“There’s this perception that sharks are invading because there’s warmer waters, but it’s not. It’s purely a seasonal movement from deep ocean water into the coastal shallows for food and for pupping.”
Researcher Dr Riley Elliott says young sharks find their way around by instinct. Photo / NZME
Shallow coastal waters are a nursery ground for fish, and almost all fish species will use them, Elliott said.
“So everything from your snapper to your kingfish to your sharks come in to drop their pups or their eggs.
“And you’ve got, predominantly, bronze whalers, but also baby great whites and other species coming inshore.
“They’re coming inshore to pup, coming inshore to mate, coming inshore to drop eggs – and also coming inshore to feed on all of those things that are coming inshore to do all that.”
Their migrations back and forth mean they can be found around Stewart Island and elsewhere around the country during our summer, and then back to Tonga, Fiji, New Caledonia and Great Barrier Reef as we head back into winter, he said.
“We’re starting to see some of those start to come back down now, just as the season is warming up.”
The young sharks find their way around by instinct, he said. Their mothers birth them and then leave them, plus they do not spend time with their siblings.
“It’s kind of built into their DNA. It’s beyond comprehension, really.”
Sharks are thought to use the bathymetry, the topography of the underwater environment, to find their way, Elliott said. They are also believed to do so visually and electromagnetically.
“They’ll use the continental ridges where you’ve got tectonic plates pushing together. It’s also where ocean currents run along, so they can jump on them like finding Nemo does with the turtles.”
Sharks also have certain behaviours that have been observed as they hang about the shallows. Elliott said they like to sit behind the surf break because the waves churn more oxygen into the water. This, along with the movement of the water under the waves, makes it easier for sharks to “breathe” without having to swim as much.
The number of sharks around New Zealand is really pronounced during the El Nino phase of the El Nino-Southern Oscillation, Elliott said.
Riley Elliot regularly swims with sharks as part of his research. Photo / Amber Jones
“El Nino produces a lot of productivity. The water is colder because of westerly winds, and as it blows away from the shore, it creates upwelling in the ocean, it creates a lot of food.
“That’s when we get really good numbers of kingfish, snapper, other pelagic fish, and with that comes the flow of the common sharks we have, the bronze whaler, the blue shark, the mako shark, and a little bit more recently, the great white seems more prevalent along the northeastern coast.”
And as those sharks come inshore in summer, and as we head for the beach, we end up seeing more of them.
“It doesn’t mean there are more fish or more sharks, it’s just the distribution of them,” Elliott said.
With more of them around, how should we react? Should we be worried? How do we keep ourselves safe?
Elliott said it was about respecting them as wild animals and respecting them as they went about their business.
“If you catch a shark while fishing, just cut the nylon as close as you can to the hook. You don’t need to bash it with a club.
“You don’t need to bring it on to the boat. And don’t target them in the first place. You don’t want to eat shark meat because it’s filled with mercury, and most of them are threatened species.
“And the other thing is, if you don’t want to have an adverse interaction with one, never swim where you are overlapping with where people are fishing or disposing of fish or processing fish.”
Elliott said it was “common sense”. If you see a shark, he said, “honestly, just enjoy it”.
Raphael Franks is an Auckland-based reporter who covers business, breaking news and local stories from Tāmaki Makaurau. He joined the Herald as a Te Rito cadet in 2022.
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