As a marine heatwave approaches this summer, more tropical fish may be swimming into Northland waters.

Niwa has forecast sea surface temperatures between Tasmania and the Chatham Islands could be 1.1 degrees above normal in
January.

Summer sea temperatures last year were the highest on record in northern and eastern parts of the North Island, higher than the previous record marine heatwave in 2017/18.

Niwa marine ecologist Dr Darren Parsons said migratory species such as tuna, marlin and mahimahi may increasingly arrive on our shores.

“It could be that a warmer summer means that we get more of these tropical migrants over summer, but we don’t know.”

Initial research suggested traditionally North Island fish such as snapper, John Dory and kingfish were making their way south and others, such as red cod and maomao were becoming less abundant in the North.

Any immediate changes to fish species in the region would not be dramatic, Parsons said.

“Generally speaking, the fish that are presently abundant in Northland will continue to be the fish that are abundant in Northland, even during or after a marine heatwave.”

Warmer waters could also lead to more “tropical vagrants”, such as tropical wrasses, butterfly fishes and surgeon fishes that drift from tropical waters.

“These tropical vagrants are not migrating here in the summer and then leaving. It’s a one-way trip for them, they drift here as larvae from the warmer waters that they usually inhabit, and some will be able to survive in our colder waters,” Parsons said.

“When we have a warmer summer, it’s possible that more of these tropical vagrants are able to survive, or survive for longer, so they become visible on our reefs as they grow into juveniles or adults.”

Climate change was not just about temperature, he said, and wind patterns, currents, ocean productivity and acidity would all change as well.

Coastal waters around New Zealand had not warmed as much as some places, such as eastern Australia, Parsons said.

If temperatures did rise more than they had been, fish communities would gradually change.

“Fish species will likely respond over generations so that their overall distribution shifts further south to where temperatures are more suitable to the physiological preferences of each species,” Parsons said.

A severe marine heatwave in 2017/18 led to reductions in mussel and kelp beds, and tropical fish drifting into normally colder climes.

Marine heatwaves are likely to become longer and hotter in the coming decades, Niwa warned earlier this year.

Niwa ocean modeller Dr Erik Behrens said in March the idea of marine heatwaves becoming permanent was worrying.

“The impacts of climate change are happening all around us and New Zealand isn’t immune,” he said.

“We’re just coming off the back of one of our most intense marine heatwaves, like what we experienced in 2017. Our work indicates that this will start to become the norm as time goes on.”

Q and A: Matt Watson on Fishing

TV fishing personality Matt Watson, who lives in the Far North, told the Advocate about a few of his favourite spots and the changes he’s seen in fishing in recent years.

What’s your favourite fishing spot in Northland?

West Coast probably, Hokianga, whenever weather permits. I would stress to anyone going across there that it’s a bar crossing and to be careful. I’ve seen some boats get in strife and end up upside down out there.

What changes have you seen in the marine environment in recent years?

The biggest changes I’ve seen are dirtier water and more sedimentation, like in the Bay of Islands, from basically run-off. A lot of dive spots have silted up. This is over the course of 20 years. It’s not in the last couple of years, it’s been a gradual thing. It’s affecting water clarity, which affects the marine ecosystem.

Are there different fish around now than there used to be?

With the marine heatwave, the water is warmer than usual but we have had those spikes in marine temperature because of that. And you get smaller marlin turning up as well. Because New Zealand’s at the extremity of the migratory range of species like marlin, typically only the ones with larger body mass that carry more fat can get down here.

You’re still getting the big ones as well but there’s been smaller blue marlin turning up in the last couple of years. And some really small striped marlin, one of which I saw swimming straight in front of my house chasing mullet up against the beach, which is almost unheard of.

Is it going to be a good summer for fishing?

I say yes every year. You can’t go into a new summer season saying it might be rubbish but there’s no real way of knowing what the currents are going to do. Some of these fish are coming from halfway across the world and who knows what’s going to happen to them on the way here.