The huge mortality rate has come to light through two studies, one Danish and one Anglo- Irish, in which observers were stationed on small boats using these nets to monitor accidental catches of dolphins and porpoises. Their findings chime with post-mortem analysis of apparently healthy carcases washed up on Britain's beaches, which mostly appear to have drowned in nets. Fishing rather than pollution appears to be mankind's main threat to these highly intelligent marine mammals.
It was difficult to persuade the fishermen to allow the observers on to their craft in the first place. Now that the scale of the accidental deaths has come to light, they are even more reluctant to collaborate with researchers.
The Marine Mammal Research Unit at Cambridge estimates that boats from Ireland, Devon, Cornwall and South Wales are catching nearly 2,000 porpoises in their nets on the Celtic Shelf - the waters off southern Ireland and the West Country. The study was funded by the European Commission.
A Danish fleet using the same fishing technique is estimated to drown about 7,000 porpoises in the North Sea. Boats from other ports in Britain and elsewhere will add to these numbers.
The nets in question are a sea-bed version of the 'wall of death' drift nets used by French, British and Irish boats in the Bay of Biscay and eastern Atlantic, but with a wider mesh. Weights pull the lower edge to the bottom while the upper edge is held some 20 feet above the sea bed by buoys. The technique has only been used for about 15 years.
'We think drowning in nets must be the most important non-natural cause of death,' said Dr Nick Tregenza, a Cornish GP and chairman of the Cornwall Wildlife Trust, who organised the British observers. 'We knew a few porpoises were being killed, but we thought it was close inshore.'
In fact most of the porpoises were caught by boats far beyond the sight of land. In 85 fishing trips lasting a total of 328 days, 43 harbour porpoises and four of the larger common dolphins were caught. Scaling that number up to reflect the total fishing effort - number of boats, days at sea, length of net used - gives a rough estimate of 1,500 porpoises killed each year by the Irish fleet and 450 by Welsh and West Country boats.
Observers who joined 37 voyages by Danish boats fishing for turbot, cod and plaice recorded 117 dead porpoises. Some of these boats leave nets on the bottom for an entire week. Danish government fisheries scientist Morten Vinther estimated some 7,000 porpoises were being drowned per annum.
He presented his as yet unpublished findings at a conference of experts recently but said he would rather not discuss them publicly for fear of offending fishermen and jeopardising future studies.
The harbour porpoise is by far the most common cetacean (the sea mammal group that includes dolphins and whales) in the North Sea. The extent of the threat from fishing nets is unknown because there is no reliable popultion estimate.
Scientists know more about the porpoises' larger, longer- nosed and more photogenic relatives. Although they live shorter lives, porpoises have much of the dolphins' intelligence and social complexity. Their habit of feeding on the sea bed leaves them prone to entrapment in gill nets.
Dr Tregenza said there was an urgent need for research to make the nets more 'porpoise friendly'. The drift nets used to catch tuna in the Bay of Biscay are not allowed to be longer than 1.6 miles - largely for the sake of dolphins - but the bottom-set nets can be up to seven miles long.
Dr Tregenza praised the fishermen for their co-operation but Mike Townsend, chief executive of the Cornish Fish Producers Association, strongly disputed the estimate of 2,000 porpoise drownings a year on the Celtic Shelf. 'The sample the observers saw was far too small to make any kind of reliable estimate. We're going to look very seriously at whether we will co-operate in future. 'Porpoises are caught sometimes, but it's very sporadic and we certainly don't want to catch them. If we didn't catch any we'd probably be accused of having wiped them out.'
French tuna fishermen are estimated to kill about 1,500 dolphins a year in their drift nets in the Bay of Biscay and in the eastern Atlantic. The Spanish use methods which kill fewer dolphins, but when caught they are sometimes eaten at sea.
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