Bull SharkSharks are fascinating to many people, but to others sharks are primitive, vicious eating machines to be killed at every opportunity. Many people mistakenly believe that sharks are a significant threat to human life. The majority of the 370 species of sharks are harmless, are less than 2 m long, and many species are less than 1 m long. The largest species, the whale and basking sharks, are plankton feeders, and no threat to humans.
Sharks and their close relatives, the rays and the ghost sharks, belong to the class Chondrichthyes, the cartilaginous fishes. They are distinguished from the bony fishes by having only cartilage in the skeleton. A more easily seen feature is five to seven open gill slits on the side of the head in sharks (in rays the gill slits are underneath and the pectoral fins joined to the head), while bony fishes have their gill slits covered by an operculum or flap. The ghost sharks also have a cartilaginous skeleton, but their gill slits are covered by an operculum, or flap.
One hundred and sixty five shark species, slightly less than half the world total, are found in Australian waters. Those families with the largest number of Australian species are the dogfish family Squalidae (40 species), the catshark family Scyliorhinidae (32 species) and the whaler family Carcharhinidae (30 species). Our sharks range in size from the 27 cm Pygmy shark (Euprotomicrus bispinatus of the dogfish family Squalidae) to the 12+ m Whale shark (Rhincodon typus of the family Rhincodontidae), largest of all fishes. Sharks are found in most parts of the ocean, from the surface to depths of more than 2000 m and from mid-ocean waters to the shallows off almost every coast in the world. In some areas, notably Central America and Africa and even tropical Australia, sharks swim many kilometres into fresh water.
The oldest known fossil sharks are from the late Silurian more than 400 million years ago. Sharks are older than dinosaurs and are still successful as one of the top groups of predators in the ocean. They are highly adapted and play a vital role in the marine environment, similar to that of birds of prey or big cats. As well as sight, smell, hearing, touch and taste, sharks have a number of sophisticated prey locating sensory systems that include vibration detection and bio-electric reception. This latter sense, only discovered in the last 30 years, enables sharks to detect small electric fields given off by animals buried in the sand.
All sharks have internal fertilisation, with males possessing claspers - modifications of the pelvic fins - for sperm transfer. A few sharks, such as the Port Jackson shark (Heterodontus portusjacksoni) and cat sharks (Scyliorhinidae), lay eggs with horny shells (the spiralled case of the Port Jackson (right) is often washed up on our southern beaches). All other sharks give birth to living young. Two patterns of development are recognised. In most sharks the developing embryo is nourished by the yolk sac, although development is completed in the uterus. This is termed ovoviviparity. Some sharks, such as hammerheads (Sphyrna spp) and whalers (Carcharhinus spp), have true or placental viviparity, where the yolk sac of the developing young is much reduced and nourishment of the embryo comes from the uterus via a placenta. The number of young varies with the species and the size of the female.
In some, such as the Australian Grey Nurse shark (Carcharias taurus), the bizarre egg-eating and intra-uterine cannibalism, where the first hatched eats the other eggs and embryos in the uterus, results in only two well-developed pups being born. More than 80 embryos have been taken from large Tiger sharks (Galeocerdo cuvier), while the Whale shark recently has been found to have as many as 300 young. However, this is far fewer than most bony fishes, some of which can produce millions of tiny eggs in one season.
Most sharks are estimated to live to 20-30 years and become sexually mature at 6-7 years, while a few such as gummy and whaler sharks are mature at 2-3 years and live only 10-15 years. The White-spotted spurdog (Squalus acanthias) lives to 70 years with sexual maturity at 20. A tagging study of the southern Australian school shark (Galeorhinus galeus) had a recapture after 36 years, indicating ages over 50 years.
Sharks eat a variety of foods. The largest of sharks, the Whale and Basking (Cetorhinus maximus) sharks, eat plankton, while the Port Jackson shark (above) often feeds on sea urchins that are crushed by flat teeth and account for the purple stain found around the mouth. Many of the larger, fast-swimming sharks are true carnivores and an astonishing variety and amount of food have been found in some. For example, a 3.5 m whaler shark taken near Sydney was reported to contain eight legs of mutton, half a ham, the hind quarters of a dog, 135 kg of horseflesh, a ship's scraper and a piece of sacking. This was in the days when abattoirs dumped remains in the water, a practice unthinkable today. The numerous accounts of indigestible items taken from sharks' stomachs are probably the result of the feeding frenzy which occurs when a group of sharks indiscriminately attack anything that comes their way.
