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Great White Shark Diving - Stewart Island, New Zealand

It's not that well known for New Zealand to be a mecca for Great White Shark diving. But this is where all the big boys come to visit every year.

Range and habitat

White sharks (Carcharodon carcharias) are globally distributed in temperate and tropical waters, including the Mediterranean Sea. In New Zealand waters they occur throughout the Exclusive Economic Zone north of about 53oS (Campbell Island). Little is known of habitat use in New Zealand waters. Juveniles and adults occur in shallow coastal waters, including large harbours and estuaries. Sub-adults and adults also occur in the open ocean, as well as around offshore islands and banks.

Genetic and satellite tagging studies demonstrate movement of white sharks between New Zealand, Australia and the southwest Pacific, and Australia and South Africa. Details of these movements are complex and the motivation for them remains uncertain. They involve rapid movements along continental shelves, rapid transoceanic return migrations, temporary residence in what appear to be favoured feeding areas and possible differences between mature females and the rest of the population.

Satellite tagging of New Zealand white sharks has shown that juveniles and adults migrate seasonally, from March to September, between aggregation sites at Stewart Island and the Chatham Islands to the tropical and subtropical Pacific (i.e. northern New South Wales and Queensland, islands and seamounts in the Coral Sea, Norfolk Island, New Caledonia, Vanuatu, Fiji and Tonga). These sharks appear to spend at least 5-7 months north of New Zealand before returning, often to the exact place where they were tagged.

Interestingly no movement between New Zealand tagging locations or between the Chatham Islands and the east coast of the South Island has been documented. However the movements of these sharks overlap with each other north of New Zealand. Little is known of the movements of white sharks, including small juveniles and large females, which occur off Northland and the west coast of the North Island.

In South Africa and Australia juvenile white sharks have been shown to make extensive return migrations along the continental shelf. The timing of these is thought to be related to the migrations of important prey species such as the sardine run in South Africa, and spawning of snapper in Australia. Genetic research indicates that like many other sharks female white sharks return to the same areas they were born in to give birth.

Population

The North Atlantic white shark population has been estimated to have declined by more than 90%. Although it is widely accepted that there has been a substantial decline in abundance in the North Atlantic this estimate remains controversial, and there are signs that the population is recovering following protection. Data from protective beach meshing (shark net) programs in South Africa and Australia indicate abrupt declines in abundance following introduction of the nets, however the South African population appears to have stabilised.

No data that could be used to determine population trends in New Zealand waters exists. As commercial fishers do not target this species and it is a relatively rare by-catch non-reporting is common, making any estimates of abundance based upon reported catch or landings unreliable.

Diet and foraging

White sharks are apex predators and may play an important role in controlling the populations of important prey species. Although they may be encountered anywhere within New Zealand’s Exclusive Economic Zone, they are most frequently observed in areas close to large seal colonies.

Species recorded in the diet of white sharks caught in New Zealand waters includes bony fishes, other sharks, stingrays, eagle rays, blue penguins, New Zealand fur seals, New Zealand sea lions, southern elephant seals, leopard seals, common dolphins, Hector’s dolphins and whale blubber (presumably scavenged from whale carcasses).

Their large size, habit of feeding on large prey such as marine mammals and their propensity to investigate objects floating at the surface by biting them makes shark attack a potential risk for anyone swimming, diving, surfing or operating a small vessel (such as a kayak) in areas frequented by white sharks. Despite this attacks by white sharks on humans are relatively rare in New Zealand, even in areas where they are seasonally abundant.

Life history

Males are estimated to reach maturity at 3.5-4.1 m total length and females between 4.2-5.2 m. Size at birth is 109-165 cm total length and 20-30 kg weight. Litter size is between 3-14 pups but usually less than 10. The gestation period is unknown but is estimated to be about 18 months.

Age is estimated from counts of calcified rings on the vertebrae. It is assumed that, like most other sharks, these rings are laid down annually. Maximum age is unknown but likely to be between 30 to 40 years. Female age at maturity is estimated to be around 15 years, and male age at maturity around 7-9 years. All age and growth studies of white sharks have so far been confounded by relatively small sample sizes and few samples from mature females.

Female white sharks reach about 7 m total length (the largest reliably measured was 6.4 m), and males to 5.5 m. Reports of exceptionally large white sharks between 9-11 m total length are impossible to verify. The maximum accurately recorded weight was 2500 kg. A 4.7 m female white shark caught off Port Waikato in 2005 weighed 1,628 kg.

Little is known of white shark breeding in New Zealand however most records of pregnant, newborn, and 1-2 year old white sharks are from shallow coastal waters around the top half of North Island. Sexual segregation does not appear to occur, with both mature and immature males and females being observed together at seal colonies in southern New Zealand. Small white sharks generally avoid larger individuals.

Tagging studies suggest white shark movements are independent of each other indicating they generally lead solitary lives but multiple individuals are regularly observed around seal colonies, feeding on whale carcasses and at whale strandings. 

© Pete Mesley's Lust4Rust and Shock&Awe Big Animal Diving - Auckland, New Zealand, 2103