Reducing ambient ocean noise is hardly something that most people would think about on a regular basis. However, imagine your upstairs neighbour was having work done on their apartment and you had an important work presentation to give over a video call. You’d find it quite difficult to hear and communicate with your colleagues, and do a proper job. That’s what marine animals who live or migrate near anthropogenic noise (man-made ocean noise) endure most of the time.

     In the aftermath of the terrorist attacks on 11 September 2001, a great lull fell over North America, above and below the water. Understandably, fewer people were traveling by air, but ship traffic was also significantly reduced. In the Bay of Fundy in Canada, underwater ocean noise dropped by an incredible six decibels, to below 150 Hz: That’s equivalent to soft rain or cat’s purr.

     The Bay of Fundy is frequented by North Atlantic Right Whales, so scientists at Duke University decided to see if the quieter waters had any impact on the giant mammals. Sure enough, after analysing their fecal matter for stress hormones, they found the lower human-caused ocean noise was leading to lower stress levels in the whales.

     Marine animals, like whales, use sound to do everything, from communicate and travel, to looking for food and finding safe environments. Sound travels faster and further in water than in air, and marine animals take advantage of it. However, this also means that when there’s a near-constant hum of underwater ocean noise pollution from things like boat traffic, it can severely impact their way of life. Over the past 50 years, increased shipping has contributed to a 30-fold increase in the low-frequency ocean noise along major shipping routes. Scientists around the world have been studying just how this noise can affect marine animals, and they are beginning to identify the measures that could save many species from the impacts of this overlooked form of pollution.

     Anthropogenic ocean noise comes from a huge variety of sources, from military sonar and aircraft landings, to the construction of offshore wind farms, and seismic surveys used to explore for oil and gas. But the most common source is boats, specifically from their propellers. When propellers, especially older ones, turn at high speeds, they can create a drop in pressure on the backside of the propeller that results in a lot of bubbles, and low-frequency noise – an effect called cavitation. Cavitation also makes boats less efficient because the propeller is expending a lot of energy, some of which isn’t helping to push the boat forward.

     This low-frequency sound has a long range, so it can disrupt marine animal communication across a wide area. Bottlenose dolphins, for example, use all sorts of sounds to communicate with each other, some detectable by other dolphins over 20km away. “We have found that dolphins adjust their calls when it is noisy underwater, most likely so they can be heard better by other dolphins,” explains Helen Bailey, research professor at the University of Maryland’s Center for Environmental Science. “This is similar to when we shout louder when we are talking in a noisy bar. However, it is safe to assume that, just like in a bar, a lot gets lost when dolphin’s have to “shout”. Chronic, low-frequency sound also impacts the ability of young fish to find home. Juvenile fish use sound to understand their ideal marine ecosystem.

     Ocean noise pollution is an especially big problem for whales, who regularly use sound to touch base with each other. One 2012 study of blue whales found the mid-range sound from ships’ sonar overlaps with their calls to each other, forcing them to repeat themselves as if they were losing connection on a mobile phone. “It literally shrinks the whales’ world,” says Rob Williams, marine biologist and founder of Oceans Initiative, a non-profit working to protect marine life. Williams believes anthropogenic ocean noise is just as much a threat to whales as deforestation is to grizzly bears.

     According to a 2017 study Williams co-authored, anthropogenic ocean noise can keep killer whales from feeding as much as they normally would, if the noise was not present. Ocean noise also impedes killer whales’ ability to catch chinook salmon and herring, their primary food sources. A recent study that evaluated the behavior of these two prey fish found that they often react to boat noise as if it were a predator. They flee or change their migration patterns, making it harder for the killer whales to catch them.

     The good news is that ocean noise problem is one of the few human-created pollution sources that has several relatively straightforward solutions. One of the easiest ways to reduce ocean noise is to simply get ship and boat traffic to slow down when moving through areas rich in marine life. “Something as simple as slowing down a few knots makes a major drop in the noise level”, reports Williams. “And the killer whales feed more. Moving ship lanes away from areas populated by sensitive species could also be beneficial”, he adds. “But in order for noise abatement endeavours to have a widespread effect on marine life, they will have to be backed by sweeping public policy”.

     The Navy has learned how to solve the cavitation problem for submarines, but this technology has not yet been required for commercial vessels. Time to act politicians……..yet again!

     As long as humans are around, the noises they bring with them will likely remain a part of ocean soundscapes. But like any other type of harmful pollution, ocean noise needs to be regulated in a meaningful way if we are to keep marine animals’ sound-driven worlds from shrinking away.

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