Demystifying shark myths
Most people think of sharks as vicious predators. This state of mind was popularized by the movies Jaws and Deep Blue Sea. However, the perception is both unfortunate and incorrect, since there are relatively few unprovoked shark attacks on humans worldwide. Of the more than 350 species of shark in the worlds oceans only a handful of them are even considered dangerous to humans. Dangerous species found in Canadian waters include the great white, mako and oceanic whitetip. Overall, the chance of being attacked by a shark is considerably less than that of being hit by lightning or of being eaten by a crocodile. Most of those attacks that do occur are in the waters off of South Africa and Australia. In Atlantic Canada, there have been four accounts of sharks attacking small boats, three of which have been attributed to the great white shark by positive identification of teeth or tooth fragments found embedded in the hull. See also Shark Sightings.
What do sharks eat?
Most sharks are predators, although the two largest species (the basking shark and whale shark) have no obvious teeth and eat only plankton (tiny floating animals). The majority of sharks eat fish and invertebrates, while some feed upon marine mammals such as seals and sea lions. Other animals have been found in shark stomachs as well. Bones and vertebrae from cows and reindeer, chickens, dogs, penguins and other birds have all been found, as well as a number of more intriguing items: tin cans, a wristwatch, an engine block, a partial suit of armor, parts of a rocking chair, bottles, buttons, shoes, belts and a handbag. Most sharks in Atlantic Canada are too small to eat large items, but fishing line, rope, balloons, lobster bands, candy wrappers and an oil filter have been found in stomachs of the porbeagle shark.
How big can sharks grow?
Sharks come in a range of sizes. The largest shark known is the whale shark, which reaches lengths of 12 metres (almost 40 feet) or more. The smallest shark, the dwarf shark, is only 25 cm (10 inches) long. The basking shark is the largest shark in Atlantic Canada, routinely reaching lengths of 8-9 metres. The largest confirmed great white shark caught in Canada was more than 5 metres long. It was caught off of PEI in 1983. Before this, a white shark was caught in a herring weir in the Bay of Fundy in 1930 which was reported by local fishermen as being 11 metres long. For awhile, this was considered the largest specimen ever caught, but when the jaws were compared to that of other jaw specimens, they were more similar to a shark of around 4.8 metres. The world record is 6.3m.
How long have sharks been around?
Sharks have been around since the Upper Devonian, approximately 409 million years ago. The subclass Elasmobranchii, to which the sharks belong, have contained three branches through time. The original and most primitive branch is the cladodontoids branch, which is now extinct. The second branch is the hybodonts, who are also now extinct, but who eventually gave rise to the modern sharks and rays contained in a grouping called the elasmobranchs. There are still a few shark species alive today that represent a transitional position between the hybodonts and today's modern shark.
Do porpoises attack sharks?
It has become close to an urban myth that dolphins and porpoises regularly attack sharks that come too close. The reality is somewhat different. In some parts of the world, dolphins are frequently preyed upon by large shark species such as whites, bull sharks and tiger sharks. In general, dolphins will avoid close encounters with predatory sharks, and ignore those not viewed as being a danger. Although there have been at least two documented occasions in which groups of dolphins have chased away threatening sharks, such occasions appear to be more the exception than the rule.
How are sharks helpful to humans?
Sharks have many uses to humans. Nearly every part of the shark can be used: its flesh can be eaten, the skin can be used as leather, and the teeth used for jewelry and ornaments. Oil extracted from the liver has been used forhigh grade machine oil, vitamin A supplements and in ladies cosmetics. The fins have also been used in shark fin soup, although this practice is being discouraged, since it is contributing to overfishing of some shark species. Parts of the shark have medicinal value as well. The shark cornea has been used in eye surgery (since its cornea is similar to our own). Shark cartilage has become a popular natural source of possible anti-cancer agents, since sharks have low cancer rates. However, there is no evidence that shark cartilage actually has anti-cancer activity. But the most important aspect of sharks is their role in the marine ecosystem: as apex predators, they help control many fish and marine mammal populations, and thus help keep the ecosystem healthy.