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 , 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.
(Spectrograms courtesy of Carrie Wall Bell and Francis Juanes of NEPTUNE Canada)
In the figure there is a thick cyan line just between the 30 kHz and 40 kHz index lines going across the entire year. This is from an upward-looking echo-sounder used to measure ocean currents. This signal is way above our hearing range, so we call it “ultrasound.” But this sound is right in the middle of the hearing range of orcas, dolphins, and porpoises.
The echo-sounder signal is not complex, but it is persistent. Communication signals on the other hand are necessarily complex and can sound quite obnoxious (these examples are in the human auditory range).
Increasingly, these types of sounds are being used to control equipment, report on sensor conditions, and even monitor the movement of tagged sharks. Given that some of these high frequency signals are designed to broadcast up to 10 kilometers (6 miles), the increasing density of these signals in the ocean may be cropping up as a problem for the animals that can hear them.
Yet relief may be within reach. While the ocean is getting louder with the sounds of mechanization and technology, it was not necessarily quieter before the industrialization of shipping. The ocean was probably a pretty noisy place before the 20th century due to biological noise. Since the industrialization of whaling and fishing millions of whales and perhaps 90% of all fish have been pulled out of the sea–along with all of their noises.
The difference, of course, is that these “legacy noises” were natural. This may or may not be significant with the broad-band mechanical noises of ships or the loud pulsing of seismic surveys, but it is possible that technical communication signals could be crafted to sound more like animal communication sounds, and less like the antagonistic sounds currently in use.
We know that some of these community animals can be quite loud, and that for the last 30 million years they have been swimming around in large groups. If technical communication signals sounded more like animal communication signals they may fit right in!
Subsea telemetry acoustic transceivers used in offshore oil operations Illustration: Nautronix