Experts study effects of sea heatwave
Warmer Tasman could affect marine life
While it’s made for ideal beach conditions, a “marine heatwave” that’s engulfed the Tasman Sea may have been damaging for ocean species, scientists say.
The latest heatwave kicked off in November and has persisted for most of this month, pushing coastal water temperatures to more than 2C above average and growing to its most intense at the west of the country.
Climate scientist Dr Jim Salinger said, among other factors, La Ninadriven winds had forced a stronger East Australian Current, sending warmer waters further south into the Tasman Sea.
“The current marine heatwave will have had impacts on coastal and near-shore environments — and possibly on fisheries.”
Professor Simon Thrush, head of the University of Auckland’s Institute of Marine Science, wasn’t yet aware of any direct ecological impacts in New Zealand, but said extreme events could “profoundly” affect how marine ecosystems functioned and the species they contained.
“Often high temperature events combine with other stresses causing major changes,” Thrush said.
“The type of things we could see this summer are shellfish die-offs and sick-looking kelp forests — in the past we have seen these changes not just because of higher temperatures alone, but in combination with disease and overfishing.”
“We might expect to see more warm water species, especially in the north-eastern part of New Zealand, so you might see some unusual species when you are snorkelling.
Salinger said an anomalously warm event became a marine heatwave if it lasted for five or more days, with temperatures warmer than the 90th percentile based on a 30-year historical baseline period.
Four had been recorded so far this century: in the northern Mediterranean in 2003; off Western Australia in 2011; in the northwest Atlantic in 2012; and off southeast Australia in 2015-2016.
Salinger, who specialised in marine heatwaves while at Australia’s CSIRO, said 2011’s “Ningaloo Nino” brought sea surface temperatures between 2-4C above normal for 10 weeks, along 1000km of coastline.
At sites close to shore, scientists observed changes among some ocean species, including more tropical macroalgae and fish species, and fewer temperate species that preferred cooler waters.
Salinger expected the current marine heatwave to linger for some time yet.