March 13, 2008
New Device Helps Military and Dolphins Alike
Britain's Defence Science and Technology Laboratory (DSTL) believes a device it developed to guide warships through mine fields may also have applications in reducing the estimated 300,000 small whales and dolphins caught in fishing nets each year.
Although originally designed to mark underwater locations such as channels that have been cleared of explosives, developers of the new believe a smaller version could be fitted to fishing nets to reduce numbers of cetaceans (whales, dolphins and porpoises) caught in nets every year."Most of the time they swim into the net because they can't see it on their own sonar," DSTL's Carl Tiltman told BBC News.
Devices strung along the net would reflect the animals' own sonar, alerting them to the net's presence.
"It will know something is there and will not just swim straight into it," said Mr. Tiltman.
The DSTL is hoping to conduct future trials of the device for preventing by-catch.
"That's where we will go," Mr. Tiltman said.
Conservationists believe the device could be useful in reducing by-catch, but say that other means, such as moving fisheries, should also be considered.
The spherical devices consist of a glass-reinforced plastic shell with a silicon gel core that splits when hit with acoustic energy.
"Some of it goes through the shell and some of it goes through the central core," Mr. Tiltman told BBC News.
The acoustic waves travel at different speeds, propagating through the device to meet at the back 180 degrees from the entry point.
"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," Mr. Tiltman said.
Then, the waves undergo a "constructive interference" that creates a more intense signal than the individual waves. The stronger signal is then reflected back from the device towards the ship or dolphin, with 80% of the received energy beamed back.
The beacons replace older devices made of metal discs filled with chlorofluorocarbons (CFCs) and were marketed mainly to 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."
DSTL has set up a company called CESALT to explore new uses for the technology, and is in ongoing discussions with oil firms about using the devices to mark undersea pipelines, something 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 warn cetaceans of the presence of fishing nets.
"There is evidence that pingers work, at least with harbor 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."
But the group ran into problems keeping the boxes intact and the batteries working, he said.
Another approach is in using nets filled with barium sulphide to make them more visible, but this has been met with only 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 explained that the DSTL device would not prevent large whales becoming entangled in nets as the majority of whale species, excluding 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.
"Despite lots of hot air about doing something about it, we haven't had any huge action to prevent the thousands of dolphins killed off the UK every year," he said.
"We have to be careful about technological fixes that seem to have promise but which further inhibit other action such as closing fisheries and moving fisheries."
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