Swimming around in Clinton Duffy's head are facts about sharks. The last fatal attack (1976), the type of shark, the name of the surf lifesaver who disappeared in the 60s ... All that was left was his belt.

Unlike some, this dangerous knowledge doesn't stop Duffy going back in the water.

"When you see them underwater, you can't deny the grace of their movement," says Duffy, one of a handful of New Zealand marine scientists who have gone into bat for sharks.

After a summer seemingly awash in shark encounters - the latest an attack last Sunday on a Sydney surfer - the thought that these merciless marauders may themselves be at risk, and deserve our sympathy, takes some swallowing.

But Duffy, a Department of Conservation marine ecologist, and others including fisheries scientists Malcolm Francis of Niwa and John Holdsworth of Blue Water Marine Research are part of an international effort to understand the behaviour and needs of sharks and other big fish.

Their work involves attaching electronic tags to sharks, including great whites and porbeagles, which transmit data about where sharks go, water temperature and depth.

There's mounting international concern that our underwater nemesis is in danger of disappearing from the seas. In our waters, great whites were added to the protected species list just two years ago. Makos, blue sharks and porbeagles are vulnerable to longline and set net fishing as bycatch, or are targetted for shark fin soup - the fins removed at sea and the bodies discarded.

Our quota management system has helped but these highly migratory species are being hit hard around the globe by surface longline fisheries for tuna and billfish. In the Pacific, the oceanic white-tipped shark is among several threatened shark species. New Zealand has yet to ban the practice of finning at sea - with the bodies because of their limited value.

"Our shark populations are reasonably well-managed compared to what's happening globally," says Duffy. "That's not to say we won't see a major collapse in some species if we're not careful - as has happened [commercially] with the school shark in Australia.

Internationally, great whites are classified as at risk of extinction. They congregate in coastal waters off North America, South Africa, Australia and New Zealand, feeding on bony fish, seals, penguins, other sharks and whales.

If they attack humans, it is generally by mistake - the silhouette of a surfie with legs and arms dangling can look uncannily like a seal, apparently, and spearfishers who've just killed a fish and paua divers working near seal colonies are vulnerable.

As apex (top of the food chain) predators, great whites are naturally rare, not helped by their slow growth and reproductive rate. Although not targeted by commercial fishermen, they are vulnerable as bycatch, particularly juveniles, while sport fishers in Australia hit numbers hard in the past.

Many of us prefer to see them dead. A fair number turned up to watch Duffy and colleagues perform an necropsy [fish autopsy] on a great white in January. The very thought of one is enough to send most swimmers sprinting for terra firma.

But just as we lament the disappearance of big predators such as tigers on land, sharks have a right to exist, says Duffy. There is also their ecological significance.

"We know from experience on land that the removal of large predators can have unforeseen consequences - we're just extrapolating that to marine ecosystems."

In South Africa, the use of beach meshing to keep large sharks out is suspected of causing an increase in small sharks and stingrays, which have depleted recreational and commercial fish stocks. The decline of great whites and other large sharks in the United States is blamed for an explosion of eagle rays, which have decimated scallop beds.

Sharks may teach us a thing or two. Professor John Montgomery, head of Auckland University's Leigh marine research centre, says sharks have an "extremely sensitive" electro-sensory system and a sense which allows them to detect water movement and distinguish animal vibrations. "They have a very fancy noise reduction system which works like noise-cancelling headphones. We're pretty interested in seeing if we can develop new technologies, based on those sorts of learning principles in the brain."

IT has been another summer of shark sightings. Back in January, they came almost daily: sharks in the surf just metres from swimmers and boarders; sharks terrorising boaties. Then again, there were a lot of people in or on the water.

Two outboard motors have fallen prey - the second just last weekend off New Plymouth. Afterwards, skipper Boyd Rutherford and his mate did what most self-respecting fishermen do and headed for the bar. Safely inside the fishing club they eyed the whopper of a mako mounted on the wall and agreed: "Nah, ours was much bigger."

But even their mako would tip its hat to Mrs White. That's what locals call the great white known to outsiders as the Taranaki Terror. Rutherford is something of a serial shark offender. Back in 2006, his yarns about Mrs White helped spark a media frenzy.
Duffy has seen her too, while on a shark survey, and says the claims are no fishing tale. "She's enormous. I saw her out of the corner of my eye as the cameraman was filming the sunset. At first I thought it was a whale breaching, then I realised it was a great white shark. It made me believe the estimate of some experienced charter fishermen that she is 6m long."

Mrs White is the only mature female great white Duffy has seen. He's tagged 4.5m females in trips to the Chathams and Stewart Is. They can grow to 7m long and twice the weight of a male. It amuses him that most big great whites are given male names - like Brutus, the one from the Kaipara he dissected in January and which turned out to be female.

Then again, there may be more than one Taranaki Terror, spotted off the coast since 2006. "Whether it's the same one turning up each year we don't know," says Duffy. "No one's got a photograph."

Clear photos of the dorsal fin can be used to identify individual sharks. Duffy plans to build a photographic database and use repeat sightings to estimate great white populations. His knowledge of sharks (and their victims) brings to mind Hooper, the shark expert played by Richard Dreyfuss in Jaws, but Duffy maintains he's no shark-spotter.

