What do they really hear?

Earlier this year a paper by Christne Erbe et.al was published in the open source Public Library of Science (PLoS-One) that frames ocean noise exposures in a sensible and informative manner. For various adaptive reasons animals are sensitive to sounds within frequency bands that are useful to them, and are not sensitive to frequencies that are not. By way of example: it does not serve human perceptual needs to hear the high frequency echolocation signals of bats; on the other hand some moths take a keen interest in bat hunting sounds – so they can evade predation.

It is also well known that odontocetes (porpoises and dolphins) have and extremely detailed command of high frequency sounds for echolocation, but have little use for signals much below 1000Hz. Meanwhile their baleen-bearing cousins, the great mysticetes, have little use for high frequencies and focus their attention (and adapted sensitivities) on low frequencies – for long distance communication and navigation.

Colors indicate how loud the noise sounds for different species

Colors indicate how loud the noise sounds for different species

But even species within the same family or genus have varying frequency sensitivities depending on how and what they do with sounds. This explains why some dolphins are highly perturbed by certain sounds that don’t seem to bother other species in the same family.

This situation has complicated noise exposure regulations around marine mammals because the regulations have been set on how much noise is in the exposure without accounting for what sort of noise it is or what a given species can hear. Only recently has hearing bandwidth sensitivities been taken into account in setting exposure standards in broad-brush terms – segregating the hearing of marine mammals in terms of low, mid, or high frequency sensitivity. These standards will more accurately reflect potential noise impacts relative to the receiving animal and put to rest some of the contention around the disparities in regulatory thresholds, noise exposures, and observed impacts.

The Erbe et.al paper details species sound sensitivities and overlays them on modeled shipping noise in the North West from Seattle to Price Rupert (shown in the illustration). They then overlaid a species density map which pulled out “hot-spots” – areas where due to noise density and population densities the exposure risk was the highest relative to a given animal.

Mapping like this helps stakeholders and regulators understand, and thus plan activities and mitigate for noise exposure impacts in consideration of spatial, seasonal, and biological interactions. This approach represents a significant improvement over the “barge ahead and figure it out later” methodology that has dominated marine enterprise since the first use of explosives and fossil fuel in the development of the sea.