Sharks

Page - September 21, 2007

Estimated population:

The worldwide population of each species of shark is unknown.

Ways to identify this species:

About 40 species of shark are endemic to the Hawaiian Islands, 8 of which are often seen near shore. The most common sharks seen near shore include the sandbar shark, the whitetip reef shark, the scalloped hammerhead shark, and the tiger shark. Sharks have very tough skin that is generally covered in small, sharp scales varying in color depending on the species.

  • Tiger shark - has a blunt snout and vertical bars on its sides. It is dark gray or tan in color and reaches lengths over 18 feet but averages about 12-13 feet.
  • Sandbar shark - has a large erect dorsal fin set forward on its body and a sloping forehead. It can reach up to 5-6 feet.
  • Whitetip reef shark - has a white tip on the first dorsal fin, the upper lobe of the caudal fin and occasionally the second dorsal fin and lower caudal lobe. Reaches about 6 feet in length but averages 2-3 feet.
  • Scalloped hammerhead shark - has a head resembling a mallet, eyes spaced far apart, and the front margin contains 4 shallow lobes. It averages about 5-7 feet but can reach from 12-15 feet in length.

Biology

  • Sharks are a family of species that contain an internal skeleton made of flexible cartilage. This enables efficient swimming and an ability to turn abruptly in the water when seeking prey.
  • Sharks have highly evolved and complex brains. The sensory part of the brain is especially well developed giving these species great senses of smell and taste that enables them to detect their prey at great distances (>2 miles depending on water conditions) and detect pheromones for mating and locative purposes. Their eyesight, functionally similar to mammals, is also very important at closer distances but dependent on water clarity.
  • Deepwater and nocturnal sharks have huge green glowing eyes designed to take full advantage of what little light exists at ocean depths; however, most sharks lack the ability to close their eyelids for protection. Most notable is the shark's ability to detect faint electrical fields given off by living organisms and inanimate objects, through receptors in their snouts, mouths and other areas of their head. This sense allows prey detection without visibility and orients sharks within the earth's magnetic field when migrating. Sharks also have the ability to sense changes in pressure.
  • Sharks have many paired gill openings that enable them to breath underwater.
  • Sharks have multiple rows of teeth along the edges of their upper and lower jaws. This enables shedding and replacement when teeth get worn or fall out. Rows of teeth may turn over at rates as fast as 8-10 days or as long as several months.
  • The female's eggs are fertilized inside the body and young are either born live or released as eggs and hatch later depending on the species. Gestation can last up to 2 years and a litter can consist of 1 to 100 pups for some species. Pups have a full set of teeth and the ability to care for themselves. Once they are born they swim away from even their mother to avoid being eaten.
  • Sharks feed on reef fishes, smaller sharks and rays, cephalopods, crustaceans, octopuses, marine mammals and various other species depending on the type of shark.
  • The shark's function in reef ecosystems is not fully understood, but it is believed that they improve fish populations by removing sick and injured individuals. This enables reproduction of the healthiest fish.
  • Sharks can live for over 40 years.

Threats

  • Many shark populations have declined from overfishing and bycatch in pelagic longlines, gillnets, handlines, and bottom trawls.
  • Sharks are fished for their cartilage and fins. Shark cartilage is used in scientific research due to its medicinal potential for stopping tumor growth. The cartilage is often sold in pill form as an alternative medicine.
  • Sharks are frequently caught for their fins, which are used in shark-fin soup in certain cultures. Shark's fins are highly prized in international trade but the shark carcass is not used and is thus usually discarded.
  • Significant population declines pose a serious risk to sharks because of their slow maturation and reproduction process.

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