By Michael Stocker
Noise pollution has now become one of the common themes of human-generated impacts on the ocean. Shipping noise, military sonar, and seismic airgun surveys are increasingly becoming part of the public discussion in marine conservation. These noises are easy for us to understand; they are loud, ubiquitous, and they are all in the range of human hearing. We can all imagine what it must be like having an expressway of supertankers and cargo vessels plying the shipping lanes over our heads, or being subjected to ear-piercing tactical sonar signals.
But there is a flood of noises creeping into the ocean that, while we humans can’t hear them, may prove equally as insidious as the loud noises that we can hear. Dolphins, porpoises, beaked whales, and sperm whales – the “toothed whales”–use high frequency bio-sonar, so their sound frequency sensitivities reach well above the frequencies that we humans can hear.
Some of their fish prey can also hear these higher frequencies as an adaptive measure against predation. And while we don’t yet have evidence of seals using bio-sonar, we do know that many seals also hear sounds well above the highest frequencies that humans can hear.
While marine technologists don’t seem to be giving it a lot of thought, our sonar technologies are increasingly crowding out these higher frequency bands with underwater acoustical beacons, echo sounders, and underwater communication systems. The spectrogram below (and the ones accompanying the sound examples) is a method of visualizing sound with time on the horizontal “x” axis, and frequency on the vertical “y” axis. The lower frequencies are closer to the bottom.
This particular spectrogram from NEPTUNE Canada displays a year of sound near the sea floor in the ocean off of Vancouver Island.