They may seem innocuous, but the noises our machines make underwater may be hurting wildlife, according to two recently published studies.
The first study, summarized on this blog last week, found that ship noises could hurt endangered killer whales. But a second, published in Nature Communications, finds that noise pollution may have a far more severe effect on smaller fish: It can be fatal.
The effect such noises have on fish matters; as the team of British, Canadian and Australian researchers points out in their paper, the world's coastal regions are "experiencing unprecedented human population growth." That expansion brings with it disruptive technologies.
“The combination of stress and poor responses to strikes by predators is why these fish became such easy prey,” Andy Radford of the University of Bristol said in a statement.
To measure the disruption, the team of researchers monitored predator and prey fish in tanks and "small isolated experimental patch reefs on sandflats" near Australia's Great Barrier Reef. Some were exposed to common aquatic ambient noise, while others were exposed to that noise mixed with motorboat sounds. (Some were exposed to actual motorboats, too.)
The effect of the motorboat noise was pronounced, as the chart below from the paper shows. The presence of the boat was significantly detrimental to survival of Pomacentrus amboinensis, the damselfish they studied.
Of the fish exposed only to the ambient noise, 79 percent survived their first three days. Over the same period, just 27 percent exposed to the boat noise survived.
To better understand what was going on, the team of researchers, led by Stephen Simpson of the University of Exeter, identified four ways the fish were affected by the noise.
First, it appeared to make them breathe more heavily. Fish that listened to the boat noises consumed 20 percent more oxygen over half an hour than those that hadn't.
That effect was even greater when the fish were exposed to an actual boat disturbance: they consumed 33 percent more oxygen, as the below chart from the paper shows.
Second, the boat noise dramatically slowed the ability of the fish to respond to a dangerous situation.
When a motorboat was "passing," the fish were six times less likely to be startled by a simulated attack from a predator. Even those that did respond were 22 percent slower than their boat-less counterparts.
The presence of boat noise also increased a predator's success at capturing the fish.
When the boat noise was present, a predator fish — Pseudochromis fuscus, or the dusky dottyback — needed 74 percent fewer strikes to catch their first prey and 82 percent fewer strikes to capture prey in general, the scientists found in an experiment conducted in a tank where making such observations was easier.
The researchers also found that the prey fish were several times more likely to survive without a boat present.
In the tank experiment, the prey were 2.9 times more likely to be eaten when listening to a boat than when not. In the experiment in open water, that multiple was 2.4.
The researchers ultimately argue that their findings can inform efforts at environmental conservation. It may annoy humans, but noise pollution may prove fatal to some fish.