For the first time, sea sponges off New Zealand's southern coast have been discovered bleached bone-white as a result of severe water temperatures.

The sponges, which are generally a rich chocolate brown, were bleached at more than a dozen locations around Breaksea Sound and Doubtful Sound in Fiordland, according to a group of scientists from Victoria University of Wellington.

According to James Bell, a marine biology lecturer at the institution, up to 95% of the sponges were bleached in some areas.

“Our initial estimates are there are at least hundreds of thousands of sponges likely to have been bleached and maybe even many more than that,” he said.

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During a study trip in April, Bell's team discovered the extensive bleaching. The species, which is distributed throughout the lower South Island, is one of 800 sponge species discovered in New Zealand.

Sponge gardens cover the bottom all around the country, providing habitat for fish and releasing carbon that other species feed on.

There have been reports of sponges bleaching, particularly off the coast of Tasmania earlier this year, but they are normally more resistant to oceanic changes than other species such as corals, according to Bell. “It’s a unusual event,” he said.

“This just highlights the kind of climate crisis that we’re facing. There are so many species around New Zealand and we don’t know what their thermal tolerances are.”

Rise in temperatures 

Last year, the world's oceans were the warmest in recorded history, owing to climate change, which can cause the waters to acidify and destroy reefs and ecosystems, among other things.

New Zealand was no exception, with the warmest year on record in 2021. According to the National Institute of Water and Atmospheric Research, temperatures in April remained abnormally high, with coastal water temperatures up to 2.6 degrees above average.

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Those temperatures were considerably higher in Fiordland, according to Rob Smith, an oceanographer at the University of Otago who works with the government-funded Moana Project to study marine heatwaves. According to him, temperatures in the region were up to 5 degrees higher than typical.

“What we’ve seen this summer is the strongest marine heatwave on the west coast of the South Island in 40 years.”