Sharp Decline in Sri Lanka’s Blue Whale Population Raises Alarms Among Experts – By Malsha – eLanka
Image Source : divemagazine
The blue whale population that has long inhabited the coastal waters of Sri Lanka is showing a significant and worrying decline in sightings, according to a report published by Mongabay.
Researchers attribute this drop to multiple stressors including heavy ship traffic in one of the world’s busiest maritime routes, unregulated whale tourism, pollution, rising sea surface temperatures, and broader impacts of climate change.
Sri Lanka is home to a unique population of blue whales (Balaenoptera musculus) in the North Indian Ocean. Unlike most blue whales that migrate seasonally, these whales remain year-round in warm waters and have a distinct vocalisation pattern, says marine biologist Asha de Vos, founder of Oceanswell — a local marine conservation and research organisation.
“They are grazers, not gorgers,” de Vos explains, meaning they feed continuously throughout the year. The southern coastal stretch between Galle and Dondra has long served as a critical feeding and breeding ground for this resident group.
Conservation biologist Ranil Nanayakkara, founder of Biodiversity Education and Research (BEAR), has studied these whales closely with de Vos, documenting a full range of life cycle behaviors — including feeding, nursing, and courtship — all occurring within 20 kilometers of the shoreline. These coastal waters are rich in phytoplankton and zooplankton, sustained by deep sea canyons and upwelling currents, providing vital nutrition for the ocean’s largest mammals.
While exact population estimates are difficult to establish, a 2023 study by Upul Liyanage from Sri Lanka’s National Aquatic Resources Research and Development Agency (NARA) reported 729 blue whale sightings — though likely with repeat observations of the same individuals. Nanayakkara’s own work has documented 81 unique blue whales based on their tail fluke patterns, while Oceanswell continues to develop a detailed photographic database.
Sri Lanka’s blue whales have been central to a booming whale-watching industry, particularly in Mirissa, Trincomalee, and Kalpitiya. A study noted nearly 80,000 whale-watching tourists annually, with tour operators once confident enough to promise sightings — or offer free return trips if whales were not seen.
However, survey data from Oceanswell, BEAR, local operators, and fishermen consistently point to a decline in whale numbers. “The reduction in numbers has been going on for a while,” says de Vos. “We’ve been observing fewer blue whales for years now, not just in 2024.”
Nanayakkara echoed this, stating that since 2018, there has been a visible decline. “This year, only four sightings were reported — one in the northwest, none on the eastern coast, and just three off Mirissa. Even acoustic monitoring using hydrophones has failed to detect their vocalizations,” he noted. In comparison to the 729 sightings in 2015, the drop is dramatic.
NARA is closely monitoring the trend. Liyanage noted that a 2024 transect study showed over a 90% reduction in sightings. Further research is planned, including a comprehensive marine study by the Norwegian scientific vessel Fridtjof Nansen. This initiative will assess marine mammal populations, plankton levels, chlorophyll concentration, and sea surface temperatures, with short-term studies also planned between Dondra Head and Galle — the core of the whales’ historical habitat.
