Declining harbour seal populations
Populations of harbour seals in Orkney and Shetland have suffered an as yet unexplained decline of about 40% over the past five years, according to new research carried out by the Sea Mammal Research Unit, University of St Andrews.
The results of the St Andrews study, due to be published in the Journal of Zoology, come from long- term monitoring of seal populations.
Declines have also occurred in the Firth of Tay and additional evidence from Eastern England suggests similar changes may be occurring there. The west coast of Scotland does not however appear to be affected in the same way.
Professor Ian Boyd, Director of the Sea Mammal Research Unit at St Andrews, said:
“Further work needs to be done to confirm the scale of the declines that have been detected but an apparent decline of 40% in five years is a cause of considerable concern.
“These are long-lived animals and this level of decline represents a loss of about 10% of the seals each year. 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.
“It is not possible to suggest, at present, a plausible explanation for this reduction in common seal numbers. It is also unclear whether this reduction is a short term or longer term phenomenon.”
Issued by the University of St Andrews Press Office Further information : Niall Scott, tel 01334 462244, mobile 07711 223062, e-mail ns30@st- andrews.ac.uk
There are two species of seals native to British waters. These are the grey seal and harbour seal (sometimes also known as the common seal). The grey seal population has been increasing for several decades. Monitoring of harbour seal populations began in the late 1980s and each region is surveyed at roughly 5-year intervals. Surveys of Orkney and Shetland carried out in 2001 gave counts of 7752 and 4883 seals respectively. In 2006 the counts were 4256 and 3021 respectively.
Britain holds about 40% of the European harbour seal population and almost half of those seals are found in Orkney and Shetland.
The Sea Mammal Research Unit (SMRU) carries out research sponsored by the Natural Environment Research Council (NERC) and advises the government about the management of seal populations in Britain.
Abstract for the Journal of Zoology paper: This study presents an analysis of changes in the regional abundance of harbour seals (Phoca vitulina) based on repeated aerial surveys of haulouts, and demonstrates the use of sparse data to deliver advice about population status and management. Generalised linear models with negative binomial errors were used to represent these overdispersed data. The shape parameter of the negative binomial distribution was directly estimated from the data where this was possible. Information from time-series of counts where there were few gaps in the data was used to improve the estimation of this parameter in areas where fewer surveys had been carried out. The results show that the number of harbour seals in eastern England has not increased since the end of the 2002 phocine distemper epidemic. There is also evidence of a general decline in most of the large harbour seal colonies around Britain. The populations in the Inner Hebrides were an exception, with numbers appearing to be stable or increasing. Between 2001 and 2006, the population in Orkney and Shetland declined by 40% (95% confidence interval: 30-50%), indicating harbour seals in these areas experienced substantially increased mortality or very low recruitment over this period. The widespread declines, ranging from Shetland to The Wash, suggest that the causes may have been present over a large part of the North Sea.