In early April, whale watchers off this country's southern coast were greeted by a disturbing sight: the lifeless body of a 20-metre-long blue whale floating in the water about 20 kilometres offshore.
The body was swelling rapidly, and suckerfish swarmed across its skin. Even more unsettling was the condition of its tail, which had been nearly severed from the body.
"It was very obviously from a ship's propeller," said Mazdak Radjainia, a structural biologist and underwater photographer from the University of Auckland in New Zealand who happened upon the whale. "It must have been a really cruel death, because it was such a massive injury."
Researchers say ship strikes are a leading cause of death among whales. Many that are killed are from endangered populations like blue whales that are barely holding on.
The problem is particularly troublesome here in Sri Lanka, where a largely unstudied population of blue whales, possibly numbering in the thousands, has come under increasing pressure from commercial shipping and from a boom in unregulated whale-watching boats.
Because these waters are poorly monitored, scientists do not know whether ship strikes are on the rise. But the whale's death in April was already the sixth of the year, according to news reports. In one grisly encounter in March, a blue whale was found draped over the bow of a container vessel in the harbour in the capital, Colombo, 145 kilometres north of this beach resort.
Last year, some 20 whale carcasses (not all of them blue whales) were seen around the island, according to Arjan Rajasuriya, a research officer with the National Aquatic Resources Research and Development Agency in Colombo. It is not known how many of the deaths resulted from ship strikes.
"These strikes likely represent only a portion of the likely true mortality," said John Calambokidis, a whale researcher in Olympia, Wash., who documents ship strikes off the West Coast of the United States. Because blue whales often sink soon after they are struck, most such deaths go unrecorded, and Mr. Calambokidis says the true number "could be 10 or 20 times" the number seen.
Twenty-five kilometres off the southern coast of Sri Lanka is one of the world's busiest shipping lanes, and whales are known to swim regularly inside them. But some scientists believe the increase in whale watching could be forcing whales to seek food farther out, pushing them into the big ships' path.
"I'm afraid the whales are being harassed by the whale-watching boats and that this could affect their movement," said Asha de Vos, a whale researcher here.
The threat to the whales has some researchers scrambling to learn as much as they can about them and to find a way to protect them.
"Having these whales right off the coast is pretty amazing," said Ari S. Friedlaender, a research scientist at the Duke University Marine Laboratory. "We know so little about blue whales in general that any place that you have easy access to animals like this, your learning curve is going to be exponential."
In 2009, Sri Lanka ended a deadly 25-year civil war that largely kept foreign scientists and researchers away from these waters. Several general surveys in the 1970s revealed that there were whales here, but it was not until the 1990s that interest started to grow. Researchers were particularly drawn by the whales' tendency to stay here year round; other blue whale populations are known to migrate vast distances.
Perhaps no one has studied these whales and promoted their conservation as much as Ms. de Vos.
Three years ago, she started the Sri Lankan Blue Whale Project, a long-term research program that she hopes will stop the carnage and raise awareness of the whales here. For the past three years, from December to May, she has been photographing the whales and using scientific instruments to better understand their feeding behaviours.
"Clearly, there's something down there that's keeping them around. But we need to know where it is and how much," she said.