Military beacons to aid sea life

A device used to safely guide warships through mine fields could be used to reduce the number of dolphins and small whales caught in fishing nets.

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.

“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 plan to do so in the future. “That’s where we will go,” said Mr Tiltman.

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.

Mine marker

The spherical devices consist of a glass-reinforced plastic shell with a core of silicon gel. When acoustic energy hits them it splits.

“Some of it goes through the shell and some of it goes through the central core,” Mr Tiltman told BBC News.

The acoustic waves – travelling at different speeds – propagate through the passive device and meet at the back – exactly 180 degrees from where they entered.

“The trick is to make sure that the speed of sound in the two materials is correct so it hits the point at the same time.”

At this point the waves undergo “constructive interference” – creating a more intense signal than the individual waves. This is then reflected back from the device towards the ship or dolphin.

Around 80% of the received energy is beamed back.

The beacons were designed to replace older devices that consisted of metal discs filled with chlorofluorocarbons (CFCs) and were aimed primarily at the military.

“When we go into places like the North Arabian Gulf and we find a mined area we need to mark it,” explained Mr Tiltman.

“If you can mark mines with a big response then ships that come through later can find them and deal with them efficiently.”

Porpoise purpose

DSTL has set up a company called CESALT to find new uses for the technology and is in discussions with oil firms about marking undersea pipelines with the devices.

This is currently done with battery-powered boxes that continually pulse a signal back.

The same kind of device – known as a “pinger” – has also been tested to discourage cetaceans from approaching fishing nets.

“There is evidence that pingers work, at least with harbour porpoises,” Dr Mark Simmonds of the Whale and Dolphin Conservation Society (WDCS) told BBC News.

“There have been trials but they have never been deployed so far because of difficulties in deploying them.”

Keeping the boxes intact and the batteries working had been particular problems, he said.

Nets impregnated with barium sulphide have also been used to try to make them more visible, but have had limited success.

“There’s nothing to say a device similar to [this] – based on strongly reflecting back the animals’ own sonar – wouldn’t do something,” said Dr Simmonds.

However, he said, the device would not help prevent large whales becoming entangled in nets as the majority of species – with the exception of the sperm whale – do not use sonar.

It would also have to be very carefully tested and not seen as a quick fix, he added.