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Common Seal Study

Published: 30 October 2012

Tagged Common Seal at Sandsayre. Photograph by Jim Nicolson Tagged Common Seal at Sandsayre. Photograph by Jim NicolsonZoomLocal photographer Jim Nicolson recently encountered this Common Seal at Sandsayre. Photographs of the seal, which showed the animal to be sporting an orange, numbered tag on its hind flipper, were sent to Shetland Biological Records Centre (SBRC) and subsequently passed on to the Sea Mammal Research Unit (SMRU) at the University of St Andrews.

SMRU's Acting Director, Dr Ailsa Hall, told SBRC that the seal was tagged on 20th August 2010 after being captured and released on Mousa as part of a study into the health and genetic structure of the common seal population around Scotland.

Scientists at SMRU monitor the populations of seals around Scotland. Their surveys show that the number of common seals counted in Shetland has decreased from about 6,000 in the early 1990s to around 3,000 individuals in 2009 - an estimated annual rate of decline of about 3.5% per year.

Dr Hall said, "It is not clear what the cause (or causes) of these declines has been and SMRU are still investigating the various factors that could be involved. These include an increase in the number of grey seals which could be competing with common seals for food; an increase in predation by killer whales; the effect of pollutants, such as pesticides and industrial chemicals; bycatch in fisheries; shooting; disturbance and stress and direct interactions with boats causing increased trauma mortality. It might also be an infectious disease or an increase in parasites that are affecting the animals."

In a 2009 and 2010 study, SMRU captured, sampled and then released seals at their haul out sites around Scotland, including 19 in Shetland, on Mousa. The results indicated that the animals were all in good condition, with no obvious signs of disease or external parasites. However, SMRU did find that animals had been exposed to the toxins from harmful algae, in the form of domoic acid. This toxin, produced by a phytoplankton, causes Amnesic Shellfish Poisoning in humans. This is not a risk to human health, since shellfish destined for human consumption are screened for the presence of toxins and, in any planktivorous fish that eat the toxic phytoplankton, the toxins remain in the its guts. However, seals eat the fish whole and are therefore ingesting these toxins which may cause them serious health problems, affecting the brain and heart particularly.

If you see a live tagged animal like this one please contact Dr Ailsa Hall at (ajh7@st-andrews.ac.uk, phone 01334 462634) as SMRU would be very interested to hear if the animals are still alive. If you find a dead seal, whether tagged or not, please call the stranded marine animal hotline on 01463 243030, Tweet @strandings together with the animal and location, email stranding@sac.co.uk or fill in the form online at  ttp://www.sruc.ac.uk/info/120150/scottish_marine_animal_stranding_scheme/318/report_a_stranded_marine_animal.