NOAA/NMFS Acoustic Guidelines reviewed

I’ve just finished the OCR review and critique of National Marine Fisheries Service (NMFS) draft “Marine Mammal Acoustic Guidelines” – a proposed set of metrics to help determine when exposing marine mammals to noise will induce “Temporary” or “Permanent”  Threshold Shift (TTS and PTS respectively). A “Threshold Shift” is when loud noise exposure compromises the hearing of an exposed animal such that it decreases its hearing sensitivity. Most of us have suffered this threshold shift after spending some time at rock concerts or in noisy restaurants to find that our ears are “stuffed up” or ringing.

The new guidelines are a significant improvement over the guidelines that have been in effect for the past decade inasmuch as they tune into the hearing frequency ranges of various marine mammals, segregating them into “Low Frequency Cetaceans” (the baleen whales), “Mid Frequency Cetaceans” (most of the toothed whales and dolphins), “High Frequency Cetaceans” (porpoises and river dolphins), seals (Phocids), and the sea lions (Otariids). One of the consequences of this segregation is that noise source impacts will be more closely correlated to the particular impacted species. So, for example, dolphins and porpoises will not figure heavily into low frequency seismic survey impact regulation, and blue whales will not be featured in high-frequency communication sonar exposures.

You might say that this is a step backwards for conservation principles as it allows humans to push more tightly into marine mammal’s bio-acoustic niches. But this segregation of hearing groups also allows for a “form-fitting” of hearing thresholds to match the specific specie’s sensitivities, permitting a lowering of some of the ‘legacy’ exposure regulation thresholds.


So good enough; we have an adjustment that is more finely tuned to species; humans are permitted to occupy more acoustic space, but we have to be a bit quieter.

Now the shortcomings with the guidelines: Aside from the basic problem that our habit of dividing our entire living planet into functional little bits has not represented an improvement for anyone; the data that the specific functional little bits in the guidelines are based upon depend heavily on just a handful of captive animals from six different species of marine mammals (beluga whale, bottlenose dolphin, harbor porpoise, harbor seal, elephant seal, and California sea lion) representing some 125+ species of cetaceans and some 33 species of pinnepeds.

Additionally, all of the subject animals are captive and habituated to receiving food (rewards) for tolerating abuse (stimulus). I am not deprecating the fantastic work of the many humans and the few animals that are performing these tasks. The work they do is indispensable in understanding  how these animals hear, and for the most part the researchers love and treat their animals really well. But as such these captive animals are a poor proxy for the wild animals that depend on hearing acuity to survive in the midst of real predators, and real prey.

Then there is the problem of the exposure signals. In the lab it is convenient to use sounds that are easy to quantify – most commonly “pure tones” and band limited “Gaussian noise.” These signal types work well in the ‘math department’ but do not really represent the types of noises that humans are putting into the ocean. This situation might be equated to measuring taste-tolerance using treacle to determine ingestion-tolerance for sawdust. Humans are putting a lot of annoying sounds into the ocean informed by lab-derived exposure thresholds, and the exposure metric is only based on how loud the signal is, not how lousy it sounds. This is not a sentimental comment; it is well known that certain types of noises induce greater hearing loss than others.

There are a number of other shortcomings in the draft guidelines. I hope that the NMFS heeds the advice that we in the NGO-conservation community provide. The new guidelines are a step in the right direction, but where data is scarce precaution should prevail.