Shark's incredible trek

Alana with shark scientist Riley Elliot.

Blue shark breeding grounds confirmed off New Zealand coast.

New research has confirmed waters off Auckland are crucial breeding grounds for a keystone shark species. Auckland shark scientist Riley Elliott long believed waters northeast of New Zealand were a crucial blue shark breeding and birthing ground.

Now, satellite tracking tags and the return of a female blue shark to local waters affirmed the University of Auckland researcher's view.

Elliott tagged blue shark "Alana" last December, shortly after she had given birth.

She journeyed thousands of kilometres to seas around Tonga, Niue and the Cook Islands, living in the tropics for six months before arriving near North Cape this week.

"She travelled the gauntlet of the fishing fleets all the way up through the Pacific," Elliott said.

"And then something in her biological clock says: 'Summer's coming soon to New Zealand so go back', and she has come back."

Elliott said Alana would now give birth off northeast New Zealand and breed again with returning males. "The pups will drift with coastal currents to the juvenile grounds further south, as far as the Chatham Rise."

Male and female sharks followed separate migration routes out of breeding season. This appeared to be an evolutionary adaptation to keep male sharks from constantly pressuring females to breed.

Elliott received about $100,000 in funding and sponsorship from the public, which helped pay for expensive satellite tags, including the one connected to Alana.

Waters only 50km from Auckland, near the continental shelf limits, were the blue shark hotspot. The animals had few rivals or natural predators there, but were under pressure from human activity.

"That same area also happened to be the epicentre of the tuna fleet, which was finning all these sharks," Elliott said.

"If you take out all the mums and babies, obviously you screw the entire population. We're talking the whole South Pacific."

Legislation to be introduced in October will make it illegal to catch a shark, kill it, remove its fins and dump the carcass at sea.

Riley said it was an improvement on the past, but loopholes existed in the new law.

"What has been implemented is far better than it was but it does still allow blue sharks to be finned as long as the body is brought back to shore."

The Florida Museum of Natural History has stated most commercially caught blue sharks were considered bycatch and 10 to 20 million were killed each year.

Shark body parts were also used in lipsticks, face creams, and health supplements, according to surf website The Inertia.

Riley said in the past two decades, 90 per cent of all sharks had been "wiped out" by shark finning and exploitation in the absence of data ensuring sustainable harvests.

Elliott features in an upcoming book and a related TV series, Shark Man, set to air on TV One this year.

Elliott hoped his research would raise awareness of the sleek blue shark's important role in the ecosystem and unique behaviours.