The football-sized "acoustic cat's eyes" reflect sonar signals. They have been designed to mark underwater locations such as channels that have been cleared of explosives.

But the developers believe a smaller version could be fitted to fishing nets to reduce the estimated 300,000 creatures caught in nets every year, BBC News reported.

"Most of the time they swim into the net because they can't see it on their own sonar," explained Carl Tiltman of the Defence Science and Technology Laboratory (DSTL) which is behind the innovation.

Devices strung along the net will reflect cetaceans' (whales, dolphins and porpoises) own sonar, alerting them to the presence of a net.

"It will know something is there and will not just swim straight into it," said Mr Tiltman.

There have so far been no trials of the device for preventing by-catch but DSTL plans to do so in the future.

Conservationists believe that they could be of use but say that other means of reducing by-catch such as moving fisheries also need to be considered.

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