Populations of harbour seals in Orkney and Shetland have declined by about 40% over the past five years, according to new research.
Researchers said the decline was a cause of concern
The decline, whose causes are unknown, has emerged in a study carried out by the Sea Mammal Research Unit at the University of St Andrews.
The results, due to be published in the Journal of Zoology, come from long-term monitoring of seal populations.
The seals on the west coast did not appear to be affected in the same way.
Professor Ian Boyd, director of the research unit, said: "Further work needs to be done to confirm the scale of the declines that have been detected.
"An apparent decline of 40% in five years is a cause of considerable concern.
"We have no evidence that there has been a short-term, catastrophic event, like an epidemic, but we retain an open mind about what might be the cause."
He said it was not possible to yet suggest a plausible explanation for the reduction, or whether the reduction was a short term or longer term phenomenon.