A new study has found increased ship traffic and dredging in India’s Ganga River is rapidly heightening noise levels in the river and stressing the river’s iconic dolphins and changing how they communicate.

Experts point out that noise levels in the world’s oceans are on the rise, but little is known about its impact on marine mammals like dolphins that rely on sound for communication. Dolphins are social animals, and they produce calls for many different reasons. They talk to each other to stay together as a group, they whistle when they feed, and they even call out their names when different groups of dolphins meet.

Earlier studies have previously tried to analyze this but have focused on marine mammals like the bottlenose dolphins and whales. But unlike oceans, where space is not really a constraint, Gangetic dolphins live in shallow, often narrow stretches of rivers. Here not much data is available on the impacts of underwater noise pollution.

The present study by researchers at the Ashoka Trust for Research, Ecology and the Environment (ATREE) in Bengaluru, India, states that these noises are only getting louder. Noises vary from the din from motors and propellers of boats and ships, dredging, churning of sediment, sounds from fishes and turtles, and noisy cities and industries on the shore that also dump waste into the river.

At times when less than five vessels moving on the river per hour, the dolphins seem to enhance their vocal activities to compensate for the high-frequency noise generated by the propellers. However, the researchers found that as vessel traffic increases and water levels fall during the dry season, leading to more intense and sustained noise pollution, the dolphins don’t seem to alter their clicks much compared to baseline levels.

For instance, as the dry season progressed from November to March, water levels went down, while boat traffic increased. Therefore, underwater ambient noise levels increased. This combination was particularly bad for the dolphins. To compensate for the intermittent, loud noise from vessels, they were calling louder, calling for longer, and producing more clicks, compared to their baseline vocal levels in a “quiet” river.

This could be because emitting clicks in a noisy world can be physically exhausting. Gangetic dolphins emit clicks almost continuously to see and sense their surroundings, and this activity consumes energy. The researchers modelled their observations and found that if the dolphins were to enhance their vocal activities to compensate for the doubling or quadrupling of ambient underwater noise levels, they would have to consume two to four times more prey per day. But there’s only so much a dolphin can eat, and the amount of prey isn’t increasing either; finding prey using clicks in noisier water is also harder.

The study points out that the Gangetic river dolphins are effectively blind as they don’t really need eyesight in the shallow waters of the rivers they inhabit. Instead, the mammals rely on sound –producing ultrasonic or high-frequency clicks in the 20- to 160-kilohertz range. They use the echoes to locate and find food, avoid ships and chart their way around the waters. They also modulate their clicks to talk to each other.

Researchers from the University of Maryland Center for Environmental Science have found that dolphins are simplifying their calls to be heard over noise from recreational boats and other vessels in nearby shipping lanes.

Research shows that normally dolphin calls have a complex sound pattern with rises and falls in the pitch and frequency in their whistles. The researchers found that ambient noise had a significant effect on the whistle characteristics. They analysed the duration, start and end frequencies, presence of harmonics, and inflection points. With background noise, such as the low frequency chug-chugging of a ship’s engine, their usually complex whistle signatures flatlined.

The study states that they found that increases in ship noise resulted in high dolphin whistle frequencies and a reduction in whistle complexity, an acoustic feature associated with individual identification. The simplification of these whistles could reduce the information in these acoustic signals and make it more difficult for dolphins to communicate. The study called for regulations and voluntary incentives to reduce the sound from vessels, with speed limits or quieter engines, could help to decrease the effects on dolphins and other species sensitive to sound.