Pilot whales mainly feed on squid, but are known to eat octopus and small fish when squid are not available, according to the American Cetacean Society, a whale conservation group.

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Thais use huge numbers of plastic bags and authorities have launched campaigns to try to encourage people to use fewer and to introduce reusable bags.

Globally, eight million tonnes of plastic – bottles, packaging and other waste – are dumped into the ocean every year, killing marine life and entering the human food chain, the United Nations Environment Programme said in December.

The effort to save the whale

Rescuers had cradled the pilot whale in a shallow canal, their arms wrapped around its slick, shuddering body. It could not eat and struggled to swim and breathe. Red umbrellas were opened above it to block the harsh sun.

A team of rescuers deployed buoys to keep the mammal from slipping into the water and drowning as veterinarians tended to it.

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Photos posted to social media showed so many long, black plastic bags that authorities were running out of room to walk in the operating room without standing on rubbish. A bundle of white plastic is shown next to innards stretching across an operating table.

Thai officials said they believe the whale mistook the floating plastic for food.

Thon Thamrongnawasawat, a marine biologist and lecturer at Kasetsart University in Bangkok, told Agence France-Presse that the plastic probably prevented the whale from digesting food.

"If you have 80 plastic bags in your stomach, you die," he said, adding that at least 300 marine animals, including pilot whales, sea turtles and dolphins, die annually after ingesting plastic in Thai waters.

A study published last year found that 83  percent of water samples from more than a dozen nations were contaminated with plastic fibers.

It was unclear whether the whale was a short-finned or long-finned pilot whale, although short-finned pilots commonly traverse the warmer waters typical of South-East Asia.