Cornish dolphins, past, present and future

Although cetaceans - dolphins, porpoises and whales - are very hard to count at sea, a surprising amount has been discovered about their populations and how they are doing.

The harbour porpoise is our smallest, and is nearly the smallest cetacean on Earth (and everywhere else I guess!). Ten years ago the number in the Celtic Sea was measured as being around 36,000; seventy years ago River Boards on several Cornish rivers were paying for the cartridges of porpoise shooters, and twice as long ago a 'fishery' for porpoises was started in the Fal but failed because the porpoises had an uncanny ability to avoid the nets, even at night - their sonar was unknown at the time. Then, in the 1950s and 60s they became much rarer and it seems that organochlorine pollution, mainly agricultural pesticides, was responsible. These chemicals have diffused over the whole earth, even getting into penguins in the Antarctic. Before the organochlorine decline has had time to pass, gillnet bycatch has emerged with the widespread adoption of this fishing method in the 1970s.

One km of gillnet anchored to the seabed on the Celtic Shelf catches porpoises at an average rate of 7 per year. Nets are not in the water all the time, and the overall rate for Cornish and Irish offshore boats alone was measured ten years ago by the Cornwall Wildlife Trust as over 2000 per year. A subsequent study by the Sea Mammal Research Unit gave the same rate, but since then the type of offshore netting has partly changed and present rates are somewhat uncertain. There is also bycatch from small boats and boats from other states. Will they recover their numbers? My guess is that a partial recovery form the 'organochlorine low' is likely but the population will then be held artificially low by gillnet bycatch. So whether we will see them regularly in our rivers again is in doubt.

The common dolphin was never seen regularly in the Helford or other rivers, as it prefers deeper waters and we have no evidence of an 'organochlorine decline' for this offshore animal. The size of this population is less clear as they seem to make a seasonal movement eastwards across the Celtic Shelf in winter reaching Cornwall and beyond. The animals seen by fishermen and others offshore probably come from a north-east Atlantic population of perhaps a quarter of a million, and nothing is known of changes in numbers. Around 200 a year die in UK/Irish bottom set gillnets on the Celtic Shelf and a similar number in UK bass pair trawls in winter in the waters south of Cornwall- Dorset. Pair trawlers from France and those fishing for other species also catch common dolphins. These fisheries are the main source of dead dolphins on our beaches in winter, but in some years when the common dolphins are unusually close to the coast a substantial proportion are from gill nets.

Extract from HVMCA newsletter No.30 Spring 2005