Exposure metrics in relief

I first dug into this ocean noise issue back in 1992, on the heels of a proposal called “Acoustic Thermometry of Ocean Climate (ATOC) which was to use long wavelength sounds to measure water density variations across the ocean much in the way that ultrasound is used to image tissue densities in the human body. As there is a correlation between water temperature and density, ATOC would be a way to measure the aggregate, as well as regional temperature variations in the sea.

The folks promoting it were physical oceanographers and weren’t really considering the impacts that basin-scale acoustical energy might have on marine life.

It didn’t assure the public that the source level was 185 decibels (dB re: 1uPa), given that the threshold for pain for in-air noise is 125dB (re: 20uPa). So not understanding the differences in how sound couples in water vs. air, and how that is expressed numerically, was the first catfight between the public and the scientists.

It also didn’t help that one of the assurances that the ATOC proponents gave to secure their permit was a promise to watch whales carefully, and stop if there was any indication of adverse reactions. And two days after the commencement of the testing, two whales washed up on the beach – and the ATOC team didn’t have any answers…

While I wasn’t a huge fan of the project, it wasn’t so much the ATOC signal that bothered me. Some of my first published work on ocean noise was trying to explain to a lay audience what ‘decibels’ were. For me the flashing red light in my dashboard was the military and industrial use of the ocean as an acoustical communication channel.

 My apprehensions materialized a few years later while the US Navy was proposing a submarine surveillance project called “Low Frequency Active Sonar” (LFAS). This time with a source level of 235dB. It was in the midst of the public hearings on LFAS that a catastrophic stranding occurred in the Bahamas, coincident to a US Navy exercise. Seventeen beaked whales and a minke were found floundering or dying on the beaches of Great Abaco Island.

Now the Navy – and the Regulators had a problem on their hands. Even while there was a lot of denial about the evidence in the Bahamas incident, similar standings started showing up around the ocean – coincident with Naval exercises.

It was the National Marine Fisheries Service (NMFS) under which the regulations fell. There was already harassment definitions – “Level A” – physical damage, and “Level B,” behavioral disruption. They just needed to be reconciled to acoustical exposure impacts. A few more catfights ensued, and initially settled on Level A being an exposure that would compromise hearing, and a somewhat arbitrary Level B on an exposure that would cause behavioral disturbance.

I say “somewhat arbitrary,” because it was clear that these regulatory thresholds were actually too simple. When dolphins were riding the bow waves of seismic survey vessels – frolicking in a Level A noise field, it was apparent that the regulations did not reflect common field conditions.

This was addressed in what became known as “Southhall 2007” which eventually informed the current NOAA Fisheries noise exposure guidelines. What the paper proposed was a segregation of protected animals into their particular hearing regimes. The dolphins were not bothered by the Level A seismic survey exposures because most of the noise was in a frequency band that dolphins don’t really hear. And it was likely that blue whales were not too bothered by high-frequency fish-finders. An these data should be integrated into the regulatory thresholds.

Given the characteristic efficiency that unambiguous regulations are hacked out between industry, the military, and the conservation community, it only took nine years for the regulations to catch up to the science. Of course the science did not stand around waiting…

Dr. Southall has probably done more to sort out the regulatory shades of gray than anyone else in the field. I mention this because last week he sent me a “Risk Assessment” document which looks at protected marine mammal species in the context of Atlantic windfarm construction and operations.

Sponsored by the Bureau of Ocean Energy Management, the beauty of this approach is that we don’t have to wait another ten years for the findings to become regulations, rather they provide some guidelines in applying existing regulations to inform current practice. Something that can be done as the windfarms are being planned, developed, built, and operated.