(CNN) Commercial ships emit low-frequency noise wherever they roam and this unwanted soundtrack negatively impacts marine life, new research suggests. Humpback whales stop singing or shorten their songs with the passing of ships, a new study published Wednesday in PLOS One reveals.
Ocean noise has been increasing in recent years, according to the study authors. One research paper proved that low-frequency noise levels in the ocean increased by 10 decibels between the 1960s and 1990s. Another study suggested that average noise levels would increase by up to 3 decibels per decade.
While there are many causes of low-frequency noise in the oceans, including seismic explosions, renewable energy harvesting and military sonar, most experts believe the main noise source is commercial passenger-cargo shipping.
A new study investigated how this increasing cacophony affects whales.
The researchers began their experiment by setting up underwater recorders in the remote waters off the coast of a roughly 9-square-mile Japanese island, from the middle of February to early May 2017.
Listening to the recordings, the researchers identified one to three singers per day and 26 singers in total. Male humpback whales sing to attract mates, the researcher explained.
Analyzing the data, the team discovered the humpback whales' singing behavior changed in the presence of ships. The main reaction of the 40-ton mammals was to stop singing or to shorten their songs whenever the ship approached, as well as after it passed. At a range of up to 1,200 meters (nearly three quarters of a mile) from the ship, the whales reduced or terminated their songs.
"Remarkably," wrote the authors, whales swimming under a ship "continued to sing as usual." And most of the whales that stopped singing did not restart for at least 30 minutes after the ship passed, they added.
The humpback whales who make their habitat near Japan, along with three other population groups, are protected as an endangered species, according to the National Oceanic and Atmospheric Administration.
Because the study focused on singing males, it did not reveal how females and whale calves respond to shipping noise, the authors noted. More research is needed, they concluded.
Despite these caveats, Spencer Fire, assistant professor in Florida Tech's Department of Ocean Engineering and Marine Sciences, described the new research as "solid."
Fire, who was not involved in the study, said, "It's one more bit of evidence confirming that the noise humans make has a negative impact on all sorts of aspects of the life of marine mammals."
Past studies have already uncovered evidence that whales respond to ocean nose. One of his favorite examples comes from researchers at the New England Aquarium, who measured levels of stress-related hormone in the feces of North Atlantic right whales in the Bay of Fundy, Canada, during the period that followed the September 11 terrorist attacks, Fire said.
Detection dogs trained to smell whale feces in the water allowed the scientists to collect samples, he explained. The team found that the reduced shipping traffic in the Bay of Fundy following the terrorist attacks resulted in a 6 dB decrease in low frequency underwater noise. At the same time, they also found "a huge drop in the level of stress hormones" in the North Atlantic right whales, said Fire.
The following year, when commercial shipping resumed, stress hormone levels in the whales returned to the usual "really high levels," he said.
Because it aligns with past research, the new study is not surprising, said Fire, still the findings are "important because of policy." He explained that shipping companies are unlikely to change unless they see hard data.
Sound is crucial to humpback whales. "They rely on sound to find food, they rely on sound to find mates, they rely on sound to find their way around the ocean," said Fire.
Because the low noise made by ships is within the same frequency as the sounds humpback whales make, shipping noise may confuse them and cause them to change their behavior, said Fire: "As a general rule in ecology, when you make a wild animal change its behavior there are almost always negative consequences."
For example, too much noise may force whales to leave their natural habitat and go "where they may be chasing food that moves too fast, or that doesn't have high enough nutritional content," said Fire. "And usually energy intake is what makes or breaks animal survival."
Another response may be the whales try using louder calls, yet this comes at an energy cost that also may be detrimental to their ultimate survival. Sometimes whales raise the frequency of their calls in response to ocean noise, he said.
"The higher the frequency, the higher the pitch, the shorter it goes," said Fire. "There's not a lot of them so they have to call really far to find a mate." Their usual calls travel far, thousands of miles, said Fire, and this allows them to communicate with mammals all across the ocean.
Sometimes whales "stay quiet" in a noisy ocean, according to Fire, who said, "if they have to wait in order to communicate, then they might not be able to breed and so that has reproductive consequences."
Some studies show the deadly impacts of deafening sounds on beaked whales -- a separate, deeper diving species from humpback whales -- made by the US Navy during training exercises.
"These big giant noises that the Navy sonar ships make may cause [deep-diving beaked whales] to freak out and they bolt straight toward the surface, which they normally would never do," said Fire. "They die immediately from this." It is basically a version of the "bends," or decompression sickness, which human SCUBA divers can get if they rise from the depths of the ocean to the surface too swiftly, he explained.
In the end, ocean noise is a "problem" for humpback whales, said Fire. "It's sort of one more thing to add to an already long list of threats for this species." Other threats include getting hit by ships, climate change and coastal transformation, he said.