Alarms help stop nets catching porpoises LONDON, Aug 6 (Reuter) - Acoustic alarms attached to fishing nets can prevent porpoises from getting caught in them and drowning, experts reported on Wednesday. In one of the first field tests of warning devices designed to protect marine mammals, U.S. researchers said they greatly reduced the number of porpoises caught in fishing nets. Scott Kraus of the New England Aquarium in Boston and a team of marine scientists said the effect was enormous. "The most serious danger to dolphins and porpoises around the world is the threat from various forms of gill-net fishing," they wrote in a letter to the science journal Nature. "More than 80,000 small cetaceans (dolphins, porpoises and small whales) are killed annually in coastal waters around the world, and at least two species are in imminent danger of extinction because of fishing activities," they added. Scientists have thought that alarms on fishing nets might help save dolphins and porpoises, which use sound to navigate and hunt. But the theory has not been widely tested. Kraus's team recruited fishermen to test their alarms. "Between 18 October and 15 December 1994, 15 commercial sink gill-net fishers from the coasts of New Hampshire and southern Maine took part in our experiment," they wrote. Some of the nets were equipped with acoustic alarms that sounded as soon as they hit salt water while others were fitted with devices that looked exactly the same but emitted no sound. Two porpoises were caught in the nets using the alarms while 25 were caught in nets carrying silent devices. The same number of cod and pollock, which is what the boats were after, were caught in nets using either kind of alarm. "At present we do not understand why the use of alarms produced such a dramatic reduction in porpoise catches," they said. The obvious inference is that the animals heard and avoided the alarms. But herring -- the main preyof the porpoises -- also seemed to respond to the alarms. "It is possible that herring reacted to the alarms by avoiding the nets, thus reducing the number of porpoises becoming entangled while attempting to capture prey," they wrote.