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Galeocerdo cuvier

Though the tiger shark is colloquially known as the waste bucket of the sea, this species of requiem shark is the largest predatory shark after the great white. The tiger shark grows to a length of over 5 metres and is the largest member of the Carcharhinidae family, which includes the other well known species such as the blue shark (Prionace glauca), the lemon shark (Negaprion brevirostris) and the bull shark (Carcharias leucas). The tiger shark gets its name from the distinctive black spots and vertical bars which run along the length of their body. Often found close to the coast and in tropical and sub-tropical waters, tiger sharks are a nomadic species that feed on a wide variety of prey including turtles, rays, other sharks, fish, sea birds, dolphins, crustaceans and carrion. Tiger sharks are a large and dangerous predator and therefore require significant resources to enable comprehensive study. Due to this reason, peer reviewed publications on its basic biology and ecology are lacking.

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Figure 1 – Distribution                Figure 2 – Tiger shark (CC Albert Kok)

Kingdom: Animalia Phylum: Chordata Class: Chondrichthyes -Sub Class: Elasmobranchii Order: Carcharhiniformes Family: Sphyrnidae Genus: Sphyrna Species: mokarran

Taxonomy and Anatomy

The shark was first described by Peron and Lessueur (1822) and was given the name Squalus cuvier(1). It was later renamed by Muller and Henle (1837) to Galeocerdo tigrinus before being finally re-named Galeocerdo cuvier1. The genus, Galeocerdo is derived from the Greek, galeos, “shark” and the latin, cerdus, “hard hair of pigs”.

Tiger sharks have a green, grey or black dorsal surface (top) and a yellow-white ventral side (underbelly). The easily recognisable dark spots and stripes of the tiger shark are most distinctive in the juveniles and range from blue to light green. The juvenile colourings are thought to act as camouflage in coastal waters as they resemble the shadows of waves in the water. Scientists have suggested this camouflage makes them undetectable from the surface, especially over seagrass beds where they will often hunt air-breathing prey such as sea birds and turtles(2). The spots and stripes fade into pale bars as the sharks mature, when they are no longer exposed to predation by other sharks and are less reliant on surface prey.

The large eyes of the tiger sharks have a reflective layer behind the retina called the tapetum lucidum. This reflects light back through the eye, allowing light sensitive cells to collect light and enhance the shark’s vision in low light conditions.

The wide and robust head of the tiger shark is wedge shaped and allows the shark to turn quickly. The tiger shark’s dentition is very distinctive compared to most sharks, possessing teeth with similar shapes in their upper and lower jaws. The teeth on the upper jaw are large with backwards curved cusps and finely serrated edges which have evolved to not only cut their prey but to saw through it also. This tooth type is specialised in cutting and rasping through sea turtle shells(3). Tiger shark tooth

The jaws are divided into 24 rows of identical teeth that decrease in measurement as they move towards the corners of the mouth. In contrast with other sharks, the shape of the jaw is square rather than round and gives the tiger shark skull its distinctive wedge shaped look.

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The serrated edges are highly effective at cutting (CC Stephan Kuhn)

Size, Age & Growth

The tiger shark is one of the largest sharks, growing to a maximum of 7.5 metres (27ft)(4) and weighing over 800 kgs(5). There is very little known about how old tiger sharks can grow and this is one aspect of tiger shark biology that scientists are currently studying. Once fully grown, tiger sharks no longer have any natural predators due to their size and the speed at which they can swim, which has been estimated at 32kmh (20 mph) or short bursts of even higher speeds(6).

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Tiger sharks are a resilient species and able to tolerate a wide variety of habitats. They are most commonly found close to the coastline in tropical and sub tropical waters worldwide (Figure 1), however they have also been recorded in warmer temperate waters(7). On-going studies have suggested that tiger sharks undergo seasonal migrations, traveling into temperate waters during warmer months and then returning to the tropics for winter. Their behaviour is primarily nomadic, meaning they tend to move widely, undergoing large voyages between large island chains to areas where prey is more highly available.

Even though they are found to make these voyages, tiger sharks are not considered a truly oceanic species(8). Due to their large range in diet and a preference for shallow water habitats, tiger sharks regularly come into contact with human populations in the tropics. Tiger sharks are curious to humans in the water and can be aggressive so must be considered with a great deal of respect.

Bulb.jpgEven though scientists have a basic knowledge of tiger shark behaviour and habitat type, there is still a lot to learn about specific behaviours such as their movements and the factors that determine them. Preliminary research suggests that differences in both behaviour and general biology do exist between tiger sharks located in different regions of the world.

