Dolphin deaths in fishing nets to be investigated through sonar detector

A new device could help to prevent vast numbers of whales, dolphins and porpoises becoming ensnared and drowning in fishing nets.

A new device could help to prevent vast numbers of whales, dolphins and porpoises becoming ensnared and drowning in fishing nets.

The new system, called T-POD, is being developed with the support of the British organisation Wildlife Trusts in an attempt to show exactly how and when the mammals meet their deaths.

The system was invented by Dr Nick Tregenza, a research fellow at Plymouth University's Institute of Marine Studies. He said: "One of the big problems is that, although it is known a lot of cetaceans are dying as a result of fisheries, we have very little idea what goes on below the surface.

"This device will give a clearer picture of the nature of this tragedy, for instance whether they are caught at any time when fishing is in progress or just at the point when the net is closed.

"It may help to identify when and where they are at risk. Knowing exactly what is happening will help the conservation movement draw up a strong case for measures to provide greater protection for marine mammals, a goal that is always going to be difficult to achieve because it will require multi-national agreement."

Whales and dolphins deliberately swim through the 40-metre-wide net openings to take the fish caught by boats.

But many pay a high price for the apparently easy pickings. An Irish observation on mid-water trawling for tuna recently found 30 dead dolphins in one haul. Observers from the Department for Environment, Food and Rural Affairs noted 53 dead dolphins from 116 bass-fishing hauls.

Throughout the whole international fishing industry this "by-catch" is huge. Conservationists estimate that in European waters alone, 10,000 harbour porpoises are killed each year in gill nets. The number of dolphins killed also runs to "thousands".

T-POD, which scans through six frequency settings each minute, is a self-contained system that logs the echo-location clicks made by porpoises and dolphins.

These mammals use sonar, sending out very short, high-pitched pulses of sound in a beam ahead of them and then listening to the echoes from everything it strikes. The sonar is so sophisticated that a dolphin can probably "see" whether another is pregnant or has fish in its stomach.

The time and duration of up to two million clicks can be recorded by T-POD and transferred to a computer where the data is interpreted.

One successful test, in Hong Kong waters, involved the use of a rig to tow the device behind a vessel two metres below the surface. It detected all porpoises within 250 metres.

Researchers are expected to find out about cetacean habitat use, interactions between dolphins and porpoises, trends in their activity levels at specific locations and their responses to possible or proposed by-catch deterrents.