Warming waters could be cause of Fiordland's disappearing marine sponges

Scientists from Victoria University of Wellington say warming waters in Fiordland could be responsible for the loss of up to 10 percent of one of the most abundant marine sponges in Pātea-Doubtful Sound.

More sponges may have been lost further south in Tamatea-Dusky Sound and Te Puaitaha-Breaksea Sound.

Marine biologist Professor James Bell said widespread bleaching of sponges in Fiordland was first discovered by university researchers in May 2022 following an extreme heatwave in the area.

Researchers - Bell, Dr Valerio Micaroni and PhD candidate Francesca Strano - returned in January to see whether the sponges had recovered.

"At the six sites we surveyed in Pātea-Doubtful Sound, we found nearly all the sponges had recovered their colour and were no longer bleached," Bell said.

But he estimated another 5-10 percent of sponges had died, based on video and photographic data.

He said when they had visited Fiordland last May, they had noticed lots of fish "seemed to have munched and chewed holes" into the bleached sponges.

However, on their return in January, he said they did not find any chewed sponges.

Bell said since it was "unlikely they could have regenerated the tissue they lost in such a short period of time" the most likely explanation was that the sponges had died or been eaten.

The team will be revisiting Dusky and Breaksea Sounds in late March to assess the extent of the decline in sponge populations.

Bell said towards the end of their trip they started to see evidence "that some of the sponges were losing their colour again."

Fiordland experienced another severe heatwave in late December and through much of January.

He said reports from the Southern Fiordland Initiative supported the findings.

"Initial surveys in Breaksea Sound by Katherine Mitchell from the Southern Fiordland Initiative suggest a much lower number of sponges than seen last year and many of the sponges have not fully recovered from the bleaching."