There is conflicting evidence concerning the digestive rates in sharks. The gastric juices of sharks are highly acidic and cases have been reported where the head of an eaten shark had been digested almost beyond recognition while the tail was still protruding from the mouth of the attacker. On the other hand, a large tiger shark died after a month in the Taronga Zoo in Sydney and at the autopsy, two 3 m dolphins were found in an almost perfect state of preservation in the stomach; they must have been eaten at least a month previously. There is also the famous Shark Arm Murder of Sydney, where a captive shark was found to have had a human arm in its stomach for between eight and eighteen days, so well preserved that the victim was identified by a still recognisable tattoo on the forearm. There is still much to be learned about shark biology.
When sharks use humans for food, a serious conflict arises. Sharks are undoubtedly important in the ecology of the oceans, acting as scavengers as well as top carnivores. Reaction to shark attack often is not based on facts, but on emotion. Figures clearly indicate that sharks are an insignificant cause of the approximately 120,000 annual deaths in Australia. Considering the number of sharks and the number of bathers in the water, shark attack on humans is an exceptionally rare occurrence.
Sharks and their cartilaginous relatives, skates and rays, are facing an increasing threat from humans. For every person killed by a shark, over 23,000 tonnes (that is 23 million kg) of sharks and rays are killed through commercial and recreational fishing and shark control programs such as meshing of swimming beaches. We target sharks commercially, both for their meat and for their fins. Sharks are also killed as bycatch, when some other species is the target. Enormous numbers of blue sharks are caught by driftnets set for tuna. We catch sharks for recreation, although increasing numbers are tagged and released. Recreational spear fishing in New South Wales has put the Grey Nurse shark under threat by greatly reducing its numbers. We kill sharks in meshing programs off New South Wales and Queensland beaches to make ocean waters safe for all swimmers. These nets, unlike those in harbour swimming areas, do not physically keep sharks from the beach but are designed solely to kill sharks.
Sharks have a low reproductive capacity and are not replacing their numbers as fast as humans are killing them. The southern Australian commercial fishery for school and gummy sharks has greatly reduced quotas as a result of overfishing. The Grey Nurse shark has been a protected species in New South Wales since 1984 and its numbers are still low. Falling shark numbers throughout the world have prompted the International Union for the Conservation of Nature to establish as part of their Species Survival Programs a Shark Specialist Group to address the problems. A program of public education is underway, and in recent years the media have changed their "feeding frenzy" reporting about sharks. The White shark (Carcharodon carcharias) was declared a protected species in South Africa in 1993 and in California, with widespread public support, in 1994. Tasmania, New South Wales and Queensland recently have protected this species in their state waters.
|
Activity |
Total Deaths |
Average/Year |
|
Crocodile Attacks |
8 *** |
0.7 |
|
Shark Attacks |
11 ** |
1 |
|
Lightning Strikes |
19 * |
1.7 |
|
Bee Stings |
20 * |
1.8 |
|
Scuba Diving Accidents |
88 **** |
8 |
|
Drownings/Submersions |
3367 * |
306 |
|
Motor Vehicle Accidents |
32772 * |
2979 |
|
* from the Australian Bureau of Statistics, Canberra |
||
More protection is needed. Marine ecology is complex and it is difficult to predict accurately the consequences of eradicating a top carnivore like the White Shark. One outcome might be that prey species like seals and sea lions would increase, and reduce the number of their prey species like smaller fishes and squid, some of which are commercial targets. The beach meshing programs that cost in excess of $600,000 per year in Australia should be re-evaluated. The early catches of 1000+ sharks per year in New South Wales have been reduced to about 200 per year as populations have been drastically reduced, even though many more beaches are now meshed.
The conservation of large carnivorous animals that affect human populations, like tigers, crocodiles and sharks, is a contentious issue. Few informed people would urge that these species be slaughtered to extinction. Many of these species are endangered, and some like the tiger and crocodile, are protected. For sharks, what is required is enlightened public debate on the value of a species, the actual vs perceived risk to humans, how best to minimise that risk and the methods and consequences of protecting threatened species.
John R. Paxton
Principal Research Scientist
Australian Museum
Copyright © Australian Museum, 2003
This page: http://www.amonline.net.au/factsheets/sharks.htm
