For many animals in the ocean, sound is everything. Some depend on it to find food, some need it to communicate with others, some need it to navigate. But the ocean, once known as the silent deep, has become a much noisier place.
Scientists at the Scripps Institution of Oceanography in San Diego say the noise level in what should be a fairly quiet zone more than 180 miles offshore has risen significantly over the last few decades, apparently because ships are bigger, faster and more powerful than they were before.
"The levels are high enough that it's of concern," says John Hildebrand, professor of oceanography at Scripps, which is part of the University of California, San Diego.
Since 1964, the noise level has risen 10 to 12 decibels, according to a report by Hildebrand and colleagues in the August issue of the Journal of the Acoustical Society of America. A decibel is a unit of measurement of sound intensity, and while 10 to 12 might not seem like a lot, it could be quite significant in the ocean.
"In background noise it's roughly the difference between sitting in a library and being in a Wal-Mart," says oceanographer and co-author of the report, Mark McDonald of Whale Acoustics, a small research firm in Bellvue, Colo.
The scientists don't know exactly how much of a problem the increase in noise poses for marine animals. Since the change has taken place over decades, it's hard to isolate a single cause and effect. But what is clear is the fact that the ocean has become much noisier, and that can't be good for many critters.
Whales that use sound to communicate "won't be able to hear each other as far away," says McDonald. "But is that a problem?"
Whales, he says, have shown an ability to adjust to changing conditions.
For some time now, Hildebrand, McDonald and Sean Wiggins, also of Scripps, have been monitoring the noise level with a sophisticated instrument on the bottom of the ocean near San Nicolas Island, off the coast of San Diego. They have been able to document the change partly because of a bit of good luck.
In the 1960s, the Navy was also monitoring noise levels in the area as part of its anti-submarine warfare program. That research was recently declassified, and the scientists were able to acquire it.
That gave them the background data they needed to see if earlier projections of an increase in ocean noise had actually taken place. Their research shows that it has, and it probably extends across the North Pacific because a huge ship sends out acoustic waves that can travel across the entire ocean.
"The place we monitored is not directly in the shipping lanes," says Hildebrand. "It's remote from the shipping lanes, and the sounds we are hearing are probably propagating all the way down from the Aleutian Islands and across from Asia." The scientists think the newer giant commercial ships that ply the oceans account for most of the increase. It's in the Navy's interest to keep its ships as quiet as possible, for obvious reasons, but there's no incentive for commercial operators to do the same.
The scientists say that over the past 38 years, the number of ships in the world's commercial fleet more than doubled, from 41,865 in 1965 to 89,899 in 2003. But doubling of the fleet should produce an additional three decibels, not 10 or 12, Hildebrand says.
So what's causing that jump?
"Our best guess is individual ships are radiating more acoustic energy than they did back in the 60s," Hildebrand says. "The ships are bigger, faster, they are carrying more cargo, and all of that requires more propulsion power. Some fraction of that propulsion power gets radiated out as acoustic energy."
McDonald, who is a consultant to a number of companies and government agencies, says it's possible to reduce the noise level by such things as using rubber engine mounts, and changing the propellers, but it would be hard to force the industry to do that without strong evidence that the noise is having an adverse impact.
That's going to be hard to do, but history would suggest that the problem should not be ignored.
Gray whales migrate from the Arctic to Baja, Calif., each year, and historically, they have been particularly fond of one lagoon where they breed and calve.
"In the late 60s or early 70s, there was a salt works installed in the lagoon (to extract salt)," Hildebrand says. "There was a fairly well documented population of animals that would come into the lagoon and use it for calving. When the salt works started up, they abandoned the lagoon.
"All the time the salt works was active, the animals avoided the lagoon. The salt works failed for economical reasons. After it failed, it took about five years for the animals to start to come back and reoccupy that lagoon. So there's a case where you can document exclusion from a preferred habitat."
Did the whales stop procreating? Hardly. Their numbers have grown significantly over the years, so once again, it's hard to measure the impact, even of an industrial activity.
But there's no question that the ocean has changed through pollution, over-fishing, and now, we know, noise.
"It wasn't a pristine environment in the 1960s," when the Navy monitored the area, Hildebrand says. "So there's probably another 10 decibels or so to get back to the primordial state. So it's 20 decibels above the conditions that these animals evolved in, and that's a big number."