a huge drop in the number of eider ducks around Shetland in recent years, according to local ornithologists.
Surveys conducted in 2012 showed a 20 per cent decline in the population of the distinctive seabird compared to three years earlier.
Martin Heubeck, of the Shetland Oil Terminal Environment Advisory Group (SOTEAG), said the cause remained unknown and could involve the local mussel farming industry or killer whales.
The common eider has suffered a long term drop in numbers over the last 40 years.
A survey carried out in 1977 counted around 15,000 birds mostly at natural sites dotted around the isles.
Twenty years later that figure had fallen to just 6,000 birds, two thirds of whom now congregated around salmon and mussel farms that have grown in number around the Shetland coastline.
Shetland produces one third of Scotland’s farmed salmon and more than half of its farmed mussels.
This initial huge fall in bird numbers was partly due to oil spills from the Esso Bernicia in the late 1970s, the Braer in 1993 and from East European floating fish factories during the ‘90s.
However eider numbers remained relatively stable until 2009 when a SOTEAG survey of 80 per cent of the Shetland coastline counted 5,782 birds.
Three years later a follow up survey covering 90 per cent of the coast found just 4,627 birds, a drop of one fifth.
The largest decline was along Shetland’s west coast between Burra and Vaila, one of the most heavily used areas by the aquaculture industry, where the decline was 1,034 birds.
However Heubeck said the cause of the fall in numbers was hard to assess, and could be anything from natural predation to industry practices.
“The eider population has been declining ever since we started monitoring them in the late 1970s and certainly oil pollution at times has played its part in that, but for the large part we don’t know what has been driving the decline,” he told BBC Radio Shetland.
“A lot of people would say that duckling predation by skuas and gulls has been severe, but it’s been the case for a long time and there is no reason to believe that between 2009 and 2012 it would suddenly increase.
“Predator nets have been set at both mussel farms and salmon farms, one mussel farmer actually reported that anti predator nets were drowning eiders, but the scale and the extent of it is really a big question mark.”
He said that killer whales had been seen attacking a flock eider ducks near Lerwick in 2007 and could be causing many more deaths elsewhere.
“In a matter of 20 minutes if they can kill 50 eiders, which is what happened at the Ness of Sound, one doesn’t know how often this has been going on in more remote parts of Shetland where there isn’t big crowds of people with big cameras and watching.”