Sound sense

Fishes seem to be developing ways to detect and avoid high-frequency sonars that trawlers use to locate them

Published: Monday 28 February 1994

-- THE BATTLE of wits between the hunter and the hunted calls for increasingly ingenious strategies on the part of each. Recently, two Danish scientists reported that certain species of fish may be evolving the ability to detect sonars -- high-frequency sound signals -- used by whales, dolphins and humans alike to locate shoals of fish (BBC Wildlife, Vol 11, No 12).

For more than 30 years, fishing boats have used sonars to locate fish shoals. The sonar signal bounces off the sea bed and fish shoals in between, indicating to fisherfolk where the fish are located. But now, experienced fisherfolk say fish are developing the ability to detect these signals and avoid the trawlers.

Contrary to the general belief that fish could only hear low-frequency sounds, Jens Astrup and Bertel Mohl of University of Arhus in Denmark found the Atlantic cod, at least, has the capacity to detect higher frequencies.

Astrup and Mohl proposed that just as several species of insects preyed on by bats have evolved the ability to detect bat sonar, fish, too, should be able to evade whales and dolphins that use sonars to locate them. To test their idea, they subjected 20 Atlantic cods to a hearing test in which they were fed high-frequency sounds, similar to those emitted by whales and dolphins. To their surprise, a majority of them tuned in to the signals.

Although no sea fish species have been seen actually fleeing their echo-sounder-using pursuers, the Danish scientists say it is likely to be only a matter of time before fish learn to avoid fishing boats. So, unless whales, dolphins and humans sharpen their wits and weapons, they may find themselves empty-handed -- or empty-stomached -- soon.

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