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Whale Sharks

When I first met Jean-Philippe and Luke, they had just returned from a day's kayak searching for the whale sharks that had recently been sighted in Bahia de Los Angeles, Mexico. These sharks are often seen in large groups or alone, swimming slowly near the surface. The whale shark, Rhincodon typus, is the largest fish in the world. It has been reliably measured to over 12 meters (39 ft.) and may grow to lengths of 18 meters (59ft.). In addition to its enormous size, the shark can also be easily recognized by its broad, flattened head and distinct pattern of white spots and stripes on a dark background. Several explanations for these distinct markings have been proposed. The pattern could be linked to some form of social behavior and recognition process or it may be an adaptation for protection from the sun's UV radiation.

Although whale sharks can be found in all tropical and sub-tropical oceans and is a massive and striking species, relatively little is known of its life history. For instance, the manner in which it reproduces was not clearly known until July of 1995. It had been debated for decades whether whale sharks laid eggs or gave birth to live young.

A whale shark that had been harpooned in a Taiwanese fishery was examined and found to contain 300 embryos that were developing inside thin egg cases within the mother. The whale shark was determined to be ovo-viviparous, meaning that the eggs hatch within the uterus of the mother and remain there until their development is complete. Much is still to be learned of the gestation period and of the early survivorship of newborn whale sharks. Although a large number of young appear to be given birth to, only nine juvenile whale sharks have ever been recorded. These ranged in length from 53-93 cm (20.9-36.6 in.). One was found in the stomach of a blue shark (Prionace glauca), and another in the stomach of a blue marlin (Makaira mazara). The movements, growth rates, and feeding habits of young whale sharks remain largely unknown.

Very little is known of the longevity and growth of whale sharks. Several specimens in captivity at the Okinawa Expo Aquarium in Japan have provided some information, but it is difficult to know how different the growth patterns in the wild may be from these captive animals. How long these giant sharks live, how old they are at the age of maturity, and how quickly they grow remain unanswered questions.

Like the great whales, the whale shark is a filter-feeder. It consumes a wide variety of plankton, small fish, and squid. Whale sharks do not appear to simply swim through dense patches of plankton with their mouths wide open in an attempt to capture prey. They are able to create a powerful suction and efficiently draw food items into their mouths and filter water back out over their gill rakers. Young whale sharks in Bahia de La Paz, Mexico were found to lift there mouths partly out of the water, turn their heads from side to side, and gulp in plankton where concentrated patches were found on the surface. After the sharks swam and gulped their way through the densest patches, they turned sharply and circled back. Whale sharks have often been seen vertically positioned in the water column, suction-filtering prey from dense patches of plankton as well.

Significant fisheries for whale sharks have developed in some regions. The flesh is considered a delicacy in Taiwan and large markets for flesh and fins from whale sharks exist in Singapore, Korea and Hong Kong. Seasonal migrations of whale sharks off the west coast of India have recently been targeted by fishermen there who export meat and fins to Asia. The resale value of frozen whale shark flesh for export to Asian markets has reached US $1/kg. The whale shark is referred to as the tofu shark in Taiwan because of its soft white flesh and it is now the most expensive of the shark meats available. A set of four dried fins runs US $400-500 in Taiwanese markets. Bans on the fishing of whale sharks were implemented in the Maldives in 1993 and the Philippines in 1998.

Other countries have found that the seasonal aggregations of whale sharks provide economic benefits in the form of tourism. At Ningaloo Reef in western Australia, steps have been taken to protect whale sharks from being disturbed by the numerous divers and snorkelers that frequent the area. Swimmers and divers are restricted to a distance of 3-4 m from the animals and may not use flash photography or motorized propulsion. The popularity of such encounters with whale sharks, as well as other sharks, is rapidly growing, and steps to appropriately manage the tourist industry need to be considered.

 

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