It all began when the rotting carcass of an 11-metre-long (36 feet) Bryde’s whale was found on a beach on Bali’s southwest coast on April 1.

Then, last Wednesday a live sperm whale measuring 18 metres (59 ft) was found stranded on a beach in the southeast. Locals managed to push it back out to sea but hours later it was stranded on another beach, where it died.

The most recent event took place over the weekend when the carcass of a 17-metre-long (56 ft) sperm whale – a deep-sea species that does not commonly become stranded – was discovered on Bali’s southwest coast.

The events are part of a wider phenomenon that has seen 21 unexplained whale strandings across Indonesia since the start of the year, according to the Ministry of Fisheries and Marine Affairs. They include the partial carcass of a 10-metre (32 ft) sperm whale that washed ashore on Bali’s southern coast on January 19 and the remains of a 10 metre (32 ft) sperm whale found floating off the coast of the Kangean Islands, a small archipelago 120km (75 miles) north of Bali, on Monday.

Permana Yudiarso, who has coordinated government responses for marine mammal strandings in Bali since 2012, said the frequency of recent strandings on the island was abnormal.

“Last year, we had nine incidents in Indonesia. Normally we have less than 20 every year. But three cases in a week only in Bali – it’s quite concerning,” Yudiarso, who is the head of the Bali office for coastal resource management at the Ministry of Fisheries and Marine Affairs, told Al Jazeera.

Post-mortem examinations are being conducted on samples taken from two of the three whales found on the island. But even when the results are made public later this month, they are unlikely to provide a definitive answer for the spate of incidents.

Some wildlife campaigners point the finger at plastics. Indonesia is the world’s second-largest source of marine plastic pollution after China, according to Indonesia’s Coordinating Ministry of Maritime Affairs.

“Plastic pollution, when plastic is found inside whale stomachs, and noise pollution, when sonar the whales use for navigation is harmed by underwater noise and they get confused and beach themselves, are two of the leading causes of death,” said Femke den Haas, a wildlife paramedic and co-founder of the Jakarta Animal Network.

In 2018, a sperm whale was found dead in the waters of Wakatobi National Marine Park some 1,000km (621 miles) northeast of Bali with 115 plastic cups, 25 plastic bags, four plastic bottles and two sandals in its stomach.

A smaller amount of plastic was also found inside the stomach of one of the sperm whales that beached itself in west Bali earlier this month. “We still cannot say if the cause of death was plastic. It could be disease,” Yudiarso said.

Still, he noted there was something of a pattern in the strandings, which tend to be more common in the transition period between the wet and dry seasons.

“We are in the middle of that period now,” Yudiarso said. “It could be related to the tropical storm we had in Java last month or a more recent storm north of Australia in the Timor Sea. We also can’t rule out the effects of underwater earthquakes – we have them all the time in Bali. On Monday morning, there were two earthquakes and that might have disturbed the whales’ sonar.”

‘Whale trap’

Sumarsono, the head of the conservation section for the Bali Natural Resources Conservation Agency, who like many Indonesians goes by only one name, shared an alternate theory.

“The south of Bali has many steep sloping tidal flats where the difference between high tide and low tide is extreme, creating a natural trap,” Sumarsono explained.

“Many marine creatures get trapped close to the shore and by the time they realise something is wrong, it is too late for them to return to the deep ocean. Bali is in the middle of the migratory route between Indonesia and East Timor, so getting trapped is a more likely cause of death than disease. It’s statistically improbable that three whales would die of disease in one week.”

Then there is the issue of rising sea temperatures caused by the burning of fossil fuels and the associated depletion of oxygen levels in the ocean from soaking up carbon dioxide.

A study, published in 2019 in the scientific journal Nature Climate Change, warned the warming oceans were driving extinction risks higher and marine biological richness lower. “Multiple regions in the Pacific, Atlantic and Indian Oceans are particularly vulnerable to marine heatwave intensification due to the co-existence of high levels of biodiversity,” the study said. It identified the waters of Indonesia as one of the five worst affected areas.

Ocean heat hit a record high in 2022.

Karen Stockin, a professor of marine biology at Massey University in New Zealand, said it was important to differentiate between climate change and regular marine heat waves – periods of abnormally high ocean temperatures relative to the average seasonal temperature caused by short-term weather phenomena like El Niño events.

“They are quite different but they both have the potential to change the distribution of prey like squid, and that runs the risk of predators like whales that are dependent on squids to change their distribution in response,” she said. “If changes in distribution bring whales closer to shore, that can potentially increase their risk of stranding.”

Sumarsono noted that two of three whales that most recently beached in Bali had large amounts of squid inside their stomachs.

But that is not necessarily a smoking gun, according to Stockin.

“The important thing to consider in strandings like these is that the causes are quite complex. It’s rarely easy to put your finger on any singular cause. In most cases, strandings are caused most often by a multitude of factors.”

Vanessa Pirotta, a wildlife scientist who has studied whale beachings extensively in the Australian state of Tasmania, an island that like Bali has been described as a natural “whale trap”, said the phenomenon remained largely a mystery.

“What makes the recent strandings in Bali even more interesting is the two very different species which have stranded: sperm whales have teeth and use high-frequency sonar to communicate and navigate, and Bryde’s whales, which are toothless and use low-frequency sonar. So each stranding event might be completely independent and occurred due to a variety of different reasons,” she told Al Jazeera.

“They may be connected or they may have just been a bizarre week of coincidences where three whales have stranded on one island. Maybe something spooked them. Maybe one of them died of old age or was ill but it’s far too soon to make that call. The results of the post-mortem examinations may help understand the causes and identify a link, but that’s not a given. In short, we will probably never know what caused their deaths.”