Amidst honking horns and the bustle of traffic, you may have sometimes yearned for the quiet of the oceans, where you don’t have to shout to be heard. But the oceans are not the silent havens they once were. Increased shipping traffic has made them noisy, and this does not seem to bode well for some ocean citizens – like the humpback whale. According to a new study, the mysterious song of the humpback whale is silenced when they stray close to commercial ships.
Most marine mammals like dolphins and whales use sound as their chief mode of communication, as other senses are diminished in water. The sounds of some whale species are complicated pieces, often referred to as songs, comprising multiple units. Only the male whales sing during mating season, so it is generally believed that they sing to attract mates. They also produce other types of sounds thought to help in feeding. Whale songs are unique to populations in a geographic area, where all the males of the group sing similar songs, and are very different from whales from a different region.
However, the ocean is no longer quiet. Ships, seismic testing, military communications using sonar, and oil and gas drills are only a handful of acoustic disturbances created by man. “It’s so noisy that by human standards whales should be wearing earmuffs to deaden the noise or else go deaf,” wrote Christopher Clark of Cornell University, who has been listening to whales for a long time, in an essay This man-made noise makes it difficult for whales to breed, feed and survive in the loud oceans.
Several studies in the past have documented the effect of extraneous noise on whale sounds. Using recordings made of humpback whale songs off the coast of Hawaii in 1998, researchers figured out that the whales sang for longer in response to US Navy sonar broadcasts.
A 2012 study reported that blue whales called more during days when seismic exploration activities, which use acoustic pulses, took place, suggesting the whales were trying to compensate for the louder ambient noise. A more recent study reported that ship noise reduces the number of dives humpback whales take to find food at the bottom of the ocean off the coast of Massachusetts, a busy shipping route.
Now, a new study reports that shipping noise completely silences the whales. The frequency range of whale vocalisations is a few tens of Hz to about 4 kHz, which overlaps with the noise frequency from ships. Researchers studied the effect of noise from passenger ships on humpback whales off the Ogasawara Islands in Japan. “This study is fascinating because of its unique setting in such a pristine area with very little ship traffic,” said Denise Risch of the Scottish Association for Marine Science, who studies how marine animals are affected by sound. Hence, noise source from single ships could be identified and the response of a single humpback whale could be monitored. “In most other, busier areas, it is often difficult to disentangle several potential causes for an observed change in behaviour,” she said.
The researchers used hydrophones, underwater microphones, to record underwater noise from February to May 2017. The recordings were analysed to identify whale songs and ship noise and determine the location of the whale in relation to the ship’s path. Over the period, 26 singing whales were identified, with about 1-3 singers in a day.
The data showed that whales in close proximity to the ships, up to about 1,200 m, reduced singing or stopped completely when the ship was approaching or after it was gone. Most of the whales started singing again only after about 30 minutes after the ship passed by. Surprisingly, if the whales were even closer to the ships, less than 500 m, there seemed to be no change in their singing behaviour.
“Humpback whales seemed to stop singing temporarily rather than modifying sound characteristics of their song under the noise, generated by a passenger-cargo liner,” said the authors in a statement. In the Ogasawara water, the noise generated by the ships was at most 8 dB above the ambient. Hence, it may be that it is more efficient for the whales to simply stop singing and wait for the noise to pass, rather than trying to be heard over the noise, suggest the authors.
Although it is clear that ship noise has an effect on whale songs, the study does not tell us how the songs affect other non-singers such as females and calves, as only the males sing.
“Few studies have investigated the impact of ships of known source level on individual animals and in the context of a specific behaviour. Such data is vital in order to assess and model longer term and larger scale impacts of noise,” said Risch. This is particularly important for a species like the humpback whale, as their songs are generally supposed to be breeding display, and any noise affecting this might affect their mating and future success of the species.
Noise pollution affects a wide variety of species both on land and in water, including us. A common response among both land and marine animals is the modification of vocal behaviour. Urban birds are known to sing or call at higher frequencies than their rural counterparts, just to be heard above the din. Other responses include increased stress, loss of hearing and in extreme cases changing their behaviour so dramatically as to cause death. This happened when 16 whales were stranded off the Bahamas in 2000 in response to Navy sonar exercises, several had bleeding ears and some were dead. A few years later a similar mass stranding of three species of whales, at least 37 in number, was reported around the coast of North Carolina, US, again in response to Navy sonar exercises. Most of the whales were dead.
The evidence for how noise pollution affects marine animals is growing. The behavioural response of these animals to noise is complex, being dependent on species, the context, the noise source and how familiar the animals are with the noise, said Risch. Sometimes the animals reduce calls, sometimes they call more, or at higher frequencies and for longer durations.
However, this study only adds to concerns that once again, humans may be interfering with how other animals behave, even in the deep of the ocean. Not only does this hurt the rich diversity of marine animals, it also affects some communities that are dependent on the ocean for their livelihood. One method Clark suggests to help the animals is to create acoustic sanctuaries, areas of the ocean where human noise is restricted, leaving the natural soundscape as it is. An increasing body of evidence showing how we are affecting the animals around us means only a concerted effort by everybody concerned can make a different. And in the process, we may actually be helping ourselves, too.
Lakshmi Supriya is a freelance science writer based in Bengaluru.