Killing a porpoise can haunt a man -- even a fisherman accustomed to life and death at sea.

Erik Anderson, who has fished New England's waters for 30 years, would haul up his gill net and occasionally discover a harbor porpoise, entangled and dying, trapped in the mesh along with his harvest of cod and flounder.

Every time it happened, six or seven times a season, Anderson worried. Each death of a porpoise heralded deep trouble for fishermen.

Up to 2,000 of the animals were drowning in gill nets in the Gulf of Maine each year -- enough to eventually wipe out the species. And if nothing was done soon to stop it, the already struggling fishery would be shut down by federal authorities.

To avoid that fate, fishermen -- first in New England and now in harbors around the world -- decided to take matters into their own hands.

Following a hunch about acoustics from a Canadian whale behaviorist a few years ago, Anderson and some of his colleagues started experimenting with their nets.

At RadioShack, they purchased a batch of the devices that sound a beep on school buses when they back up. They tied the bulky boxes onto their gill nets, and set them into the Gulf of Maine. It was decidedly low-tech, and risky at that. But amazingly, porpoises heard the low-frequency alarms and avoided swimming into the nets. Rarely were they ensnared.

Word is spreading

Now, word is spreading around the globe, fisherman to fisherman.

"Ping, ping, ping" can be heard in ocean waters from South Africa to the Irish Sea.

Called "pingers," net alarms are considered so successful in protecting marine mammals that a federal order recently mandated them on drift nets off California and Oregon and sink nets in New England.

Deaths of whales and dolphins have dropped by two-thirds on the West Coast -- with 90 common dolphins dying in 1997, compared with a past average of 271 annually. Killings of some whale species, including sperm, humpback, beaked and killer whales, dropped to zero. On the East Coast, porpoise deaths in the Gulf of Maine have declined by more than 90 percent.

"You just can't be a fisherman these days without being attentive to these other [ conservation ] issues," said Anderson, who spent $3,000 equipping the gill nets on his 40-foot vessel with pingers. "Ultimately the fishing industry has the solutions to these problems right at their fingertips."

At first skeptical of the fishermen's idea, many marine scientists are now confident that pingers save the lives of many whales and dolphins. Yet they do not fully understand why they work, or what impact the noise might have on other ocean creatures. Cautioning that pingers are not a cure-all, they worry that their use is spreading faster than the science.

A few questions

Conservation groups endorse pingers, but they also wonder whether the undersea cacophony could disrupt nature in ways not yet understood, or whether cetaceans could eventually get so accustomed to the pings that they start ignoring them.

"Right now we know that they work, but we have a lot more questions than we do answers," said Nina Young, a research scientist at the Center for Marine Conservation. "Whenever you have a new technology, you have to enter into it slowly and determine the effects."

Fishery managers believe that a few low-decibel pings in the ocean, usually inaudible 300 meters away, have to be better than killing porpoises in Maine or sperm whales in California.

"I'm a firm believer," said Tony West of Los Angeles, who is one of 90 California fishermen who must use pingers when deploying drift nets for swordfish and thresher shark.

"We're in the infancy stage, so there's a lot of improvement needed from a practical standpoint from the fisherman's point of view," he said. "But I'm ecstatic about the success we've had. . . . And they haven't hurt us one bit in terms of catch." Pingers may offer the last, best hope for troubled fisheries around the world.

Gill nets -- which can stretch a mile across the sea -- are the world's most common commercial fishing gear. But the mesh often unintentionally ensnares marine mammals, perhaps hundreds of thousands a year globally.

In the United States, the Marine Mammal Protection Act forbids fishermen from killing mammals in numbers considered unsustainable for a population. In some cases, government officials believed that the only way to meet those rules would be to shut down a fishery. But the invention of pingers has averted such draconian steps.

"Anyone involved with this issue . . . knows there wouldn't be a gill net fishery in New England if pingers didn't come along," said Roland Barnaby, a fishery education specialist at the University of New Hampshire's Sea Grant program.

The pings may be the undersea version of highway reflectors, warning marine mammals to slow down and pay attention. Scientists hope that the noise is not loud enough to scare them, but instead warns them that an obstacle lies ahead.

The ocean is a noisy place, and the devices are programmed to broadcast the pings at a level as close to background as possible while still being loud enough for an animal to hear at least 100 meters away -- in time to change course. The signals are sent at 15 to 30 decibels over ocean noise, or 132 decibels (equivalent to 72 decibels above ground, the level of a light conversation). "We wanted the minimal amount that would still work but wouldn't make the ocean sound like a lot of racket," said Scott Kraus, a scientist with the New England Aquarium who is a leading expert on pingers. "You could put your head right next to one of our pingers and it wouldn't hurt you."

The pulses are dispatched every four seconds. For undersea animals, who use sound to help them "see," the pings illuminate a net like a street lamp.

Whales or dolphins apparently blunder into nets when they are swimming along, perhaps in a sleepy daze. Hearing the unusual ping, the animals might turn on their echo-locators -- their sonar -- and then they can sense the net.

"We hope they are alerting devices, but we don't know exactly what it is the animal is responding to," Kraus said. "Is it like a yellow line on the highway that alerts you that you're running off the road, or is it actually scaring you, making you run away from it?"

Despite the success stories, fishermen and government regulators must be cautious in how and where pingers are deployed. They probably will not work for every fishery or for every species of marine mammal, so each area must be studied first.

"We've never said the pinger is the panacea for the by-catch issue. It's just another tool," Kraus said. "There will always be animals that make mistakes. We will never eliminate by-catch altogether unless we eliminate fishing."

© 1998 Philadelphia Newspapers Inc.