"My interest developed out of self-preservation - I was a spearfisherman and at the time there were a number of attacks in Australia ... ."

The recent sequence of attacks around Sydney has gained international attention, causing the cancellation of an annual harbour swim and boosting sales of electronic shark repellents. It may be disconcerting to learn that their great whites may head over here to gorge on fur seals, while ours sometimes pop over there for a bite.

Four years of satellite tagging in New Zealand has proved a goldmine for scientists' understanding of great whites, although just 15 have been tagged. The researchers lay burley to draw the sharks to the surface then use a long pole to attach "pop-up" tags, which are programmed to release after a few months and transmit data via satellite.

Spot tags, which provide real-time data, are another option but the shark needs to be removed from the water. Tagging has revealed great whites are highly migratory rather than the coastal inhabitants they were taken to be. Their ability to vary their body temperature according to the surrounding water temperature helps.

Kerri, the name given to a great white tagged off Stewart Island, experienced temperature variations from three to 26 degrees on a 3000km journey that took her south to the Auckland Islands and then northwest to the Great Barrier Reef.

While tagging by Australian and New Zealand scientists has revealed a transtasman flow, this is thought less popular than the north-south movement between New Zealand and the tropics. Great whites tagged in the Chathams have turned up in New Caledonia, Vanuatu and the Louisville seamount chain, which extends well into the Pacific.

"It turns out that our New Zealand sharks are South Pacific sharks," says Niwa's Malcolm Francis. "We don't have our own population here - we share it with Australia, Fiji, Vanuatu, Tonga ... We still don't know how much time they spend in the tropics and what they're doing up there. It's possible they are honing in on humpback whales giving birth to calves, which may be a good source of food for them over the winter period."

The scientists have also learned that great whites dive to great depths during their open-ocean wanderings. One went to 1000m, probably in search of food. It's thought pregnant females may also hang out in deep water. Francis says protection measures may need to be extended internationally, not just around coastal waters.

Duffy has, this summer, concentrated on juveniles around the Northland coast and harbours including the Manukau and Kaipara. It's thought that the warm, fish-laden waters of the northern North Island are favoured by females to rear their young; as they grow they head south to cooler waters and an abundance of seals. Fiordland, Stewart Island and the Chatham Islands are great white hotspots.

Understanding where sharks go and when, what habitats they prefer and what they eat, will help scientists to recommend fisheries management strategies to help them survive.

Francis says interaction between fishers and, say, great whites could be regulated by ordering closed seasons in certain areas at certain times or by restrictions on the types of gear most likely to catch the sharks. "But we need more data from tag recaptures to answer that question of where they occur and when."

The researchers have applied for increased funding to widen the scope of the programme. "We have tagged 15 great whites over five years, which is pretty slow going. We want to increase the rate of tagging so we can start looking at seasonal patterns of distribution - when are they in the tropics, when are they around seal colonies in New Zealand, do they stay there or shuffle between?"

Francis and John Holdsworth of Blue Water Marine Research tagged five porbeagles off Fiordland last year and are about to repeat the process. Porbeagles, which grow to about 3.5m, exhibit similar migratory behaviour to great whites. In the Northern Hemisphere, they are targeted for eating; in Pacific waters they are killed for their fins and as bycatch.

Duffy wants to extend the tagging programme to the threatened basking shark, which is being considered for protection. That will depend on funding for more pop-up tags, which cost US$3000 ($6000) each. Applying pop-up tags with a pole is relatively safer than Spot tags, which must be fixed to the dorsal fin of a shark held in a cradle.

Swimmers' fears that there are indeed more sharks around may prove correct, thanks to the quota management system and a decline in tuna longlining economics. Holdsworth notes that mako and blue shark landings and taggings reported by big game fishers fell sharply from the mid-90s but have levelled off in recent years. Reported commercial bycatches of mako fell from 319 tonnes in 2001 to 76 tonnes in 2007 but the reliability of the data is questionable.

Holdsworth and others hope shark numbers are beginning to recover. But no one knows for sure. Not enough research has been done to provide a baseline and the migratory nature of many species makes the process fraught.

If numbers are increasing, researchers can at least reassure us that shark attacks on humans are much less frequent in New Zealand than in Australia. For one thing, there are more large shark species there, and more people in the water. There may be other reasons.

Duffy says unprovoked attacks on divers or swimmers have averaged about two a year for the past decade and most have resulted in minor wounds. The last serious attack was off Bench Island near Stewart Island on New Year's Eve in 1999. He can understand the fear of great whites by paua divers in particular.

"But white sharks do appear to be able to tell the difference in most instances - they recognise that we are not their natural prey."

As for sharks' penchant for outboard motors, that is thought to result from the electric current generated by outboards, which they may mistake for a struggling fish.

"Personally I think it's more dangerous driving to the beach than going in the water," says Francis. "Both Clinton [Duffy] and I are scuba divers. Knowing there are sharks out there doesn't stop us going in the water.

"If sharks really wanted to eat us we just wouldn't go in the water. There are heaps out there - great white sharks occur right around New Zealand - and we are so defenceless in the water that if they really wanted to eat us there would be people dying every day."