New Zealand's fishing industry could hit a snag if it doesn't plan for warmer oceans significantly affecting marine life, say Niwa scientists.

As the ocean gets warmer it could affect phytoplankton - a key part of the oceans, seas and freshwater basin ecosystems - and could see a reduction in food supply for fish.

"This has already happened in the water just off Tasmania and the south-east corner of Australia, which is warming rapidly as the East Australian current pushes warmer water further south causing huge changes to the ecosystem," said Niwa marine biogeochemist​ Professor Cliff Law.

SUPPLIED A Nasa satellite image of phytoplankton in bloom along the The Chatham Rise. The area provides 60 per cent of New Zealand's commercial catch, and could be at risk as oceans continue to warm.

Tasmania's coastal ecosystems were changing with an increase in subtropical species, which can impact the economy, he said.

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"The average warming around New Zealand is 2.5 degrees [Celsius] by the end of this century, which will affect how the ocean mixes and the nutrients available for plankton growth, with knock-on effects on the foodweb and fisheries.

PETER MEECHAM/FAIRFAX Tasmania's coastal ecosystems are changing with an increase in subtropical species, say Niwa. (File photo)

"People tend to think of climate change from a terrestrial angle but obviously, as the ocean is a big part of the globe, there will be significant changes. As it's also a significant part of New Zealand's Exclusive Economic Zone we need to start planning for this now."

Near-record sea surface temperatures described as "off the charts" were reported on Monday.

A La Nina pattern and higher than normal atmospheric pressure are warming sea surface temperatures by more than 6 degrees Celsius in some areas, compared to the average for this time of year.

KENT BLECHYNDEN/STUFF Professor Cliff Law says the ocean near Tasmania is a good indication of how the water around New Zealand will change as the planet warms.

Higher pressure in the atmosphere creates tranquil weather, which calms conditions over land and sea, and leads to less wind.

Winds are what churn up the sea and bring colder seas to the top, but with high pressure the sea at the top is stagnant and heated more effectively by the sun.

Law said that the surface ocean has a 'density step' between the surface water and the lower, deeper water where the nutrients come from, which acts like a physical barrier to nutrient movement between the layers.

BLOOMBERG When water is too warm corals turn completely white - this is called coral bleaching. Corals can survive a bleaching event, but they are under more stress and are at risk of dying.

"With further warming, there's a stronger density step and less nutrient supply for the surface waters so phytoplankton doesn't grow so well and productivity drops down."

Law, the lead author in a recent paper published in the New Zealand Journal of Marine and Freshwater Research, urged authorities to look at the ocean near Tasmania as a good indication of how the water around New Zealand will change as the planet warms.

"Scientists have used two earth system models and four climate change scenarios to project how New Zealand waters will be affected by 2100.

JAY BOREHAM/STUFF Phytoplankton swarm at Swann Beach Cove on the Whangaparāoa Peninsula.

"The best-case scenario involved a cessation in the production of carbon dioxide in the next few years, with the worst-case being business as usual in which carbon dioxide levels keep rising."

One of the most affected areas for nutrient decline will be the Chatham Rise, which lies to the east of New Zealand and currently provides about 60 per cent of the country's commercial fish catch.

Also vulnerable are the sub Antarctic waters to the south of New Zealand, which are also home to some commercial fisheries.

DELWYN DICKEY/STUFF Snapper numbers could increase as they spawn in the warmer waters.

"All regions will see a reduction in food supply, because of a decrease in particulate material sinking from the surface – and that is what links climate change to our fisheries," said Law.

The fishing industry may need to think about being more agile in terms of the species it catches and where it catches them, he said.

"For example, if fish go deeper to maintain their temperature, they will have to adjust their practices to account for that."

The warning comes on the back of a report issued by independent public policy think tank, The New Zealand Initiative, recommending big changes were needed for recreational fishing.

The Future Catch report stated a five-year study on the state of New Zealand's recreational fishing sector was needed, with a focus on preserving fish stocks for the next generation of recreational anglers.

Minister for Fisheries Stuart Nash said of the report: "We all want sustainable shared fisheries for current and future generations to enjoy, and we all have a role to play in making that happen.

"This report contributes to an important conversation, and I will be discussing with officials on how we can all work together to improve our shared fisheries.

"Managing our fisheries needs to include customary and recreational interests as well as commercial, and we should have a system that takes all interests into account."

In September the Ministry for Primary Industries announced it will review snapper numbers over the next three years by electronically tagging 100,000 of the fish - a decision welcomed by experts from various fishing sectors.