It was a bit over a decade ago that a catastrophic whale stranding event brought the public’s attention to the fact that noise in the ocean could be lethal to marine mammals. The particular stranding was a consequence of mid-frequency navy sonar used during military exercises in the Bahamas. It was a tragic coincidence that the stranding occurred just when the public was reviewing a US Navy proposal to ensonify the entire ocean with Low Frequency Active Sonar.
This coincidence had an amplifying effect on the public’s concern about the impacts of human-generated ocean noise pollution. While there had been some scientific research on the issue it was spotty. There was little more than anecdotal evidence of noise impacts of any sort, but the public concern did drive the inquiry. Soon the Office of Naval Research was funding studies on the impacts of sonar on whales, but also on the larger questions of what other noises might be problematic – and what other animals may be at risk.
It didn’t take much of a ‘look under the hood’ to understand that in addition to new, powerful military sonar systems there were other noises polluting the sea. The mass globalization of trade had been expanding the shipping fleet since the 1960’s with the global dependence on fossil fuel introduced super-tankers into the mix – to the extent that right now there are over 100,000 large ships producing noise in the ocean – making it ten times louder than it was in 1960.
And then there is the oil… Far from the “peak oil” concern of yore, offshore oil deposits promise enough fossil fuel to cook our planet a few times over. Finding and retrieving this oil brings in its own set of noises. One of the more significant noises in the ocean is the din of seismic airgun surveys used to map the reservoirs of oil beneath the seafloor. Survey ships trawl large arrays of airguns that kick endless trains of explosions which in many cases can be heard for thousands of miles across the sea.
These surveys are not just occasional events either; as you are reading this there are over 50 seismic surveys being conducted around the world. If you put a hydrophone at the equator over the Mid-Atlantic ridge you could record seismic surveys occurring off the coasts of Angola, Argentina, Brazil, the Gulf of Mexico, and up in the North Sea.
As the petroleum is found and exploited additional noises are introduced by seafloor mounted extraction equipment which pump and process oil, gas, brine, sand, and drilling fluids often under extreme pressures (not a formula for quiet). These installations look much like underwater cities grinding, whirring, humming, and roaring away. And even this equipment has its own set of intentionally introduced noises – as they are often situated on multi-nodal acoustic communication networks which exchange data across that same frequency bands that dolphins use for their bio-sonar.
Unfortunately unlike airborne noise that is loosely coupled to the environment and attenuates by interference with features in the landscape, waterborne noise couples quite well to the environment and easily propagates over really long distances. It does not take a lot of energy to make a noise that can be heard a thousand miles in the ocean, but if you made a noise in New York City that could be heard in Pensacola Florida, you’d have some explaining to do.
The fact that water transmits sound so efficiently is why this issue is important. Animals use ocean water as a communication channel – to find food, communicate with kin, avoid predators, and sing out their breeding passions. Marine animals have evolved these communication channels over millions of years – establishing finely detailed and adaptively situated “acoustical niches.” All of a sudden, in a matter of 50 years all of these really noisy neighbors move in, blasting all manner of noise, across all bandwidths, without regard to the existing acoustical life in the sea.