"Navy sonar got our attention, but now we're looking at low-frequency noise and thinking, 'Wow, this could be very important,' " said Aran Mooney, a zoologist with the Woods Hole Oceanographic Institution in Massachusetts.
Marla Holt, a marine biologist at the Northwest Fisheries Science Center in Seattle, said, "We've opened up this whole new area of research and are looking at acoustic exposure in new ways."
Now, rather than the discreet, damaging event, "it's that consistent, ubiquitous exposure that may be the most concerning," she said.
Pumping up the volume of background noise has well-known physiological effects on humans. It can alter hormone levels and increase blood pressure, and prompt us to change our behavior. Same, too, with some fish, which boost the release of the stress hormone cortisol in response to white noise.
"The analogy I use, having grown up in New York City, is when you see someone jackhammering a sidewalk, do you go out of your way to walk around it or do you just walk right by," said Art Popper, a biology professor who runs the aquatic bioacoustics lab at the University of Maryland.
But the impact of sound varies by species. Goldfish have excellent hearing. Salmon and trout don't. Humpback, fin whales, right whales and bowhead generally flee all types of noise. But how important a sign is that, really? Does it mean animals are driven from important feeding or mating grounds, or is it merely a nuisance?
A chief concern is the potential for "auditory masking." Killer whales, for example, have been shown to raise their voices when their group-specific calls are being drowned out by noise.
But is that as harmless as humans talking a little louder -- or is it the equivalent of regularly screaming at the top range?
"Is there an energetic cost to all of this?" Holt asked.
Scientists don't really know.
One of the most dramatic findings came just last month, when Spanish researchers captured cuttlefish, squid and octopus and exposed them to low-frequency sounds at low-intensity -- similar to what may be found in shipping lanes. The animals developed lesions in nerve fibers within their sensory systems.
"What was surprising was just how massive the trauma was, and at small levels of exposure," said scientist Michel Andre, at Technical University of Catalonia in Barcelona, Spain.
It was the first evidence of significant harm to an invertebrate species that plays a key role in the ocean food chain.
"It was a cool first step. I think most people didn't really expect that," said Mooney. "But they made some mistakes. It needs to be repeated."
Popper was more blunt. The researchers, he said, made so many errors that he can't trust the findings.
But Popper agreed the impact of noise on cephalopods like squid is a significant unanswered question. "It's really important," he said.
Popper is focused largely on fish, studying noise impacts on them in lakes and rivers.
"The results I'm getting so far is that fish may be hurt a little, but it's more like getting a cut on your finger," Popper said.
Will that always be the case?
"I don't know," Popper said. "I don't know whether or not to be concerned because I don't have the data."
At least not yet.
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