Researchers are using killer-whale sounds to scare dolphins away from nets
RESEARCHERS are now communicating directly with dolphins and porpoises as a way to keep them out of fishing nets. They are also trying to help these animals "see" escape routes from nets using acoustic techniques.
The accidental netting of dolphins and porpoises is one of the hazards of fisheries, explains Dominic Rihan, marine technical executive of Bord Iascaigh Mhara (BIM), the Irish Sea Fisheries Board.
"One dolphin caught is one too many. Fishermen don't want to catch them either," he says.
The dolphins and porpoises are at risk of drowning if caught in nets and so, for some years, Ireland - together with the UK, Denmark and France - has developed and tested methods for keeping these animals away from nets. Partners in this work include University College Cork and the Irish Whale and Dolphin Group.
Most of the effort and success so far relates to the use of "pingers", devices attached to nets that emit high-frequency sounds that drive them away.
Simple pingers emit periodic high frequency pulses in the 10khz to 80khz range, sounds that displease the animals. The latest designs, however, remain silent until they detect the "clicks and whistles" made by approaching dolphins and only then begin to ping.
This helps prevent the animal from becoming "habituated" to the sound, causing it to disregard the ping.
It is difficult to assess their effectiveness, however, given that nettings here are relatively infrequent. Dolphins are at greatest risk of the trawled nets used to catch species such as mackerel and herring, while porpoises are most in danger from the static gill nets used to catch cod, hake and monkfish, says Rihan.
BIM monitors "by-catch" and has found that one dolphin might be drowned every 60 to 100 trawls while one porpoise might be caught every 40 to 60 days of fishing, according to Rihan.
Nor does BIM know exactly what puts the cetaceans at risk. "Most of it is down to feeding, but we don't know for sure. It is a difficult thing to solve as the deaths are very sporadic," says Rihan.
Then there is the complexity associated with deciding what noise will give the best effect. "Different species of cetaceans have different reactions to noises," explains Rihan.
This is made more complicated given that sounds can be species-specific. A bottlenose dolphin might respond differently to a particular sound than a common dolphin. For this reason, BIM staff are trying to identify the ideal sounds to keep the animals away.
"We are concentrating on the signals and making them more efficient," says Rihan.
One idea is to use sounds emitted by predators to keep the animals at bay. "We are looking at using the sounds made by killer whales to deter the dolphin species," he says.
THE BIM RESEARCHERSare also examining an as yet theoretical approach, helping the dolphin to "see" escape routes from trawled nets.
The animals make extensive use of echolocation to guide them through the water. The emit high frequency sounds and then pick up on the fine signals coming back as the sound waves bounce of objects ahead of them.
In the same way, the lack of a bounced back signal would indicate to the animal that there was a gap or opening.
"Acoustics is really what they use to guide themselves," the BIM researcher says.
They also like to feed close to the entrances to the nets, but this then puts them at greater risk of being caught up in them, according to Rihan.
The plan would be to "light up" the net opening using an acoustic signal that could be detected by the dolphin. The animal would read the signal and make for a gap or opening. This should help guide it out of an enclosing net and allow it to escape.
