Underwater under siege

For a marine animal that relies heavily on echolocation to communicate, feed and breed, an increase in sound pollution could spell doom. Reuters

In 1970, an album titled ‘Songs of the Humpback Whale’ was released with 35 minutes of sound recordings of these whales. The operasque sounds created by the whales, dolphins and most other cetaceans continue to fascinate scientists even today with many recordings revealing the extreme frequencies these sounds can be emitted in. Scientists have also found through several studies that the sounds are not just for social interactions in the underwater world. They are also fundamental cues for feeding, navigation, and communication in the ocean.

What happens then if a mother whale is trying to teach the baby whale to swim across a difficult waterway and suddenly the baby cannot hear the mother’s cues because there are multiple other sounds coming its way? What if the other distractions are so loud and unfamiliar that the baby whale actually gets alarmed on hearing them and gets lost?

Increased noise from shipping traffic, motorised fishing vessels, underwater oil and gas exploration, offshore construction and other human activities are creating exactly this kind
of ruckus for the whales and other marine
species that can leave them confused and disoriented. And because the way sound travels underwater is so different from the way it travels in air, there are multiple levels at which such noise pollution can interact and affect the lives of all marine animals.

Unable to find habitats

Ecologist Francis Juanes and marine ecologist Arlos Duarte recently analysed decades of data sets and studies of the effects of noise pollution on marine creatures. They found that human-made sounds are impacting all marine dwellers in a negative way — so much so that some fish larvae are unable to find their habitats or homes.

Prof Duarte says that while the importance of sounds has been studied in detail in humpback whales, who are able to communicate through thousands of miles using complicated songs, there is also evidence that miniscule fish larvae ‘hear’ the call of their habitat and follow it to find home when they are drifting on the waves. Unfortunately, he says, that call is no longer being heard.

“Imagine having to raise your kids in a place that’s noisy all the time. It’s no wonder many marine animals are showing elevated and detectable levels of stress due to noise,” said Joe Roman, a marine ecologist.

The scientists found that fish and some invertebrates avoid certain areas of the Red Sea where the frequency of ships travelling is high. They also noted that the overall number of marine animals has declined by about half since the 1970s. “In some parts of the ocean, there were fewer animals singing and calling than in the past – those voices are gone,” said Duarte.

The sound impact

There are several reasons why sound tends to become a greater stress for aquatic animals than those living on land. Sound travels almost five times faster through sea water than through air, and low frequencies can travel hundreds of kilometres with little loss in energy. In addition to this, the hearing range of marine mammals is far greater than their vocalisation range as they rely on sound cues much more than visual cues to avoid predators too. Also, because the whales and dolphins utilise a wide band of acoustic frequencies — from low-frequency sounds down to ~15 Hz used by Blue Whales to 120–150 kHz used by several species of porpoises — the broad range can easily intersect with almost all sounds introduced by humans in water. All this gives rise to conditions that can be quite complex to maneuver for all kinds of animals be it tiny shrimps or massive whales.

According to a 2018 study by Pune based Maritime Research Centre, ship movement in the Arabian Sea, Bay of Bengal and Indian Ocean led to increase in decibel (dB) levels in the seas from a maximum of 185-190dB to 210dB over seven decades. In this period, the minimum noise levels along major shipping routes, too, had gone up to 110dB from 90-95dB. The sources of these sounds included > 200dB: Sound from ships for communication purposes or to detect any danger, 200-250dB: Seismic surveys using airguns to illuminate the sea bottom to understand the nature and presence of oil activity, and 100-150dB: Sounds of various machines that aid movement of large ships and vessels. The researcher warned that such sounds could not only lead to discomfort but also internal injuries, bleeding, haemorrhages, or even death among the marine animals.

A 2019 study noted another harsh effect of human sounds. Increased ship traffic across the Ganga is stressing the river’s iconic Ganges river dolphins and changing the way they communicate. For a mammal that relies heavily on echolocation to communicate, feed and breed, this kind of stress could be devastating.

Turning the volume down

An interesting observation Juanes and Duarte additionally made during their study was the effect of the global lockdown on the reduced human-made sound levels and thereby the activity of marine creatures. They found that when 60% of people were under lockdown in 2020, there was 20% reduction in the human noises created underwater. Almost immediately, large marine mammals were seen around coastlines and areas of the seas where they had not been observed for decades.

The scientists strongly feel that when it comes to the various environmental challenges like climate change and plastic pollution that are also impacting ocean health, noise pollution is often given less attention whereas tackling this marine “anthrophony” was the “low-hanging fruit” of ocean health. “If we look at climate change and plastic pollution, it’s a long and painful path to recovery,” Prof Duarte said. “But the moment we turn the volume down, the response of marine life is instantaneous and amazing.”

While the whales and fish do not have a mute button or ear plug to muffle the alien human noises, the responsibility to lower the marine ‘loudspeakers’ for our marine neighbours lies with us and is not difficult to execute.