As recently as the 1950s, porpoises were regularly seen, even in estuaries like the Fal, Fowey and Tamar, and were a familiar sight on inter-island crossings on Scilly. Our best guess is that pollution with organochlorine pesticides, affecting the porpoises, and river pollution in general, affecting fish stocks, brought about a huge decline in inshore sightings. The Trust's research on fishing boats suggests that "by-catch" in gill nets may now be frequent enough (over two thousand a year in the Celtic Sea) to cause a further decline in numbers.
Since that original work on the fishing boats in 1992-94 the Trust has stayed closely involved with cetacean issues through Seaquest South-West, through the International Whaling Commission's Scientific Committee and by assisting the Sea Mammal Research Unit. Through the work of Stella Turk and staff at HQ, the Trust is also the centre of the network for reporting strandings in Cornwall. So far this year nearly half of cetacean (whale, dolphin and porpoise) strandings in England have occurred in Cornwall.
But the evidence we are collecting is not encouraging. Most of the cetaceans that strand on our coast have been killed by fishing gear - and many more die than ever wash up on coastlines. The two big problem fisheries are gill nets anchored to the sea bed - these catch porpoises - and mid-water trawl nets for mackerel, bass, hake, tuna and other species. These huge towed nets catch common dolphins, pilot whales, striped dolphins and occasionally bottlenose dolphins. Bottlenoses are very large dolphins, and although there are only a few of them around our coast they are the species people most often see as they pursue their prey or play in shallow waters very close to the shore, or surf beside human surfers in the breakers.
With evidence of the dangerously high level of death in fishing gear, and few good ideas on how to prevent it, the Trust launched an ambitious project to develop a submersible device, the POD, capable of withstanding the rigours of life on a fishing boat and able to detect and log the numbers of sonar clicks made by dolphins and porpoises. The POD is now, after many struggles, working well, and has been rapidly taken up by conservation workers around the world. One is being towed through the waters of Hong Kong to detect the elusive finless porpoise - it has no dorsal fin, perhaps to enable it to enter water so shallow that any dorsal fin would be damaged by drying or even sunburn.
Another POD is on its way to the Sea of Cortes that lies between the long, narrow peninsula of Baja California and the Mexican mainland. In the northern end of this sea live a few hundred of one of the world's rarest and smallest cetaceans - the vaquita. The vaquita is a porpoise similar to our own harbour porpoise but it has attractive dark shadowing around its eyes and lips, and distinctly larger fins. Despite its charm, the vaquita is in deep trouble from gill-net by-catch; hopefully the Trust's experience and POD will be valuable to the Mexicans working for its preservation.
Back in the UK the Harbour Porpoise Biodiversity Action Plan is getting under way. Research is showing that the "pingers" developed to frighten porpoises away from gill nets bring more problems than successes, and new kinds of solutions are needed. But how can we find what is needed to alert a porpoise to a net? What features of a net make porpoises decide to stay clear of it? How often are porpoises and dolphins not using their sonar? The Trust's POD has a role in answering those questions and is opening up a new approach to non-invasive research on truly wild and free porpoises which we hope will be able to contribute to answering conservation questions that affect many small cetacean species and many fisheries around the world.
Nick Tregenza
