Explosion in fish numbers as sea warms

Adecade ago, catching kingfish in Otago Harbour was rare.

This year, it appears they have become "almost prolific" along the coastline, and a University of Otago marine ecologist is calling them "sentinels of climate change".

Dunedin recreational fisher Brent Russell caught a 72cm kingfish at the weekend near Quarantine Island.

He was delighted with the rare catch but put it back because it was 3cm short of the legal size.

"The last caught in the harbour that I'm aware of was a small one in 2015."

Commercial fisher Allan Anderson, of Karitane, said he and his son had been set-netting off Karitane for years and in the past few years kingfish had become more common.

"We would catch maybe one or two in a summer season 10 years ago. We're catching one or two every day now, if not 10 sometimes. Twelve is the most we've caught in a day.

"It's been something that's been turning up in our waters more regularly every year. They're becoming almost prolific."

They were set-netting more than four nautical miles off the coast, but kingfish preferred to be closer to the coast. He felt even more could probably be found closer in: "We're only on the fringe of what's truly there."

Kingfish are most abundant in the northern half of the North Island, and can reach more than 1.5m and 30kg.

University of Otago marine ecologist Steve Wing said the fish were becoming more prevalent in the south because the water was getting warmer. Coastal water temperatures were between 2C and more than 6C above average at the moment.

"One of the things we are seeing now are a lot of the species we associate with sub-tropic waters."

He had heard reports of kahawai being caught around southern parts of New Zealand and snapper in Fiordland this year.

"Those fish are real sentinels for what's going on. They show sub-tropical waters are here and they indicate a larger, physical change is going on.

"This is what we expected to happen in a warming ocean ... under climate change."

A Niwa meteorologist said La Nina was causing much of the water heating this year. It brought large ridges of high pressure to the atmosphere across the Tasman Sea and over New Zealand, resulting in sunny, warm weather and tranquil seas. Without storms to mix up the sea, the surface layers of the ocean had warmed dramatically.

Wing said sea currents were causing sub-tropical water to move south.

"The waters are so warm because we've got this big injection of sub-tropical waters into the Tasman and it's coming up along the Southland Current."

It was difficult to know if it was a one-off event or would continue.

"The ocean is quite variable from year to year. But the overall trend - the trend over 50 years - is that our winter-time temperatures are increasing.

"What that means is, the sub-tropical water that's normally here in the summer is spending more time here in the winter as well. Overall, the temperatures are increasing."

There would be "winners and losers" with climate change.

"Things will be different - it's the only definitive thing we can say. It's not necessarily all bad.

"There will be some changes that are quite dramatic in some places, and in some places it will be quite bad.

"For us, there may be some improvements ... like catching kingfish off our coast."

However, it was costing commercial fishers thousands of dollars each season. When they netted a kingfish they were fined, because there was no quota so far south.