Wave Lenght Matters

We have more and more noise in our oceans. Much of the noise is shipping noise that is always present and quite often the dominant noise (example).

Admittedly most of my work is a result of legislation forcing proponents to show that they don't violate noise limits. But ultimately it is a result of political decision to protect our wildlife and the ecosystems both for their intrinsic value, but also because we realise the need to protect our planet for the future.

Sometimes the motivation for legislation comes from a non-logic human feeling of wanting to protect something pristine or cute. I think many people believe that when we talk about limiting ocean noise we do it for the little whales - think dolphins and porpoises. This is of course not untrue, but you need to be quite close to a porpoise for it to hear you!
The main reason for this is that porpoises' hearing is very well adapted for shutting everything but their own echolocation clicks out (see this blog post). In other words, they don't hear very well at the frequencies where we're most noisy. This is also true for most other echolocating whales (read more here).

It's a little hard to obtain hearing thresholds for baleen whales (they need a really big pool!), but their hearing has been estimated from their vocalisations (source).
Figure 1. The estimated hearing thresholds (green lines) for a baleen whale (minke whale, left graph) and typical hearing thresholds for toothed whales on right graph, both superimposed on marine noise plots. Notice how minke whales' most sensitive area coincides with shipping noise.
I wanted to show this in dBSea by modelling different frequencies, and see how far they would spread in the Atlantic. Below is a model of noise in the Atlantic (Figure 2). To get any sort of spread of noise from the higher frequencies I had to amplify them substantially (see figure discription for details). Even though the 25-160 kHz noise source is 40 dB (100 times the pressure) higher than the 12.5-80 Hz noise source, no levels are visible on the map. The reason for this is that high frequency sound attenuates at much greater rates than low frequency sound, and the scale of the map is simply too big to show the limited extend of the noise.
Figure 2. The spread of noise from multiple sources in the Atlantic, weighted for minke whale hearing. The frequencies are shown on the figure. The noise levels are not the same for all sources. From left to right. noise levels are: 150, 155, 160, 170 & 190 dB re 1┬ÁPa.
All this means that on a global scale, high frequency noise is only ever a very localised problem, but low frequency noise, because it spreads so much further is almost always a regional problem.
I want to make it clear that local noise pollution can indeed be very harmful as many of the species using high frequencies live in coastal areas with many higher frequency noise sources from human activity.

But what's the big deal - why is it a problem that we mask the sound communication of the big whales?

According to EU legislation, countries are to aim for marine environments where:
“Introduction of energy, including underwater noise, is at levels that do not adversely affect the marine environment” - EU legislation

So it's illegal to "adversely" affect the marine environment. That still doesn't tell us why it's important. One very interesting reason why it's important is highlighted in this article (popular, scientific). The paper shows how big mammals in the sea are vital for nutrient circulation in the seas, and for nutrient transport to land, where we use it to grow food.

Thanks for reading - post to be updated!

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