周越

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， even as far north as the Bay of Fundy in Canada. As a result， underwater noise in the bay dropped by an incredible six decibels， and sounds at pitches below 150 Hz were significantly reduced. The area 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 faecal matter for stress hormones， they found the lower human|caused ocean noise was leading to lower stress levels.

Marine animals like whales use sound to do everything from communicate and travel to look for food and find safe environments. Sound travels faster and further in water than in air， and marine animals take advantage of it. But this also means that when there's a near|constant hum of underwater 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 noise present along major shipping routes. 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 endure most of the time.

For decades now， scientists around the world have been studying just how impactful this noise can be on marine animals. Now， they are beginning to identify the measures that， if widely adopted， 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 reverse side 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， and are often impacted. The scientists have found that dolphins adjust their calls when it is noisy underwater， most likely so they can be heard better by other dolphins.? This is similar to when people shout louder when they are talking in a noisier bar.

By "adjust" means simplify， as anyone might do to try to relay a message when there's a lot of ambient noise. In a 2018 study， the researchers recorded underwater noise caused mainly by boat traffic in the western North Atlantic up to 130 decibels， on par with a busy highway. If dolphins are regularly trying to communicate with such interference， it's safe to assume a lot gets lost in translation.

Chronic， low|frequency sound also impacts the ability of young fish to find home. Juvenile fish use sound to suss out their ideal marine ecosystem. They listen for a diverse soundscape， which indicates that there's abundant resources for lots of different life. When anthropogenic sounds block these natural soundscapes， they may end up in inhospitable environments. （Sadly， at the same time， mass bleaching events are killing off fragile coral reef systems， leaving less life to make sounds there and attract the juvenile fish | a harmful feedback loop which can further damage the resilience of reefs.）

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 | it fundamentally impacts every aspect of their way of life. "I think sound is as important to whales as all of our senses put together，" he says. "They can feel it vibrating throughout their whole body."? Williams has studied killer whales for decades， including the southern resident killer whale in the northeast Pacific Ocean， which is among the world's most critically endangered animals due to a dwindling food supply， pollution and ocean noise. According to a 2017 study Williams co|authored， anthropogenic ocean noise can keep these whales from feeding as much as they normally would if the noise was not present.

"We're showing that when boats are too close， and the boats are making noise， the killer whales are spending 18-25% less time feeding than if there were no boats around，" he explains. Williams says the team also found that the whales' calls only reached about 62% of the distance they would in a marine environment not populated by boats and ships.

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 by fleeing or changing migration patterns， making it harder for the killer whales to catch them.

Anthropogenic noise affects other prey fish in different ways. A 2016 study found damselfish don't hear their predators approaching as well when motorboat noise is present | making them more susceptible to being eaten. In fact， more than twice as many fish were eaten when motorboats were passing than when they weren't， which suggests anthropogenic sound is directly linked to an increase in fish mortality.

It's difficult to assess exactly how detrimental anthropogenic noise is to all ocean wildlife since it appears to impact some species more than others. Most studies on the subject， however， suggest that the auditory interference inhibits marine animals' abilities to hear and react to biological sounds， which in turn decreases their ability to survive. And， in the case of critically endangered species， like the southern resident killer whale， experts like Williams believe such an impediment could ultimately put the species on a fast track to extinction.

The flip side of the ocean noise problem is that it is one of the few human|fuelled pollution sources that has several relatively straightforward solutions.