What Additional Factors Affect the Success of Shark Migration?

Sharks are the most successful predators in the ocean. Their only real enemies are other sharks and humans. Of these two, humans are the only serious threat to the migration success of sharks. Humans threaten sharks and their migration success in several ways. These include killing sharks for their commercial value, for sport, in response to the media, and to protect swimmers at public beaches.

Predation: The most common factor affecting migration is predation, the likelihood that small, slow or weak animals will be eaten by another larger predator. This is equally as true for sharks as it is for other animals with one exception -- since sharks are the most successful predators in the ocean, their only enemies are other sharks and man. Sharks do feed on each other, though this is less likely than we would think since sharks are highly specialized hunters with preferred food sources. Humans are the only really serious predators affecting shark survival.

Environmental Factors: Because they are strong ocean swimmers, sharks are able to escape unharmed from many environmental events that will do serious damage to slower or more fragile animals lower on the food chain. For example, due to its keen sense of smell, swim speed and strength, a shark is likely to identify and swim around an oil spill. Newborns and juveniles would not be so fortunate, especially since sharks frequently give birth to their young in estuaries or lagoons, "nurseries" where juveniles may remain for the early period of their life-span. Size, endurance, wide territorial range and a powerful immune system also make sharks less likely victims for devastating killers such as red tide, tumors and diseases caused by environmental pollutants.

Toxic chemical spills and dumping, on the other hand, could harm large sharks as well as all other marine animals in the area. Chemicals are not as easy to detect as oil, and animals may not try to escape. Toxic chemicals can be absorbed by organisms and passed up the food chain through predation. Top predators like sharks may be at higher risk since some kinds of toxins accumulate in each predator along the food chain, and become most concentrated at the top of the food chain.

Commercial Fishing: Many species of sharks are actively commercially fished for their fins, meat, livers, skins and cartilage by commercial enterprises all over the world. Many sharks are also killed accidentally when gill nets are used by commercial fishermen. In recent years, dramatic increases in shark catches have occurred worldwide. It is currently estimated that 100-200 million sharks per year are fished. This has a powerful impact on shark populations because sharks have a slow rate of growth and have few young. Some species take as much as 12 or more years to reach adulthood, when they can reproduce, and many species give birth to only one or two pups at a time. Therefore, over fishing of sharks can have long term effects on their numbers.

Finning: The main reason sharks are fished commercially is their fins. A traditional Chinese delicacy, shark fin soup is made using only the fins of shark, which have a thickening property. In a brutal practice called "finning," the fins are cut from the shark's body while the shark is still alive. The shark may be sold for the rest of its meat, but since the fins alone bring such a high price (about $20.00 a lb.), the shark is usually thrown back to die slowly. One of the sharks that is most preferred for its fins is the great white shark. The United States has outlawed live finning, and protects 39 species of sharks by law. Trade in great white shark parts has been banned by Australia, and South Africa has banned fishing great white sharks, declaring them a protected species. In spite of these regulations, a huge black market exists and widespread poaching continues. Other sharks fished for their fins include hammerhead sharks, blue sharks, bull sharks, mako sharks, sandbar sharks, basking sharks and tiger sharks.

Shark Tournaments: For decades large numbers of sharks have been killed every year at sport shark fishing tournaments along the Atlantic and Gulf of Mexico coastlines. From the 1960's to the 1980's, the numbers of sharks caught in these tournaments increased dramatically, bringing estimated catches into the many millions of pounds. Because of the large drop in the shark population in recent years, many of the traditional tournament fishing events have been discontinued. In addition, the conservation movement in sport fishing has introduced a new ethic into recreational shark fishing, by educating fishermen about marine resources, including sharks. In the place of previous "catch and kill" tournaments, "catch and release" tournaments are frequently held, and all sharks caught are tagged and returned to the ocean. The increase in shark tagging programs has given fishermen a positive alternative to killing their catches. Catch and release tournaments are a cooperative enterprise that benefits both sport fishermen and scientific research.

Media: The media has affected attitudes and also causes a threat to sharks. Imagine the effect on shark populations if the Hollywood film industry came out with another sequel to "Jaws" called "SHARK ATTACK!" This very successful film, as well as many "tabloid" documentaries, have had a devastating impact on sharks, causing thousands to be slaughtered. Films like these have little basis in scientific fact and distort reality in order to entertain an audience. Great white sharks are not single-minded man-hunters. It is extremely rare that a shark will attack a human, and if it does, it is almost always a mistake which occurs because the human is behaving like the shark's preferred prey. Surfers wearing wet suits and riding surf boards look like sea lions to sharks. Most often, if a shark does mistake a human for a sea lion, the shark will let go and not pursue the human victim further.

While serious scientists are trying to find the real answers to why sharks are resistant to disease, other people have made sensational claims in order to make money. Irresponsible claims that "Shark Cartilage Cures Cancer!" or "Shark Liver Oil Cures Arthritis!" have created an industry which sells medications made from shark body parts. Media coverage of these false claims has helped create a market for these drugs among people who are desperate for a cancer cure and sharks are now being fished in the Pacific Ocean for their cartilage.

Beach Nets: Finally, beach nets kill sharks and can affect their migration success. Nets to protect swimmers at public beaches from sharks are used extensively along the South African and Australian coastlines. The most extensive net system is near Durban, South Africa, where there are 21 nets, each 305 meters long (1,000 feet). Frequently, when hunting for fish, sharks as well as other marine animals get over or between the nets into the areas near the beaches. The animals are then trapped and they drown.