Cornish Dolphins – an update (Dr Nick Tregenza)

Saturday, 23rd January 2010

In his talk to an appreciative audience of 57, Dr Tregenza began with the largest cetaceans, the whales, showing photographs of stranded animals, alive and dead, and mid-water sightings. It was a surprise to hear of the great variety of whales which have been recorded in Cornish waters, for example a Baleen whale at Sennen, Fin whales around the Lizard and Lands End and Sei whale in Carrick Roads. Sightings of Fin whales over the last 10 years may reflect growth in the population since it was cut by more than 90% by commercial whaling, which ended in the 1980s. Strandings of Minke whales have been recorded and these are seen regularly. Sowerbys Beaked whale, an animal that can dive to depths of 2-3 km, is rarely seen from the coast, but there have been live sightings in Falmouth Bay (reported by fishermen as crocodiles). Pilot whales are more often seen. Orcas also visit Cornwall and there have been 30-40 sightings. They tend to associate with basking sharks and have been observed to tear them apart as food.

The commonest cetaceans are Common dolphins, often seen by fishermen 2 or more miles offshore but rarely in shallow water. Hence the tragic stranding in June 2008, when 26 healthy Common dolphins died in Froe Creek, was a most unusual occurrence. A naval exercise using anti-submarine sonar had ceased earlier, but helicopter noise may have been contributory in driving the animals inshore in panic. Such mass strandings are very rare. One involving Long Finned Pilot whales occurred in Mounts Bay in 1905 and there was a near-repeat in 1962, when the whales entered the Helford and Gillan Creek but were driven back to sea.

The Harbour porpoise is the smallest of the cetaceans, but widespread, extending to Greenland. Sightings decreased from 1955 onwards, caused by the agricultural use of organo-chlorine pesticides. Entering the marine environment via surface run-off, these pollutants move up the food chain from algae to fish to cetaceans and other animals and, as they are soluble in fats, the chemicals become concentrated in the female’s milk and are transferred to the babies. As a result, although the females became relatively unpolluted, the death of contaminated males and babies damaged the breeding process. The pesticides were eventually banned, except for daffodil growers who claimed exemption to combat Narcissus bulb fly, but even they were forced to concede after a campaign by CWT and others exhorted children to ‘buy chocolates not daffodils’ for Mothers’ Day! The well-being of peregrine falcons, otters and cetaceans subsequently improved.

Accidental entanglement in fishermen’s nets, known as the by-catch problem, is a serious issue for dolphins and porpoises, with the animals suffering injury and death. Over 2000 by-catch incidents were recorded by Cornish and Irish fishermen each year. They occur all over the Celtic Sea, showing that the creatures occupy shelf waters, not just coastal zones. In an attempt to understand and reduce the death toll, devices which recorded cetacean communication sounds were developed by Dr Tregenza. The POD (porpoise detector) is housed in a cylinder and installed on the sea bed. A smaller, ‘bomb-like’ version can be attached to nets. The instruments record the presence and number of cetaceans and can help to chart their movements. Porpoises are known to have moved southwards since 1994, increasing the numbers found off Lincolnshire and East Anglia and in the Celtic Sea. This movement is probably a response to shifting fish patterns. PODs have been used in the Sea of Cortez, Baja California, where by-catch has reduced the Vaquita, a small endemic porpoise, to 200-300 individuals; and in New Zealand, where inshore gill net fishing has been restricted to halt the decline in Hectors dolphin.

Another important instrument is the Pinger, which emits a sound pulse every 4 seconds. In trials under the auspices of the Cornwall Wildlife Trust, four fishermen have attached pingers to some of their nets. These act as a warning to cetaceans; and accompanying POD records show that fewer porpoises are detected in the vicinity when pingers are employed. Freed of the stigma of cetacean by-catch deaths, the fish catch can be marketed at better price as environmentally-conscious (Pisces project) and the fishermen now want to attach pingers to all of their nets. Other fishermen contend that the pingers do not work, or break, and work is in hand to develop a more robust version.

Although pingers are effective in reducing porpoise by-catch, the evidence is uncertain in relation to dolphins. Common dolphins get caught in nets not infrequently and Bottlenose dolphins are now present in such small numbers that by-catch trends are impossible to evaluate. Past observations record how Bottlenose dolphins would drive fish into Hooe Lake, Plymouth, then wait for the tide to fall to catch them as they swam out. Then Bottlenoses disappeared, apart from a few lonely individuals, and did not reappear until 1991 when a pod of about 20 arrived, probably after splitting off from a more distant group. Now they have been reduced to perhaps 10 and are on the verge of extinction. These are the inshore form of the species and have been recorded from Dorset around Cornwall to Ilfracombe. An offshore form exists in larger numbers, mainly along the edge of the continental shelf. Bottlenose dolphins sometimes play with surfers and swimmers and have a similar life-span to ours. They are highly intelligent creatures with exceptionally strong social structures. They are one of the few animals which can recognise themselves immediately in mirrors. In a comparison of brain size relative to body mass, man registers as 7.44, dolphins 5.31, chimpanzees 2.49 and dogs 1.17, but if only lean body mass is used dolphins and man are very close together.

The key issues which have faced the cetaceans have been (1) organo-chemical pollution, the reduction of which is now showing some success; and (2) fishery by-catch, which hopefully can be reduced by further development of pingers and fishery management measures. The animals most at risk and in danger of disappearing entirely are the inshore Bottlenose dolphins.

On behalf of the HMCG, I would like to record sincere thanks to Dr Tregenza, not only for his absorbing talk but for helpful corrections in the above report.

© Helford Voluntary Marine Conservation Area | | Jackplug Web Design