Diet

The diet of the tiger shark has been more thoroughly researched than many other sharks, due to the broad range of species this opportunistic hunter preys upon. As the shark ages, its diet changes (called ontogenetic diet shift) and scientists are still working to better quantify this change with shark size. Young sharks (<200cm) prey on a lower diversity of species, generally feeding upon fish, birds, crustaceans and sea snakes(9). Smaller sharks are thought to target benthic (bottom) dwelling species because of physical limitations such as a thin elongated body giving low swimming speeds(10).

Bulb.jpgTiger sharks are a species that are known to surface regularly, which makes them an ideal species to track with satellite tags.

As the sharks age, the diversity of prey increases to include turtles, other sharks, rays, squid, and marine mammals (dugongs, seals and dolphins). This increase in range indicates that larger tiger sharks are not only shifting their diet to include larger prey items but also expanding their diet to include a wider variety of smaller prey(9). A larger number of pelagic fish and squid species found in the stomachs of larger sharks suggests a shift in behaviour to more open and deep water habitats(9). Research on tiger sharks in Western Australia has suggested that the sharks primarily use stealth to attack prey items, rarely engaging in high speed chases. The sharks were tracked and observed to use a ‘bouncing’ method, where they moved from the top layers of the ocean, to the bottom in order to maximise their potential to catch prey such as turtles and birds(2).

Bulb.jpgInterestingly, studies have shown larger tiger shark stomachs to contain kitchen scraps such as chicken, ham, bones, a sandwich, steak, tin foil, tin cans plastic bags, cellophane, paper and cardboard(9). It is becoming increasingly important that people become more aware how their activities impact on the marine environment. All types of plastic have been found in the stomachs of marine species including the baleen whales, turtles, sea birds and sharks.

Reproduction

There is currently little known about tiger shark reproduction, with estimates of tiger shark sexual maturity occurring at ~10 years of age(11). Males appear to mature at approximate lengths of 2.3 – 2.9 metres, and females 2.5 to 3.5 metres, although these estimates do vary significantly in different regions around the world. The breeding cycle of tiger sharks is also subject to debate, with publications quoting both a biennial and triennial reproductive cycle. It is known that internal fertilization takes place, although whether female tiger sharks can store sperm like other species is also unknown. In the northern hemisphere mating occurs between March and May and in the southern hemisphere between November to January. The male tiger shark uses his teeth to hold the female’s fins and inserts one of his claspers into her genital opening (cloaca). This aggressive process often causes the female considerable discomfort, leaving scars on their flanks and pectoral fins. Female tiger sharks often have thicker denticles than their male counterparts for this reason. The gestation period is also currently believed to be around 15-16 months and litters can range from 3 to 80 pups but average around 33 pups(12). Scientists have reported a large variation in pup birth size, anywhere from 50 centimetres to 90 centimetres(13).

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Tiger Shark Pups © Bonnie Holmes

Tiger sharks are the only species the Carcharhinidae with aplacental viviparous (ovoviviparous) reproduction(8). This means that the shark’s eggs hatch internally and the young are live and fully developed when born. An interesting fact about aplacental viviparity is that the growth of the pups is supplemented by uterine milk, which is secreted by the lining of the mothers’ uterus. It is believed that tiger shark pups absorb the remaining milk sac into their stomachs prior to birth, with the remaining milk to sustain them in the first few days or weeks of life.

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Near Threatened

Conservation Status

Tiger sharks are caught as a target species and as bycatch by many fisheries around the world. They are directly targeted in the western Atlantic, Australia, India, Papua New Guinea, Brazil and Taiwan fisheries(14). Tiger sharks are considered bycatch in tuna and swordfish longline fisheries, in particularly those operating close to continental and insular shelves14. Recreational fishermen have been recorded catching this species in South Africa, Australia and America, and trends suggest these sharks are now more commonly tagged and released than destroyed (IUCN website). A study by Julian Pepperell(15) estimated that tiger sharks represent 10% of recreational shark in New South Wales, Australia. Shark control programs that exist along Australian, Hawaiian and South African beaches attempt to limit tiger sharks from areas where bathers would come into contact with this species. The use of heavy gillnets and drumlines reduce the abundance of larger sharks in coastal areas. The International Union for Conservation of Nature (IUCN) website lists this species as Near Threatened, which indicates tiger sharks are not in immediate danger but are likely to become vulnerable in the near future. Luckily, the tiger shark has a slightly higher resilience to exploitation than many other slower growing species, due to its wide distribution, diverse diet and relatively high growth and reproductive rates. Currently in Australia there are no specific conservation or management measures in place, however the US administers the shark as part of their Fisheries Management Program 1993(14).

For a full list of references (#), please visit our website (TRACS Australia).

Source by Brendan