By Gabrielle McCulloch for Stuff
A bleached sea sponge found in Fiordland in 2022. Photo: Dr Valerio Micaroni and Francesca Strano / Victoria University of Wellington
The water temperature in the South Island is set to soar this week as a marine heatwave sweeps the West Coast.
It's expected to reach a whopping 18.4C - five degrees hotter than the average water temperature for this time of year.
A marine heatwave is when water temperatures stay in the warmest 10 percent of what has been historically observed for at least five days.
MetService oceanographer Dr Joao de Souza said although the balmy waters would be good news for holidaymakers, they could harm marine life.
"Last year, a similarly warm marine heatwave in Fiordland reached great depths and caused widespread bleaching of sea sponges," he said.
The native sea sponges, which should have been a velvety brown, were bleached white en masse as the heatwave ripped through the lower south Island.
Scientists were left in shock, but sea sponges are hardly the only ocean critter to be blighted by rising temperatures.
Salmon farms in the Marlborough Sounds were plagued with higher than usual deaths in the warm water last year.
Southland fishers have also reported seeing larger and more exotic fish than ever before in the deep south, which can threaten local species like Hector's dolphins.
De Souza, who has been tracking marine heatwaves as part of the Moana Project, said the coming temperature rise was a "big deal".
"Since 2010 we've been having more heatwaves and longer heatwaves. The water is way warmer than it should be," he said.
By Friday and Saturday, the water temperature would have reached its peak, but de Souza wasn't certain what would come next.
"Marine waves, they extend far below the ocean's surface. The warmer water is around 200 metres deep.
"What's happening below can be even more impactful [on marine life] than what's happening on the surface," he said.
Niwa predicts by 2100, marine heatwave days in Aotearoa will increase from about 40 days a year to between 80 and 170 days.
The southern tip of the South Island will be the worst hit and could see marine heatwaves start to last for more than a year.
The current heatwave will largely affect Fiordland, Stewart Island and the Otago Peninsula, de Souza said.
- Stuff