Floating whale carcasses are a problem – can we predict their drift?

Computer models could help predict the drift of dead whales so authorities can safely let their remains decompose naturally in the ocean.

Dead whales attract large numbers of sharks and, if washed onto busy beaches, are extremely smelly and unsightly, creating logistical problems for local governments. If ocean currents take them into shipping lanes, they pose a major hazard to passing vessels.

While most dead cetaceans never wash ashore, every year, 30 to 40 carcasses get stranded on the coast of Australia, for example. The numbers have been increasing since commercial whaling was mostly abolished in the 1980s.

These carcasses may be left to decompose where they lie, buried on the beach, taken to landfill, composted, rendered into biodiesel, blown up into smaller pieces with explosives or taken out to sea, where they will drift for days or weeks before sinking.

Sometimes, after being towed and released, the carcasses get washed ashore again somewhere else, simply moving the problem.

Computer models can predict where floating objects will drift, but this is hard to do accurately for dead whales because of their unusual size and shape, says Olaf Meynecke at Griffith University in Queensland, Australia.

“The most important thing is to know whether the whale will drift back to shore,” says Meynecke.

To learn more, he and his team closely monitored the drift of a dead humpback whale off the Queensland coast.

On 16 July 2023, the 14-metre-long floating whale, weighing up to an estimated 25 tonnes, was spotted by a volunteer coastguard. The whale was headless, which led researchers to speculate that it may have been killed by a boat strike.

By the next day, the carcass had drifted 4 kilometres. After locating it, Meynecke and his colleagues attached a satellite tracker.

On 18 July, the whale washed onto land. It was then towed 30 kilometres offshore and released again, after which the team followed its course for 150 kilometres over the course of another week. Finally, either the carcass sank or the tracker failed.

His team found that in the first few days after death, when the carcass was floating highest in the water, up to 1.5 metres above the surface, wind strength was the greatest factor in the drift direction.

“Current was only important when the carcass was decomposed and the body was less buoyant,” says Meynecke.

After the satellite data was collected, the team used a search-and-rescue computer model to see what paths it simulated for various objects that resembled the profile of a dead whale, including a skiff, a life raft and a small vessel called a panga, based on the same location and weather conditions.

The predicted locations were accurate in the first few days, but between 10 and 20 kilometres off after six days, says Meynecke.

He hopes to repeat the study with more carcasses to assess different scenarios and provide more accurate estimates of where whale remains will drift.

Moving a dead whale from a beach to a landfill can cost authorities more than A$10,000 (nearly US$7000) and it also removes large amounts of nutrients from the ocean food chain, says Meynecke.

“The goal is to provide local authorities with a tool to be able to quickly determine whether it is feasible to tow a whale out to sea and know where it will end up.”