Featured image: An olive ridley turtle. Photo: Ar.Shakti Nanda/Flickr, CC BY 2.0
Colombo: Conservation experts in Sri Lanka have been left puzzled by a recent spate of marine turtle fatalities, where the animals’ bodies have washed ashore without any indications of the typical causes of death.
Three carcasses were found Oct. 3 on Mount Lavinia Beach in the commercial capital, Colombo; the next day, two dead turtles were found in the Wellawatte of Colombo, and five more at Galle Face in the heart of the city. All 10 bodies were found along a 10-kilometer (6-mile) stretch of coastline, indicating something unusual had occurred.
The government’s Department of Wildlife Conservation (DWC) conducted necropsies on several of the bodies to ascertain the cause of death. Suhada Jayawardana, a veterinary surgeon with the DWC who examined the bodies, said all of them were olive ridley turtles (Lepidochelys olivacea), the most common marine turtle species found in Sri Lankan waters.
Entanglement in fishing nets as bycatch is the most common cause of marine turtle deaths in Sri Lanka, and this was initially speculated to be the likely reason for the recent deaths. But Jayawardana’s necropsies ruled this out, with the soft tissue on the flippers and heads not exhibiting any of the cuts that occur when a turtle tries to disentangle itself from a fishing net. On some occasions, pieces of the net can be found embedded in the body of dead turtles, but none were found in this case.
“The turtles appeared to be healthy at the time of the death,” Jayawardana told Mongabay. “We found food in their gut, which means they continued feeding until the last few days. If they contracted a disease, that would have probably stopped them from feeding.” He added the turtles likely died three to four days before their bodies washed ashore.
Perera previously operated a dive center from Mount Lavinia Beach and was a frequent visitor to the area where the turtles washed up dead. He said it was not uncommon to see three or four dead turtles beached or floating in the shallows, but not this many within the space of just two days.
Causes of death
Another initial suspect for the turtle deaths is a fuel oil spill from the crude carrier MT New Diamond after it caught fire off Sri Lanka’s eastern coast in September. A week-long operation eventually doused the fire without any of the crude oil spilling out, while the leak of the fuel oil was quickly contained.
“There is no connection between the New Diamond oil spill and the turtle deaths,” said Charitha Pattiaratchi, a professor of coastal oceanography at the University of Western Australia. Pattiaratchi said Sri Lanka is experiencing the end of the southwest monsoon, so winds and currents in the vicinity of the New Diamond would have pushed the slick northeast, away from Sri Lanka’s landmass. That would put the pollution out of the range of where the turtle carcasses were found. Additionally, it would mean that if the turtles had died in eastern Sri Lankan waters, then their bodies would have drifted away from the island and not washed up on land.
Pattiaratchi, an expert in the oceanic currents around Sri Lanka, said the turtles would have originated from the west to the northwest.
Arjan Rajasuriya, a former research officer at the National Aquatic Resources Research and Development Agency (NARA), told Mongabay that the turtles would have died within about 30 kilometers (18 miles) from the shore where they were found, if indeed a single event had killed them. “If the turtles died far away from the shore, then the oceanic currents and waves would have dispersed their carcasses across a much larger area,” he said.
Blast fishing suspicions
DWC veterinary surgeon Jaayawardana said an explosion could be among the possible reasons for the deaths. With the bodies exhibiting no signs of external wounds, Jayawardana has sent the turtles’ internal organs away for further laboratory analysis.
A possible explanation is that the animals were killed by the shockwave from blast fishing, which would be consistent with the lack of external injuries, Jayawardana said.
Blast fishing using dynamite is illegal but still practiced in many parts of Sri Lanka, though it has slowed down following restrictions on explosives following the 2019 Easter bombings. But if blast fishing was the cause of death, then there should be far more cases of dead turtles washing up on shore, Perera said, adding that blast fishing is not common along the coast of Colombo. However, Pattiaratchi said the blast fishing theory is plausible if the turtles died of an explosion. Blast fishing occurs throughout the year, but it’s only during the present period that the bodies would wash ashore. In January and February, the winds and currents would push the bodies away from land, Pattiaratchi said.
There’s another possible explanation that could involve an explosion. Naval exercises have been reported in Sri Lankan waters from around Sept. 23, although it’s not clear whether these involved the detonation of any explosives or whether they occurred in the general area where the turtles would have been affected.
Dwindling turtle populations
As the search for answers in the mystery deaths continues, the case has highlighted the need to protect the threatened marine turtles that frequent the rich waters around Sri Lanka.
Of the eight marine turtle species found in the world, five are known to nest on Sri Lankan beaches. These are the olive ridley turtle, green turtle (Chelonia mydas), loggerhead turtle (Caretta caretta), hawksbill turtle (Eretmochelys imbricata), and leatherback turtle (Dermochelys coriacea). The population trend for all five species is declining rapidly, said Thushan Kapurusinghe of the Sri Lanka Turtle Conservation Project (TCP).
“Turtle deaths are very common in our waters,” he told Mongabay. “Some turtles are directly being hunted, while many fall victim to bycatch of fisheries. Getting entangled in fishing nets is the biggest threat to turtles.”
In some cases, up to 50 turtles have been found snagged in a single large trawl net, Kapurusinghe said. “In such instances, a lot of turtle carcasses may get washed ashore within a shorter stretch of a beach area within just a few days,” he said, indicating this might explain the recent deaths.
In January and February, thousands of olive ridley turtles flock to India’s Odisha state to nest. During this time, the olive ridley population increases in Sri Lankan waters, and with it the number of dead turtles washing ashore on the island’s eastern, northern and northwestern coasts, Kapurusinghe said.