Little known about timid porpoise
IN the opening chapter of An tOileánach, Tomás Ó Criomhthainn recalls an incident from his childhood. He and his sister came upon “a great school” of “muca mhara” that the islanders encircled in boats, forcing one of them onto a beach.
“Some able fellow drew his blood, and when the rest of the porpoises smelt the blood they came ashore, helter-skelter, to join the other high-and-dry on the sand. It wasn’t long till the men there were as bloody as the porpoises, and the islanders drove them down the strand covered in cuts and wounds. The islanders had no lack of pork for a year, and it would have lasted two years if it hadn’t been for the relations they had everywhere on the mainland.”
It’s a strange story. Porpoises hunt alone or in small groups. “Great schools” are rare, and not concentrated so tightly that they could be herded onto a beech.
Nor could they be trapped using rowing boats. Although the porpoise is the commonest cetacean around the Blaskets, Tomás’s “muca mhara” were probably pilot whales.
The harbour porpoise was guest of honour at the Irish Whale and Dolphin Group’s conference in Killiney, where experts from Canada, Denmark, Germany, and Britain joined their Irish colleagues for an in-depth look at our smallest cetacean.
According to Dr Simon Berrow, we know remarkably little about porpoises. It’s fairly clear what they eat and we can crudely estimate how long they live, but we are in the dark as to where and when they breed.
The general life strategy of the animal is fairly clear; in Simon’s phrase, a porpoise “lives fast and dies young”. Big creatures lose heat more slowly than small ones, which is why whales are large. But, as with cars, size is expensive. More prey must be caught to support a big body.
The porpoise exploits a niche at the bottom end of the size market, opting for a heat-demanding, rather hectic lifestyle. Being small and agile helps when fishing, but there’s a limit to how small a cetacean can afford to be; newborn babies can’t tolerate cold. The porpoise baby, therefore, is as large as the mother can produce and only one baby per year is possible.
Unlike other toothed whales, porpoise females are bigger than males, enabling them to produce bigger, more cold-resistant babies.
According to Professor Andy Read, a world authority on the species, a person might survive for four or five hours in the sea off Ireland in winter. A porpoise, however, must keep the cold at bay throughout every moment of its life. Heat is lost from fins and flippers, and so, in porpoises, these are small.
Females in Read’s study population in the Bay of Maine mature at about the age of three, and are pregnant every year after that. Few individuals live longer than 15 years. According to Dr Emer Rohan, of UCC, Irish porpoises don’t reach maturity until they are about six. A few may live to 14, but one Scottish animal reached the ripe old age of 24. The diet, here, consists almost entirely of fish, with 11 species recorded to date. Carcasses examined by Emer showed evidence of bullying by bottle-nosed dolphins.
Entanglement in nets is a problem for porpoises and for fishermen. Attaching electronic pingers to nets every 100 to 200 metres reduces mortality; porpoises are timid and the sound frightens them away.
Ronan Cosgrove, of Bord Iascaigh Mhara, has been experimenting with pingers. Some makes are superior to others. The devices are effective, but can become entangled in a trawler’s traction gear.
There will always be a temptation not to use them, so fishing grounds will have to be policed to ensure compliance. Pingers add to marine noise pollution. If used excessively, porpoise-feeding opportunities may be seriously reduced.
The devices are promising, but more research is needed.
Electronic listening-and-tracking devices may soon reveal the movement patterns of porpoises. PhD student, Joanne O’Brien, has been recording porpoise vocalisations in the west of Ireland. Signe Sveegaard, of the Danish Environmental Research Institute, presented some intriguing results obtained by the satellite monitoring of tagged porpoises in Denmark.
Our porpoises may be holding their own, but they need protection and support. The Whale and Dolphin Group’s website is at www.iwdg.ie.
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