How noisy ships change dolphins’ language

In the deep waters of the eastern Arabian Sea, bottlenose dolphins (Tursiops truncatus) are changing the way they communicate to bond, hunt, navigate and signal danger. The reason: Not to get drowned in the noise from ships and other human activity.

One of the world’s busiest maritime corridors, the region sees a steady passage of container ships, tankers and bulk carriers linking the Persian Gulf, India and East Asia. Beneath this traffic, dolphins depend on whistles, clicks and other vocal signals for social coordination and survival. A new acoustic study by the National Institute of Ocean Technology (NIOT), Chennai shows this balance is being disrupted.

The researchers, who analysed passive acoustic data over six months, found that when vessels were present, dolphin whistles shifted to higher frequencies, became longer in duration, with fewer inflection points compared to quieter periods. “Their whistles became less complex,” said R Kannan, first author of the study published in Acoustics Australia.

The study suggests that when ship noise overlaps with the frequency bands dolphins use to communicate, they shift the pitch and length of the calls to compensate for background noise. Scientists call it the Lombard effect.

The study also found dolphins whistled less frequently when ships were present, and whistle diversity declined, with the descending-ascending contour type dominating under noisy conditions.

Some scientists call for more studies. Marine biologist Isha Bopardikar from the Foundation for Ecological Research, Advocacy and Learning (FERAL) cautions against attributing the vocal changes of dolphins solely to noise. “While whistles do occur in the same/ similar frequency bands as some shipping noise, shifts can also be due to different behaviour, group sizes. This calls for focal studies with concurrent behavioural observations.”
She notes that prolonged exposure to elevated noise can impact marine life. “Chronic noise exposure can affect fine-scale acoustic communication of such mammals; if a dolphin gives out a less complex signal than normal, it means less information,” she said, adding that this could potentially affect communication between a mother dolphin and its calf, as also synchronised behaviours.

Similar behavioural shifts have been documented in other places. In 1999, research in Canada’s St Lawrence River found that beluga whales altered their vocal behaviour in the presence of vessel noise. A 2014 study in Portugal’s Sado estuary reported acoustic changes in bottlenose dolphins near operating vessels, while research published in 2013 in the northern Adriatic Sea linked leisure boating to dolphin displacement. In 2018, studies in the US showed dolphins simplifying calls in noisy environments, and work in Danish waters found that high vessel noise disrupted foraging in harbour porpoises. Humpback whales off Hawaii were shown in 2020 to increase call intensity as ambient ocean noise rose.

What distinguishes the Arabian Sea study is its focus on a heavily trafficked commercial shipping corridor rather than coastal tourism hotspots. Vessel noise here is continuous and intense, making mitigation more complex.

Globally, underwater radiated noise from shipping has become a growing concern for the International Maritime Organization (IMO). In 2014, IMO adopted voluntary guidelines to reduce underwater noise from commercial ships, but they were not mandatory. Revised guidelines released in 2023 remain non-binding, even as IMO has set a goal of reducing average shipping noise by three decibels by 2030, a target experts say will be difficult to meet without enforceable measures.

Proposed mitigation strategies include designing quieter propellers, improving hull design to reduce cavitation, installing vibration-dampening systems, reducing vessel speeds in ecologically sensitive zones, rerouting traffic away from key foraging habitats of marine life, and developing noise management plans at ports.

“We cannot stop the growing shipping traffic. But we can identify marine foraging regions and take measures to keep the waters calmer to protect the marine mammals,” says Prof G V V Pavan Kumar of the School of Naval Architecture and Ocean Engineering at the Indian Maritime University.