Climate change killing off sea sponges in Fiordland

Climate change isn't just causing more flooding, it's also killing sea sponges in Fiordland.

Scientists say warming southern waters could be responsible for the death of up to 10 percent of sea sponges in the area.

These round, flat sea sponges survived a marine heatwave. They're the lucky ones - up to 10 percent have died.

"In 15- 20 years perhaps there won't be any sponges there... that'd be concerning," marine biologist Professor James Bell said.

Millions of Fiordland sea sponges were bleached white in May by warmer ocean temperatures - one of the largest examples of bleaching in the world. And some were nibbled by fish which killed them.

"It may be because the sponges are bleached white the fish can see them easier and nibble on them because of that," Prof Bell said.

Bleaching is caused by marine heatwaves, which are fueled by climate change. Fiordland is recording water temperatures up to 4.5C warmer than usual.

"These unusually warm ocean conditions are becoming a common experience across the Fiordland marine environment," oceanographer Robert Smith said.

"The species that live there are not necessarily being given the time they need to recover."

To take a closer look, scientists have brought sea sponges back from Fiordland. Now that the bleached sea sponges have been analysed here in the lab, the scientists will head back to Fiordland next month for further research.

The little sponge plays a big role in the marine ecosystem.

"They remove food from the water column, from the seawater down onto the sea floor and create a carbon sink that other things can feed on," Prof Bell said.

So finding out exactly what's killing them and how to stop it is important work.