“The sea from close inshore was boiling with surfacing and leaping porpoises, an astounding spectacle. The density of the school was such that there may well have been more than a thousand; at least there were several hundreds.”
When amateur naturalist, Tommy Warren Davis, watched porpoises hunting herring in Milford Bay in 1928, he can have little suspected that such a sight would be unthinkable today. Over the past century it seems that one of our largest, but least–known predators, has suffered a sharp decline. No one is quite sure of the current population, but they have vanished from the Mediterranean and are now rare in French waters, along the Channel and in the southern parts of the North Sea. As a result, it is listed on the EU’s Habitats Directive and conservationists are calling for a special marine reserves to protect its last strongholds.
“Our coastal habitats are outstandingly important to the species,” says Mick Baines, Welsh co–ordinator for the Sea Watch Foundation. “We really ought to designate the best sites, such as Strumble Head in Pembrokeshire, as a Special Area of Conservation (SAC).”
Actually we know remarkably little about porpoises. At first this seems odd. After all, even though tiny by the standards of the whale family, porpoises are still one of Britain’s biggest mammals, with adults growing to two metres in length, and weighing about 80kgs. In common with all the smaller cetaceans, they are carnivores, diving for five to ten minutes to hunt fish like sand eels and herrings in water up to 100 metres deep. These are evidently energetic chases because, on rising to the surface, they rest, panting loudly, giving rise to the local nickname of ‘pufferfish’. Because medieval monks mistakenly categorised them as fish, they were also known as ‘herring hogs’ or ‘sea swine’ (the latter coupled with their abundance around the Gower peninsula probably gave Swansea its name).
Beyond this little is known with certainty. Their social structure is a mystery although males are known to wander more than females: The former also have over–sized testes and produce huge quantities of sperm which suggests multiple matings. It is also clear tht the mother–calf relationship is much the strongest bond they have.”
Copulation takes place in mid-summer in traditional breeding grounds, followed by an 11–month gestation period, before a single calf emerges, tail first (to reduce the risk of drowning) the following June. The new-born calf is very large, almost half the body-length of the mother, and will be suckled for about a year until supplanted by the arrival of a new sibling. The youngster is then left to its own devices until it reaches sexual maturity at three or four years. From then on they appear to reproduce annually.
Although they are rarely caught alive, porpoises are more frequently beached than other cetaceans, usually on gently sloping sandy beaches – probably because their echo-location system fails to recognise the danger. As a result, until recently data relied almost exclusively on post–mortems and sighting records. The latter are scientifically suspect because porpoises are remarkably inconspicuous. They spend just 5% of their time at the surface, rarely jumping out of the water or riding in the bow waves of boats. As a result sightings are normally confined to a brief glimpse of a short black dorsal fin, cartwheeling through the waves.
Although numbers are known to have declined sharply over the past century, no one is quite sure of the reason. Pollution, declining fish stocks and accidental drowning in fishing nets are all thought to be part of the reason. As a result the idea of school a thousand strong is certainly a thing of the past, but on the plus side, Pembrokeshire remains one of the best places in Europe, let alone Britain, to see this and many other smaller and mid-sized whales.
[This photograph was taken off Skomer on a trip to see its wonderful puffins in June 2012]
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