Outdoors: While common seal numbers are declining, the grey goes from strength to strength

You row on and a minute or two later he pops up again, still following the diminishing ripples but always keeping a safe distance. It is a grey seal; the large round eyes watching and evaluating. What is it thinking? Why is it so fascinated by the wee boat and its occupant?

Only the seal knows that. Perhaps it is simply curious, or maybe the disturbance of the splashing oars causes fish to dart and reveal themselves from their hiding places in the shallow kelp beds. But whatever the reason, it is just one facet of seals' behaviour that makes them one of our most interesting animals.

There are two species of seal in Scotland - the grey and the common or harbour seal. The grey is the larger of the two and ironically much more abundant than the common seal. The two can be fairly easily distinguished. The grey has a long pointed head that is almost dog-like in appearance. The head of the slighter built common seal is blunter and more rounded; if the grey seal has the face of a dog, then that of the common seal is more akin to a puppy.

Autumn is the busiest time of the year in the grey seal calendar when they begin to move to their breeding colonies to gather on remote shores, sometimes in their hundreds both to give birth and then mate. Bulls often mate with more than one cow and the breeding beach can occasionally turn into a bloody affair, with some males left with gaping wounds from their clashes with other bulls.

Common seals pup in June and July, usually on exposed rocks or sandbanks. They are found locally in the Firth of Forth, with better numbers from the East Neuk of Fife north to Peterhead. There is a good concentration around the inner Moray Firth, but the biggest populations are found in the Northern Isles, the Hebrides and the west coast. The grey seal is found virtually all round the Scottish coast with the biggest concentrations in the Firth of Forth, Hebrides, Orkney and Shetland.

Seals have hit the headlines recently following the well-publicised incidents of the bodies of both common and grey seals being found washed ashore with terrible corkscrew injuries, possibly caused by the animals being caught in some type of ducted propeller. The Sea Mammal Research Unit at St Andrews is currently researching the causes of these deaths.

A survey last year revealed the Scottish grey seal population to be at around 164,000 animals, higher than previous estimates. However, the same survey estimated the Scottish common seal population to be in the region of 32,000 to 38,000 animals, with significant localised declines since 2000, especially in the Northern Isles, Firth of Tay and Strathclyde.

The main causes for these regional reductions are unclear but predation of pups by the increasing population of killer whales in the Northern Isles and competition for food supplies with the larger and more numerous grey seal could be significant factors. In 1988 and 2002, the common seal population in the North Sea also took a knock following outbreaks of a naturally occurring seal distemper virus.

So, while the fortunes of the grey seal prosper, those of the common seal are more uncertain, perhaps in part due to the success of its cousin. Monitoring our seal populations is important as their position at the top of the food chain provides a useful signal of the cleanliness of our seas, given that seals are susceptible to the accumulation of potentially harmful marine pollutants such as heavy metals and polychlorinated biphenyls, which in turn can affect breeding success.

This article first appeared in The Scotsman, Saturday September 11, 2010