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Location: Sharks & Rays

Tiger shark (Galeocerdo cuvier)



Tiger shark (Galeocerdo cuvier)
The tiger shark is considered one of the most dangerous sharks. It is about 18 feet long and inhabits shallower water, often where people swim. The diet of tiger sharks varies widely and includes all types of sea life and even garbage. The tiger shark, Galeocerdo cuvier (Peron & LeSueur), is a large (up to 18ft) predator found in tropical and subtropical waters worldwide. Tiger sharks are one of three main shark species known to attack humans, and are responsible for most shark attacks in Hawaii. Less than one shark attack occurs per year on average in Hawaii (compared to an annual average of 40 drownings) and most attacks are non-fatal. This attack rate is surprisingly low considering that thousands of people swim, surf and dive in Hawaiian waters every day. Despite these statistics, shark attacks remain a highly emotive topic in Hawaii.



This reaction is unsurprising in a state that is economically dependent on tourism and recreational ocean use. Shark attacks are bad for business and prompt outcries for shark culling. Unfortunately such 'knee jerk' reactions have no sound scientific basis. Furthermore, killing tiger sharks contradicts traditional beliefs of native Hawaiians, who consider these animals to be sacred 'Auma kua' or ancestor spirits. Nevertheless, from 1959 to 1976, the state of Hawaii killed 4,668 sharks (at an average cost of $182 per shark) in a series of shark control programs. In spite of such efforts no significant decrease in rate of shark attacks was ever detected. Following a series of fatal shark attacks from 1991-93 attempts were made to revive the shark control program. This prompted a team of Ph.D. students, led by Dr. Kim Holland of the Hawaii Institute of Marine Biology, to lobby the state legislature to instead fund scientific research aimed at increasing understanding of tiger sharks. Major goals of this research include determining movement patterns and habitat use of these predators.
 
Tiger sharks are captured by setting lines baited with fish market scraps, at dusk, just off Honolulu harbour. Lines are recovered at dawn the following day.
 
Sharks caught are secured alongside a small boat, identified, measured and sexed, and a tissue sample is removed for genetic fingerprinting. Each shark receives a numbered identification tag and those smaller than 10ft are immediately released.
 

Larger tiger sharks are fitted with acoustic transmitters (devices which emit ultrasonic beeps). These beeps are beyond the hearing range of both sharks and humans but are detectable with a hydrophone (underwater microphone). Transmitter equipped sharks can be followed by boat using a hydrophone to continually listen for these beeps. A single 'beer can sized' transmitter is surgically implanted in each sharks peritoneal cavity (the space containing the liver and other organs). During this operation the shark is turned upside down and immediately goes into tonic immobility (a trance-like state lasting approximately twenty minutes). An incision is made in the peritoneal wall, the transmitter is inserted and the incision sutured closed. The shark is released and followed for several days.

Data obtained by tracking with boats is supplemented by using bottom monitors placed at different locations on the seabed. Bottom monitors are devices that continually listen for the uniquely frequency coded transmitters carried by larger tiger sharks. Monitors can identify each transmitter equipped shark, and record how long each shark stays in that area. Bottom monitors are regularly retrieved by divers so that recorded data can be recovered.
 
To date over 130 tiger sharks ranging from 7 to14ft in length have been tagged and released. All tiger sharks tracked swam over 10 miles offshore along a similar course in the first 24 hours after release, and showed no sign of returning to the Honolulu area in the following 48h. Bottom monitor data indicate that although most sharks return to their original site of capture, time spent away from this area varies between individuals from two weeks to ten months. These results may suggest that tiger sharks have extremely large home ranges and that control programs are unlikely to be effective in catching sharks responsible for attacks as these individuals may move beyond the fished area within hours of the incident.



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