Mating cod put navy in danger

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The Independent Online
HUGE NUMBERS of grunting cod have been blamed by a team of Norwegian scientists for posing an unexpected security risk to the Scandinavian country's military security.

According to a recent analysis that has been produced for the navy, millions of tiny grunting sounds emitted by the cod during the mating season can create a background noise loud enough to blot out sounds of suspicious maritime activity.

The cod make their grunt-like sounds approximately every 80 seconds in order to lure their mates. And the number of fish is vast - Norway's seas are famous for the huge schools during the mating season.

The study by the Norwegian Defence Research Institute said the millions of fish grunts are overloading naval sonar systems and masking sound waves that sonar transmits in order to detect objects underwater. The result is that it is almost impossible to navigate safely under the cold waters of the Norwegian Sea.

The study concluded that naval vessels would be well advised to steer clear of the region when cod spawning is at its height in February and March. The scientists used a spectral analyser to break down the background noise picked up by sonar techniques designed to register enemy submarines.

At this stage, the Norwegians say they have no answer to the problem but to advise their submarines to avoid all known spawning grounds at the appropriate times of year.

"This is the first time we have registered the frequency of sounds from wild cod," researcher Erling Kjellsby is quoted as telling Norway's largest newspaper, Verdens Gang. "The noise of cod looking for partners ruins the listening conditions."