48 Hours: Something fishy in the weather

Also called the yellow demoiselle, the tropical smokey chromis inhabits deeper coral and reef waters in the eastern Indian Ocean, western Pacific and northeast and northwestern Australia.

Irene Middleton is a marine ecologist working toward her PhD studying tropical and rare fishes in New Zealand. She has worked at Niwa and is a part-time marine biosecurity adviser for the Northland Regional Council. In her spare time Irene Middelton is an avid diver and a celebrated wildlife photographer. She wrote the following article, first published in NZ Fishing News, and she took the photographs.

The summer of 2017-2018 was a record breaker.

As we all basked in the sun on the beaches and lavished in the tepid ocean, the Tasman Sea soared to record temperatures.

The waters between New Zealand and Tasmania were 2C or more above average; and in some spots off the West Coast the water soared to between 4C and 6C above normal.

The media reported headlines like "Why our sea is scorching" and "Concerning marine heatwaves increasing".

But why the negative connotations? Warm oceans doesn't sound that bad; more mahimahi and wahoo to be caught, snapper living in Fiordland, longer, and more productive marlin seasons, and toasty warm dives.

Marine ecologist, photographer and writer Irene Middleton.

Why is everyone so fussed about these so-called "marine heatwaves" and increasing sea surface temperatures?

A marine heatwave is defined as at least five consecutive days with sea-surface temperatures in the top 10 per cent of warmth over a 30-year period. They are significant climate events and can have extensive significant impacts on food webs, ecosystems, biodiversity and fisheries.

Many marine species are very sensitive to temperature and such rapid changes in the oceans are bound to affect food webs.

To date, impacts of marine heatwaves have been most notable in tropical regions where we have seen mass coral bleaching and changes to reef functioning.

By comparison, New Zealand reefs have remained relatively unimpacted by climate change, compared to similar latitudes elsewhere. For example, in south-western Australia there has been a significant increase in tropical wrasses due to climate change associated 'heatwaves'.

And in Tasmania the southward migration of a large black urchin has been linked to ocean warming. Grazing by this urchin, along with the warmer ocean temperatures, has caused Tasmania's iconic giant kelp forests to all but disappear.

The fact New Zealand has so far avoided such impacts may be due to our oceanic isolation and our, until now, relatively low rates of sustained ocean warming.

A diver with a tiny flying fish, another tropical and sub-tropical species rare to New Zealand.

However, if the heatwaves of this summer are anything to go by this may soon change. In fact, recent evidence suggests that average ocean temperature is increasing in southern parts of New Zealand and winter ocean temperatures in northeastern New Zealand are beginning to warm.

These temperature increases are likely to result in the poleward shifts of tropical and subtropical fish species into New Zealand waters.

Yes, that includes species like mahi-mahi, but also reef fishes and deep-water species. Most of these species do not survive New Zealand winters, but because ocean warming is happening right now it might not be long before they do.

We also have no idea what the impact these range shifts of subtropical and tropical species, and even some of our native species, will have.

Seen here en masse in a defensive ball while a single snapper cruises past, starry toados, sometimes called pufferfish, are a species from a large family of subtropical fish.

Snapper in Fiordland may be great for the southern fishers, but will we start to see a decline in snapper in Northland?

Will a tropical surgeonfish or wrasse start competing with butterfish for food resources, or will a tropical predatory fish totally change the food web in northern New Zealand?

To understand the impacts we need data. However, searching for and recording the distributions of these tropical fish species is a bit of a needle in a haystack. With New Zealand's extensive coastline no one person or organisation has enough resources for this work.

We need eyes in the water, and that is where anyone can come in. Over the next few months I will be concentrating on subtropical and tropical fishes that turn up in New Zealand waters sporadically and may be early indicators of permanent change in fish diversity as our oceans warm.

I invite you to help me. I'm hoping that together we can develop an up-to date dataset of tropical fish sightings in New Zealand that will provide us with an early warning system of changes to our fish communities and marine environment.

If you spot an unusual or tropical fish species in New Zealand waters, take a photo and record the location, depth and if possible the water temperature at the time. Send the details to whatsthatfishnz@gmail.com or by finding Irene Middleton's page @WhatsThatFishNZ on Facebook. She will forward any fish she can't Identify to experts at the Auckland Museum and Niwa, then give the sender some feedback on the fish, its rarity and where it is usually found.

The National Institute of Water and Atmospheric Research (Niwa) is keeping a close eye on the ''perfect storm'' causing the Tasman's rising temperature.

The build-up looks similar to the intense mix of phenomena that caused last summer's record heat – the marine heatwave (raised water temperature), a La Nina event, and predominant warm northerly winds.

That scorching summer of 2017/18 saw record high temperatures, even hotter than the previous event more than 80 years earlier, the 1934-35 summer. While summer 83 years ago did not reach last summer's levels, Niwa said the long-ago event was more extreme because the temperatures at the time were 2.7C higher than normal.

As for this looming marine heatwave, even early in summer parts of the Tasman are already about 2C warmer than average, Niwa principal climate scientist Dr Brett Mullan said.

In last summer's marine heatwave, a large area of the Tasman Sea off the West Coast was about 2C hotter than normal right through December, January and February. In March, Niwa reported that parts of the Tasman had been up to 6C warmer than average.

While this summer the temperature has not reached that level the warm water is reaching further south: ''Further south than last time. Niwa is keeping a close eye on this,'' Mullan said.

It was likely New Zealand would more often experience the level of last summer's marine heatwave because of climate change. However, summers more than 2C above the climatology of the [future] period would continue to be rare events, Mullan said.