Harpooned by hunters and poisoned by pollution, whales now face another deadly enemy - noise.

Scientists say a huge increase in noise from ships, pleasure boats, offshore industry and military technology threatens whales, dolphins and porpoises.The confusion caused by modern noises, such as military sonar, could be responsible for "mass beachings," according to a paper presented recently to the International Whaling Commission.

Whales rely on their own wide range of sounds for communication, navigation, finding food and mating. Man-made noises can interfere with these signals or damage whales' hearing, threatening their survival.

"A deaf whale is almost certainly a dead whale," said Steve Trent, campaign director of the Environmental Investigation Agency, which produced the paper.

Toothed whales, such as dolphins, beluga and killer whales, send out ultrasonic sounds which "bounce back" off shoals of fish or other prey. Sperm whales use "echo-location" to avoid fishing nets. Beluga use it to find openings in ice to come up for air.

The biggest threat is a huge increase in low-frequency noises, which travel great distances under water and to which whales are particularly sensitive. The most dangerous sources are cargo vessels, including oil tankers, and oil and gas exploration.

In the Canadian High Arctic, ice-breaking ships, the loudest ocean-going vessels producing more than 200 decibels, have frightened belugas 35 miles away. Offshore research vessels bounce low-fre-quen-cy pulses - of more than 240 decibels - off the seabed to map areas likely to contain gas and oil. These seismic tests are thought to have caused the stranding of 24 North Sea sperm whales three years ago.

The stress of escaping from noise is also likely to shorten their life spans. Whales usually swim away from noise, diving suddenly. The effort required can affect their carefully balanced energy levels, weakening their capacity to reproduce. Whales that dive from the surface may not have had time to replenish their oxygen supplies. Advances in military sonar and oceanographic research have added to the problems.

The Environmental Investigation Agency wants a comprehensive research program into the long-term effects that noise has on the physiology and behavior of whales.