Marine Species Conservation
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Sharks (elasmobranchs) comprise about 1% of all living fishes, and share nearly all the major features of their finned relatives. Like all fishes sharks use gills to extract oxygen from the water in which they live. Sharks have five to seven gill openings along each side of the head, through which they breathe. Primarily marine fishes, a few have adapted to fresh water, such as the Northern River Shark and Speartooth Sharks (Glyphis sp A and C respectively) found in the Northern Territory of Australia.
About 350 species of sharks occur worldwide. Sharks are primarily oceanic and are widespread in tropical to temperate zones. Sharks vary greatly in size. The largest species, also the largest of all fishes, is the plankton eating giant the Whale Shark (Rhincodon typus). The largest measured specimen was 12.65 m long and weighed 21.5 tonnes, but the Whale Shark probably attains 18 metres. Some of the deepwater shark species attain only about 25cm.
Most sharks are predators. Many sharks species become active after dusk and hunt during the night. The majority of sharks feed on other fishes. Large sharks, such as the Great White Shark (Carcharodon carcharias) and Tiger Shark (Galeocerdo cuvier), prey on large marine mammals such as seals, sea-lions, dolphins as well as large fishes, turtles and even sea birds.
Several species of shark are known to be dangerous to humans: the Great White Shark, Tiger Shark, Bull Shark (Carcharhinus leucas) and other whaler sharks (Carcharhinus sp.). No shark is thought to target humans as prey, rather the majority of shark attacks can be attributed to the shark confusing us with its normal prey.
While some sharks are probably not very selective feeders, certain sharks eat some foods more than others. For example, hammerhead sharks are known for eating stingrays; bull sharks eat other sharks; and smooth dogfish eat crabs and lobsters. Tiger sharks have been called "garbage cans of the sea" because they feed opportunistically on both live food and carrion. Prey includes bony fishes, other sharks, marine mammals, seabirds, and invertebrates. Many sharks prey most often on the weak, elderly members of a population. They select this because it is easier to catch.
Many species of sharks are adapted for bottom feeding. Bottom feeders use the upper jaw to help pick up prey items. One example of a bottom feeder, the Port Jackson shark, has two types of teeth. Front teeth are pointed for grasping and back teeth are flat and molarlike for crushing. Another mechanism some sharks use for collecting food is filter feeding. The basking shark (Cetorhinus maximus) and the megamouth shark (Megachasma pelagios)strain enormous quantities of plankton from the water on gill rakers. Whale sharks also filter feed, but instead of using gill rakers, they strain plankton through a spongy tissue supported by cartilaginous rods between the gill arches. Filter feeders have reduced, nonfunctional teeth.
Mating in sharks is facilitated by the clasper organs of males, which are inserted into the female's cloaca during courtship. Fertilisation is always internal, and reproduction occurs in one of three modes:
Examples of all three reproduction methods are found in sharks living in Australian waters. The Port Jackson Shark (heterodontus portusjacksoni) lays spiral shaped eggs that young hatch from (oviparous), the Smooth Hammerhead Shark (Hphyrna zygaena) gives birth to live young (viviparous) and the Grey Nurse Shark (Carcharias taurus) gives birth to pups after they have hatched from eggs within the uterus (ovoviviparous).
Little is known about the growth and age of Sharks. Many of the conventional methods for aging animals, such as examining teeth, will not work with Sharks. Sharks grow slowly compared to bony fishes, possibly due to sharks' slow digestive time and feeding rates. There is considerable variation in age and growth rates between species, and even between populations of the same species. Growth rings are periodically deposited on the vertebrae of some sharks. Vertebrae can be stained and examined for these growth rings. Growth rings may, however, stop developing in older sharks. Examining the vertebrae of captive-born sharks after their death enables researchers to compare the number of growth rings with the shark's known age. In some areas tagged sharks are providing information about growth rates. Once a shark is caught, it is measured, tagged, and released. The shark is measured again when it is recaptured. Researchers correlate the measurements with the number of years since capture and calculate a yearly growth rate.
In Australia, species are listed as threatened species under the Environmental Protection and Biodiversity Conservation Act 1999. The listed shark species are:
Projects aimed at the conservation of these species have been funded under the Natural Heritage Trust's Marine Species Protection Program.
Shark attacks occur rarely. Only a few of the 450 or so shark species have been known to attack people. Unfortunately, some attacks are fatal.
There are some easy and commonsense precautions to take that can help reduce the risk of a shark attack. This risk minimisation advice is reproduced from the Australian Shark Attack File.
For more information on shark attacks, risk minimisation, statistics and maps, please see:
The Department of the Environment, Water, Heritage and the Arts is not responsible for the content of the following web sites.
If you would like further information on sharks listed under the EPBC Act, please check the above links before contacting the DEH.
For more information about the conservation of sharks in Australian waters contact us.