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Ancient whale song: louder than modern human-caused ocean noise?

Bioacoustics, Ocean, Ocean energy, Science, shipping Add comments

AEI lay summary:
M.S. Stocker, J.T. Reuterdahl. Is the ocean really getting louder? 164th Meeting of the Acoustical Society of America, Oct. 22-26, 2012.  ASA press release

A paper presented at this fall’s Acoustical Society of America meeting has triggered a wave of provocative headlines about how whale-filled oceans of the past may have been “as loud as a rock concert” or how “Noisy whales made FAR MORE oceanic racket than humans do.”  The paper does indeed ask an innovative question: what was the ocean soundscape like in the pre-whaling days?  And its answer, while couched in a high degree of uncertainty, is also eye and ear-opening: ten times as many whales made a lot more noise than today’s diminished populations, perhaps adding up to overall noise levels that match those caused by today’s shipping, oil and gas exploration, and other human activities in the seas.

It’s probably a bit too acoustics-geeky for me to object to the “rock concert” comparison, but for the record, the paper itself never uses those two words alone or together!  (Though somehow the phrase slipped into the ASA press release; we’ll have to blame the editor or an author’s pre-conference sleep deprivation for the slip…)  In fact, thanks to their differences in density, measurements of sound in water are about 62dB higher than a similar sound level in air; thus, the mention of 126dB of whale sounds filling the ancient oceans would not be equivalent to a rock concert, but more like the 64dB sounds of laughter or a loud conversation.  Which, as it turns out, much better captures the essence of the historic soundscape as described by the authors:

The bio-acoustic environment of the pre-industrial whaling ocean could be correlated to the animal sounds in any biologically diverse and well populated habitat wherein the riot of birdcalls, the stridulation of insects, and the mammal vocalizations are the dominant noise contributors to the soundscape.

The question of ancient ocean sound levels is relevant because much of today’s thinking about the impact of human noise is predicated on research that shows global shipping increasing the ambient noise levels in the oceans by 10dB or more since the 1950’s.  This just happens to be the era in which whale populations were at their nadir, with several species having already become rare enough that it was no longer worth the effort to find and hunt them.  But as the authors stress in their conclusion, an ancient ocean full of whale song — along with the more widespread  sounds of the onomatopoeia-ic large fish, Grunts and Drums, or the “great schools of tuna miles across (that) would churn up the sea surface for days as they migrated past California’s Channel Islands” noted by early 20th century fishermen, which the authors note “would likely be as loud as or louder than even the most tempestuous sea state” — is a very different place than an ocean full of the noise of ship engines and airgun reverberations.

Animals have evolved to fill distinct acoustic niches, with their songs and calls at different frequencies, made at different times or the day or year, and using distinct rhythmic pulses; each species’ hearing is especially tuned, or filtered, to pick out the calls of its own species from the voices of others.  But, as the authors note, “the signature of mechanized ocean noise interference from shipping is broad-band, pervasive, and chronic, and more likely to mask across animal frequency and/or time domain filters throughout large areas of the ocean.”

Or as put so well by Tim Baribeau, it’s important to remember that “we could still be disorienting whales with our bizarre and intrusive industrial sounds. But it’s incredible to think that the oceans may once have been filled with a cacophonous background chatter of animal noise.”

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