Listening out for whale song is one way that ships can avoid colliding with whales. But wh

at do you do if the whales aren't singing？ One answer, developed by researchers in the Canary Islands, is to focus on the way the giant creatures mask the clamour of ocean noise.

No one knows how many whales are killed by ships each year, but collisions are thought to ac- count for up to a third of unnatural deaths. For the northern right whale, this means one or two individuals every year, and with only 300 animals left, that's an unacceptable loss. There are three global hot spots for whale collisions: northern right whales are at risk in the Atlantic off the east coast of the US and Canada, fin whales in the Mediterranean, and sperm whales off the Canary Islands.

Last year, the Massachusetts-based international Fund for Animal Welfare attached hydro- phones to buoys and successfully listened out for right whale song in the North Atlantic. But since right whales remain silent for hours at a time, and sperm whales are silent for around haft the time, a better tracking technology is needed.

The new Whale Anti-Collision System (WACS), developed by Michel Andre, is designed to help ships in these hot spots steer clear of the whales. It uses a sonar technique called ambient noise imaging to locate the animals even when they are silent.

To do this, Andre has been testing a long lines of buoys, each bristling with 32 sensitive under- water microphones, or hydrophones, that pick up sea sounds such as breaking waves, rain and ship- ping noise. Any approaching whale blocks this background noise and creates a detectable "sound shadow".

By comparing the reflections received from the hydrophones on different buoys, Andre's system can gather enough information to work out the whale' s position and how fast it is moving. Software displays the results, on a 3D colour map. Underwater objects stand out like hot spots on a thermal image. In trials planned for next year, Andre will place two lines of 12 spherical buoys along the 120-kilometre ferry route between the islands of Tenerife and Gran Canaria. The signals will be sent to a land station where they will be analysed to create a 3D picture of the sea in real time.

If the trials are successful, the WACS software could be installed aboard ferries so that ships' masters can steer clear of whales. Failing that, the whales' movements could be tracked from shore- based installation and radioed to ships in the area.

The loss of one or two northern fight whales every year is considered an unaeeeptable loss because they ______.

A．are the only Whales that can sing songs

B．are the only whales capable of masking the clamour of ocean noise

C．provide clues to scientists to study sea currents and waves

D．are on the very verge of distinction