Sharks have survived five major extinctions that wiped out the majority of life on earth during different stages of its history. So how have sharks survived and what life functions and morphology have they evolved to be so successful? What conservation exists and what are the anthropogenic threats to sharks?
Evolution and Anatomy of Sharks
The history of sharks stretches far back beyond the dinosaurs. Fossil records indicate that sharks evolved from the Ordovician period. Unlike many fish, sharks have a cartilaginous skeleton, built in an extremely stream lined design, which allows them to cut through the water to reach their prey rapidly. The absence of more solid material like bone is also a mechanism that allows the shark family to withstand the pressure found at depth.
Another difference between the shark family and bony fishes is the absence of a swim bladder. The swim bladder is a structure that helps maintain fish buoyancy at varying depths in the ocean, however this is not found in sharks. Instead, they have developed a large oil-based liver which enables buoyancy control. This lack of a swim bladder also means that when sharks are brought up from the depths by humans they are not as likely to get damaged as fish with swim bladders.
The dreaded shark’s teeth are also well adapted for their individual purposes in different sharks. The sand tiger shark tooth is long, narrow, and pointed with serrated edges designed for holding prey and tearing off chunks of flesh. Rays, in contrast, have teeth designed to grind food and break down crustacean shells.
Sharks have evolved an array of different visual adaptations. Sharks have adapted to hunting at night and at depths when there is little light available. They have other enhanced senses to compensate.
Human Attacks on Sharks - a Major Threat
Statistically the number of sharks that attack humans every year is tiny, on average 5, whereas the number of shark deaths caused by humans globally is phenomenal at 100 million a year. For more information see the 'Shark Facts' link. This has forced many sharks into the critically endangered category of threatened animals.
In many countries, sharks such as the spiny dogfish shark are eaten. In recent decades the activity of finning has increased in frequency and the fins and oil have become valuable commodities. They are valued in the food and cosmetic industries, for example shark fin soup, where they are only a thickening agent, and in skin care products.
In some cases the majority of fishing catches are by-catch; a large percentage, on average between 10-50% is estimated to be sharks. The majority of by-catch must be thrown back. Unfortunately only a small number of by-catch survives after being returned to the water. Tourism is also a major threat to shark populations. There is even a belief that ecotourism has a negative impact on natural shark behaviour.
Tourism, Ecotourism, Alternative Energy and Sharks
Are sharks more valuable dead or alive? Ecotourism tries to promote the live value of sharks to local communities and internationally. However in some bodies of thought there is a fear that human interaction and commercialising of sharks could negatively affect their natural behaviour, although the full effects if this are not yet known.
A relatively new and popular recreational activity is shark cage diving which has become a holiday excursion in areas like the Great Barrier Reef of Australia. This interaction allows humans to get up close and personal with sharks, without really putting their own safety at risk. Most cage diving is now restricted to viewing larger pelagic sharks, like the great whites, blue and mako sharks. The water is often ‘chummed’ or scented with food and fish entrails to attract sharks to the area using their heightened sensory system. In cage diving there is still a high risk factor and the safety record of the activity is not perfect.
Select dive companies offer the opportunity for divers to be involved in a feeding frenzy as well as diving and snorkelling with sharks. During shark feeds one trained feeder will distribute fish meat to encourage sharks to swim near the observing divers kneeling on the sea bed. This is a form of ecotourism, but it is not necessarily healthy natural behaviour for the sharks. In most responsible companies, the number of divers allowed in the water at any one time is restricted to prevent too much pressure on the wildlife. This activity is a controversial one particularly as the safety standards involved and the industry by nature are difficult to regulate.
Where are Sharks found
Sharks are found in almost every ocean, and at varying depths. The deepest recorded shark was found at 3700m. Other deep water sharks include the weird and wonderful goblin shark, which has a long rostrum and pink colouration unique in shark species.
Shark Conservation
A number of trusts and organisations exist that support the conservation of this endangered organism. The shark family, though fearsome predators in their own environment, do not have the ability to battle humans and the ever growing fisheries industries and sport hunting. One of the most prominent charities in the U.K is The Shark Trust, a charity that campaigns ‘to advance the worldwide conservation of sharks through science, education, influence and action’.
References
Ocean-The World’s Last Wilderness Revealed by Fabien Cousteau-Dorling Kindersley Limited 2008
Shark-Miranda Macquitty-Dorling Kindersley Limited 1992
The Seafarer’s Guide to Marine Life by Paul V. Horsman- Christopher Helm Publishers ltd. 1985
The Shark Trust - Sharktrust.org-visited on 02/03/10
Shark facts-Marinebiodiversity.ca-visited on 02/03/10
"Water Column" by Douglas David Seifert in Dive Magazine-‘To Feed or Not to Feed'-2009
Alex Leeper - Hello, My name is Alex Leeper. I am currently studying an undergraduate degree in Marine Biology and Oceanography and the university of ...
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