- A resident population of blue whales has for many years lived in the coastal waters of Sri Lanka, but in recent years sightings of the animals have declined rapidly.
- 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.
- Sri Lanka’s leading marine researchers agree that increasing sea temperatures in the North Indian Ocean, warming at the fastest rate of any of the world’s oceans, have likely pushed the whales to new waters.
MIRISSA, Sri Lanka — Sri Lankan waters are considered a haven for the largest creatures ever to have lived, the blue whale, a rare place for a resident, year-round population to thrive. In 2021, however, news headlines claimed a population decline, and in 2023, a media report mentioned a significant drop in blue whale sightings.
For decades, the North Indian Ocean has been known to have its own distinct population of blue whales (Balaenoptera musculus). They have their own vocalization 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 specializing 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 behavior — 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.
Decline and fall
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.”
Sri Lanka remains a top destination for whale and dolphin watching. Unlike with blue whales, sightings of other cetaceans have increased: Bryde’s whales (Balaenoptera edeni) and short-finned pilot whales (Globicephala macrorhynchus) are regularly seen on whale-watching trips, and spinner (Stenella longirostris), bottlenose (Tursiops truncatus) and Risso’s dolphins (Grampus griseu) are also present in high numbers.
There’s speculation about the possible causes for a decline in blue whale sightings. One whale-watching operator from Mirissa named three: the long-term impact of the busy commercial shipping lane through whale waters; the illegal practice of some operators offering diving opportunities with blue whales; and the burning and sinking of the MV X-Press Pearl cargo ship, which unleashed several tons of toxic plastic pellets.
Sri Lanka lies on one of the world’s busiest shipping lanes, from East Asia to Europe, with up to 50,000 ships passing through each year. With whales feeding in these same waters, ship strikes, and underwater noise pose serious concerns, according to experts. A 2017 study by Nanayakkara found half of the recorded whale deaths in Sri Lankan waters resulted from ship strikes.
But de Vos said this doesn’t sufficiently explain the recent sharp decline in sightings.
“I recognize that shipping was heavy and was potentially leaving impacts. However, it is a little unusual for them to suddenly go from being in high numbers to low overnight due to shipping,” she said. “One can expect a petering out. It is not just the blue whales that get hit. Bryde’s whales also get hit, but their numbers are increasing.”
Avoiding feeding grounds
Efforts to redirect the shipping lane away from the whales’ feeding grounds have so far failed, despite lobbying by various groups including the International Fund for Animal Welfare, WWF, the International Whaling Commission, the Great Whale Conservancy and others. A formal application to move the shipping lane, opposed by the Sri Lankan government, was considered by the International Maritime Organization in 2023, triggering further consultation and research but no action.
Nanayakkara said he’s concerned about the emergence of “swimming with the whales” tours.
“It is illegally conducted, where operators charge large sums of money for people to get in the water, and see these large leviathans up close and personal,” he said.
The impacts of the 2021 X-Press Pearl disaster are well documented but not, on their own, enough to explain the decline, he said. “Although the Sri Lankan government studies into the aftermath of the maritime disaster have never been made public, numbers of blue whales had been dropping way before that. And plenty of other cetaceans are still around.”
Environmental change
So what’s driving the decline? Long-term environmental and climate change impacts and the simple reality that blue whales are ocean wanderers, experts say.
The North Indian Ocean’s surface temperature is rising more rapidly than any other of the world’s oceans, heating up by more than 1° Celsius (1.8° Fahrenheit) since 1950, and more than 0.5°C (0.9°F) in the last 20 years alone. This and other climate change factors are altering monsoon patterns and rainfall, thus changing marine ecosystems.
Both de Vos and Nanayakkara say climate change is the most likely cause of the disappearance of blue whales from the coastal waters of Sri Lanka. Using maps and satellite imagery, they note, it’s possible to see the distribution of chlorophyll, a proxy for the presence of phytoplankton, is shifting. Zooplankton are also being impacted to an extent, and it could be this that’s sending blue whales out of Sri Lanka’s coastal waters in search of more productive areas.
Increased rainfall as a result of climate change is likely also a factor, de Vos said. She noted how, between 2011 and 2012, blue whales disappeared, and some researchers blamed the growing whale-watching industry for frightening them off. A study later showed the whales were pushed farther out to sea by decreased food levels in the water as a result of high rainfall that affected salinity levels. A year later, the blue whales were back in the coastal waters.
This time it appears these ocean wanderers may have gone for good, but once again no one knows for sure; there’s no dedicated research to locate the whales. There are reports of greater blue whale numbers around the Seychelles, historically a haven for the species before industrial whaling decimated their numbers. But until scientists’ database of flukes can be compared, it’s not possible to know where these ocean adventurers have journeyed, de Vos said.
“Whales are nomadic,” Nanayakkara added. “They travel around wherever there’s productivity, wherever there’s food they can return to, just like you and me. They will keep going back to places where there is food.”
The Sri Lankan whales are out there somewhere, he said. “We may not know the specifics until our research unlocks facts. I hope they have found a place where there’s plenty of food, where they can breed and thrive. Who knows, one day they might come back.”
Simon Payne is Emeritus Professor at the University of Plymouth, U.K. and can be found on instagram @brave_new_wild
Banner Image: The distinctive flukes of a blue whale. Image courtesy of Ranil Nanayakkara.
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