Niwa marine ecologist Dr Darren Parsons said initial research had suggested that some species, such as red cod and pink maomao, were growing less abundant in the north of the North Island, while others like snapper, leatherjacket and john dory were extending their range further south.
But this work had some notable limitations: species' responses to climate changes were often subtle, while the data scientists had to work with remained somewhat coarse.
"There are also usually other influential factors operating, which can make it challenging to identify a pattern," he said.
"For example, the biomass of the snapper stock in the South Island has increased a lot over the last 10 years.
"So, it's possible that range extensions could also be a response to the increasing number of snapper in the South Island."
Parsons also noted the success of snapper spawning was positively related to water temperature in the months after spawning.
"So, while people might be seeing snapper turn up in Southland in warmer years, it might be the success of spawning events from warmer summers that is the thing to watch out for," he said.
"It will be three to four years before the snapper spawned during a marine heat wave grow to legal size and get noticed though."
Fortunately, trawl surveys were helping provide an early picture.
"A trawl survey that we conducted on the West Coast of the South Island in 2019 did indeed observe a very strong cohort of one-year-old snapper at that time, so there is a possibility that was a response to the 2017/18 marine heatwave."
Elsewhere, climate-driven shifts in fisheries were already much more apparent.
"For example, on the east coast of Australia - Tasmania in particular - there has been a southerly shift observed in many species over the last few decades," he said.
"It might be that the bigger anomalies in marine temperature that we are experiencing at the moment may drive similar distribution shifts in New Zealand fish species over the coming years, but it's too early to tell at this stage."
A recent major Government stocktake of our marine environment found sea surface temperatures have been rising at a rate of 0.1 to 0.2C per decade – and at a quicker rate in our coastal waters.
Niwa scientists have also warned that average sea temperatures around New Zealand could rise by 1.4C within four decades – and almost 3C by the century's end.
That would mean that, by mid-century, we could be facing 260 days of marine heatwaves per year – and 350 days by 2100 – compared with the 40-odd days we see now.
Generally, Parsons said the responses of fish to these gradual shifts would be species-specific.
"Some will be more tolerant - or may even benefit - compared to others."
The greatest risk, he said, was to species that couldn't move out of the way of warmer waters, such as shellfish and aquaculture.
Niwa's current modelling for the Pelorus Sound greenshell mussel industry indicated a 10 per cent reduction in meat yield over the summer months, relative to the 1997-2005 average.
That comes after the country's largest salmon exporter, New Zealand King Salmon, was recently forced to close farms in Pelorus Sound after a heatwave led to 1300 tonnes of dead salmon being sent to landfill.
Otago University physical oceanographer Dr Robert Smith told the Herald this month how shifts being observed in fisheries highlighted why it was important not just to consider the surface temperatures, but also those deeper in the water column.
"At the moment, waters off the northeast coast of New Zealand are between about 1C and 2C warmer than normal, down to a few hundred metres," he said.
"These can be really deep-reaching events that can have impacts at the shelf edge around New Zealand."
When temperatures reached several degrees above normal 100m below the surface – as had been recently observed on the West Coast – it carried implications for species like crayfish or benthic organisms that couldn't easily escape the heat.
“So, one of the interesting things to watch for this coming summer will be how deep this heat extends into the subsurface.”
