Waters warming in Fiordland may be responsible for the loss of up to 10% of one of the most common marine sponges in in the Doubtful Sound.
According to Victoria University of Wellington, more sponges may have been lost further south in Tamatea, Dusky Sound and Breaksea Sound.
Widespread bleaching of sponges was discovered in May last year by researchers at the university and was linked to "an extreme marine heatwave" which affected millions of sponges.
Marine biologist Professor James Bell led a trip to Fiordland over summer where he said "mixed results" were discovered.
"At the six sites we surveyed in Doubtful Sound, we found nearly all the sponges had recovered their colour and were no longer bleached, that’s the good news.
"However, some sponges are likely to have died. It’s hard to say how many have been lost but we think it could be 5 to 10 percent, based on video and photographic data."
He said the bleaching and sponge deaths may have other flow on effects on the local marine environment.
The team is set to revisit Dusky and Breaksea Sounds next month assess the extent of the decline in sponge populations.