Victoria University of Wellington marine biology professor James Bell said yesterday a monitoring trip to Doubtful Sound last month revealed the majority of the millions of sponges that were bleached white during last year’s heatwave had recovered.
However, when the phenomenon was first recorded in May it appeared some sponges that had been bleached had been attacked by fish and were decaying.
When the researchers returned to the area last month, they found no sign of the "chewed on" sponges.
It was considered very unlikely the sponges would have grown back all the material that was eaten and instead more likely they were completely eaten and had disappeared, Prof Bell said.
Scientists estimated there were millions of sponges in the fiords and tens of thousands to hundreds of thousands of sponges were estimated to have disappeared, he said.
Sponges provided habitat for other marine animals and their loss could have significant knock-on effects for the larger ecosystem.
"They are part of the underwater ecosystem, they are something that’s really abundant, and they’re there for a reason.
"They're doing stuff and if we lose them, then those other ecosystem impacts could be quite dramatic.
"At the end of the day, the average person on the street probably isn’t that focused on sponges, but ... they represent some bigger processes that are going on in the ocean.
"If they continue and continue to get worse, we may find that there are really big economic impacts," he said.
The variety of cup sponge (Cymbastela lamellata) that became bleached during the spike in sea temperatures off the Fiordland coast last year was among the most abundant of the marine sponges in the area.
It was a nondescript sponge relatively overlooked by researchers until the bleaching took place.
Prof Bell has been studying marine science since in New Zealand since 2006 and only recently had he seen researchers’ interests shift to heatwaves.
Before last year, New Zealand had only experienced relatively moderate heatwaves and there was little solid evidence there had been any major impacts, he said.
"Up until early last year, maybe a little bit before that, we didn’t really have any focus on marine heatwaves, or a major focus on marine heatwaves.
"People were concerned that they might happen but we hadn’t had anything like what we saw last year in terms of marine heatwave impacts.
"It’s a recent thing. "
However, last year’s heatwave — and the one the country is experiencing again now — made researchers think about some of their climate change experiments differently, he said.
Projected average temperature increases in the coming decades were not as extreme as some of the short-term marine heatwave temperature increases seen now.
Prof Bell said the effect of rising sea temperatures had been studied by taking organisms and increasing the temperatures they were subjected to beyond the average sea temperature they were used to, to simulate projected increases to sea temperatures in 2100, for example.
"What we’re seeing with these marine heatwaves is that these organisms that we’ve been exposing to 2degC [increases], they normally do all right.
"But the marine heatwave impacts and the extreme temperatures are much higher than anything we expected to see in 2100 — and they’re happening now."
The research team will revisit Dusky and Breaksea Sounds in late March to determine the extent of the decline in sponge populations there.
