The blue whale population that has lived for many years in the coastal waters of Sri Lanka, has shown a rapid decline in their sightings in recent years, said a report published by the Mongabay yesterday.

“With multiple pressures on these massive creatures — from ship traffic on one of the world’s busiest shipping lanes, to disturbance from whale tourism, pollution and surface sea temperature rise and climate change — there are several possible factors for the disappearance of the whales,” it said.

For decades, the North Indian Ocean has been known to have its own distinct population of blue whales (Balaenoptera musculus). They have their own vocalisation or dialect, distinct from other blue whale populations. They also spend all year in warm waters, unlike most other blue whales, who migrate to feed in cold waters and breed in warmer seas, says Asha de Vos, a marine biologist and founder of Oceanswell, a marine conservation research and education center in Sri Lanka.

De Vos told Mongabay that the North Indian Ocean blue whales are “grazers not gorgers,” feeding throughout the year. The coasts of Sri Lanka, and particularly the southwest coast between Galle and Dondra, has long been a hotspot for this resident population.

Ranil Nanayakkara is a conservation biologist specialising in terrestrial and marine mammals and founder of Biodiversity Education and Research (BEAR). Both de Vos and Nanayakkara have studied these whales the year around — feeding, mothers nursing calves, and courtship behaviour — the whole life cycle playing out within 20 kilometers (12 miles) of the Sri Lankan coast.

Deep waters close to the coast and upwelling currents from sea canyons have created conditions supporting phytoplankton, and in turn, zooplankton and shrimp, the ocean’s tiniest creatures that sustain its biggest. “In terms of weight it could eat a fully grown African elephant every day,” author and biologist Mark Carwardine wrote of Sri Lanka’s blue whales in his On the Trail of the Whale.

However, it’s not easy to estimate blue whale numbers around Sri Lanka with certainty. A 2023 study by Upul Liyanage from the Sri Lankan National Aquatic Resources and Research Development Agency (NARA) and others noted 729 blue whale sightings, with many of the same whales likely observed multiple times. Nanayakkara’s research has identified 81 different blue whales from their distinctive tail fluke patterns. Meanwhile, Oceanswell is carrying out a database analysis.

The population and consistency of blue whales has given birth to a thriving whale-watching industry in the eastern city of Trincomalee, Kalpitiya in the northwest, and, most prominently, in Mirissa, in the south. One published study estimated nearly 80,000 visitors to Sri Lanka take whale-watching trips each year. Blue whales used to be abundant enough that some operators guaranteed a sighting or offered a free additional trip if there wasn’t one.

Oceanswell and BEAR have both conducted surveys on sightings at sea and spoken with whale-watching operators and fishermen, who agree that whale numbers are in decline.

“The reduction in numbers has been going on for a while,” de Vos told Mongabay. “We have seen dwindling numbers of blue whales in the last few years, not just 2024.”

Nanayakkara added that since 2018, “we observe a decline in blue whale sightings. In 2024, we saw one in the northwest, zero in the eastern coast and three off Mirissa, a total of four sightings this season. Even hydrophones haven’t been picking up the sounds from blue whales.” Compared to the reported 729 blue whale observations from 2015, this marks a dramatic decline.

Liyanage said NARA, the government’s marine research agency, is paying close attention to the trend. In 2024, it carried out a transect study that indicated a decline of more than 90%. More research is planned, he told Mongabay, adding that this year, “the scientific research vessel Fridtjof Nansen” — operated by the Norwegian government — “will undertake a comprehensive coastal study of marine mammal abundance and water quality parameters including plankton, chlorophyll and sea temperatures and there will also be short term surveys focusing on the hotspot between Dondra Head to Galle.”