Does NZ's 'marine heatwave' mean more shark encounters?

It's made for balmy beach conditions, sent kingfish and rays into southern waters, and even brought forward the grape-growing season in some places.

But a Department of Conservation scientist says there's nothing to suggest a massive "marine heatwave" that's enveloped the country is behind a string of summer shark encounters.

On Sunday, teenager Alvira Repia-King was left with 52 stitches after being bitten by a seven-gill shark at Oamaru's Friendly Bay, while on Christmas Day, a small shark latched onto the toe of 9-year-old Cordelia Scott as she was swimming at Punakaiki on the South Island's West Coast.

Great white sharks have nudged fishing boats in two separate instances and last week, surfcaster Joel Gray got an unexpected shock when he landed a 2.5m juvenile at Matatā Beach.

DoC's Clinton Duffy has also fielded fresh sightings of a whale shark near the Poor Knights Islands, and a "very large" hammerhead near the Mokohinau Islands.

Yet, despite the northern North Island experiencing record-warm sea conditions, the marine scientist told the Herald this summer's shark count had so far been "fairly typical".

"I wouldn't say this has been a stand-out year for sharks," Duffy said.

"The species that typically show the biggest shifts during environmental events tend to be offshore, oceanic ones whose prey distribution are affected by things like currents, hydrology and nutrients."

Coastal shark species, meanwhile, relied on specific habitats, so returned to the same spots year after year.

"What we do see more of unusually warm years are migrants like sub-tropical sharks and rays – but so far, there hasn't been a lot of sightings of these."

While around 66 shark species have been identified living in our surrounding seas, there are only around a dozen that fishers and swimmers will regularly come across.

Despite their fearful appearance, only a handful, like the great white, mako and hammerhead, pose a threat to human life.

Many species congregate in the warm waters of the upper North Island, ranging from bronze whalers, blue sharks, makos, giant manta rays and school sharks.

Around the South Island, generally harmless species such as spiny dogfish, school and blue sharks make the marine habitat their home.

But by far one of the most dangerous stretches of coastlines in the country is in the deep south, where mature great whites, with their distinct white underbelly, make their residence.

Considered the deadliest and most dangerous shark, the great white is found around both islands, with the young preferring warmer northern waters. Adults can be found in southern waters near seal colonies.

There's also been a growing number of great white sightings in the Bowentown area of the Tauranga Harbour and also along the coastline, prompting a recent warning from DoC.

Run-ins with large sharks in coastal waters usually happen over spring and summer, when many species move inshore to pup and feed.

At the same time, Duffy said scientists had to take into account that warmer water could simply be drawing more people to the sea, which in turn brought more sightings.

"Undoubtedly, that plays a role in the number of sightings we get, and makes for a challenge when it comes to analysing data."

He said the marine heatwave seemed to have a more apparent effect on fish species – particularly with kingfish and short-tail stingrays and eagle rays being spotted in southern areas that'd usually be too cold for them.

New Zealand sea surface temperatures, as at January 8. Marine heatwave conditions remain around the northern North Island. Image / Supplied

The last time New Zealand experienced a comparable event, over the record-hot summer of 2017-18, Dunedin residents noticed a sudden jump in stingray numbers around Otago Harbour.

Niwa fisheries scientist Darren Parsons said it wasn't only fish species common in New Zealand that would be on the move right now.

"Tropical species like warm waters, so they'll be travelling further south," he said.

"Expect more species such as marlin, mahi-mahi and little tropical vagrant species to turn up in Poor Knights Islands."

Heatwaves aside, Duffy said the message to shark-wary Kiwis heading into the surf remained the same as every year.

"People need to be aware that the marine environment isn't their backyard," he said.

"If you go down to the sea it's reasonable to expect a shark is nearby, because at this time of year, several species do come in near the shore."


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