Project Passenger Pigeon

Lessons from the Past for a Sustainable Future

whale_sharkWhale Shark

Sharks in PalueShark fishing in Palau

 

Species Declining, Threatened, Gone, Saved

The Whale Shark: Waning of a Gentle Giant


Listed as Vulnerable on the Red List of Threatened Species

In 2002 the species was listed in Appendix II of the Convention on International Trade in Endangered Species (CITES). Appendix II includes species in which trade must be controlled to avoid utilization incompatible with their survival. Although this is a positive step, CITES designation does not carry the power of law, and enforcement is left to the individual member countries. Those countries that participate actively in the shark fin trade do not recognize the CITES designation. There are, however, reasons to be positive with respect to shark conservation. In 2006, Taiwan outlawed whale shark fishing, Palau declared its waters a shark sanctuary in 2009, President Obama signed the Shark Conservation Act in 2011, prohibiting the movement of shark fin through US ports, and a growing number of restaurants worldwide are removing shark fin from their menus.

Threats: Rare attacks on whale sharks have been reported from orcas and white sharks, but the whale shark has no significant natural predator other than man. While whale sharks are protected in many parts of their range, they are fished legally and illegally in some countries. Whale sharks are hunted for the oil from their livers, which is used to waterproof boats, and for their meat, which is eaten in many parts of Asia. In addition, whale sharks increasingly fall victim to the practice of "finning", in which the fin of a live shark is sliced off and the animal is thrown overboard to die. Shark fin has long been considered a delicacy in many parts of the world, where it is most commonly served as shark fin soup. The fins of larger sharks are extremely valuable, and together with the amount of meat that can be taken from a single animal, this makes them a premium species for fishermen in developing countries.

Conservation status: The conservation status of whale shark populations is unknown. While large feeding aggregations suggest healthy numbers of animals, the composition of these aggregations with nearly all subadult sharks leaves the status of breeding animals undetermined. Current evidence indicates that whale sharks are declining in number, with well-studied aggregations hosting fewer and smaller sharks over the past 10 years. This trend indicates a loss of reproductive age animals from the population, and a shift to smaller non-breeding individuals. Whale sharks’ late maturity makes them slow to recover from any breeding depression, and therefore successful attempts to mitigate this downward trend must begin while numbers of mature sharks are still high.

Background
The whale shark, Rhincodon typus, is the largest living fish and the largest shark. Whale sharks have broad flattened heads, enormous mouths, and an elongated body ending in a caudal fin that can be 5 feet tall in an adult animal. Whale sharks are blue-gray in color, with a pattern of spots and stripes that is unique to each animal. Whale sharks as long as 16 meters have been reported, though the animals most commonly seen are between 5 and 8 meters in length. They are found in tropical and warm temperate waters around the globe, and are an epipelagic species, meaning they favor the open ocean. It is believed that whale sharks are highly migratory, they may cross oceans, and even move between ocean basins. Whale shark lifespan may approach that of a human, possibly reaching 60-100 years. An animal this large does not escape notice, and while whale sharks were described to science only in 1828, local cultures throughout their range had bestowed names on the creatures, and incorporated them into folk tales and legends. In Mexico the whale shark is called “tiburon balena”, shark-whale, or more colloquially “Domino” for their pattern of spots and stripes. In India, the whale shark is called “Vhali”, which means “dear one” in Gujarati. Filipinos call the animal “butanding”, which translates to “gentle giant” in Tagalog. In Kenya, whale sharks are called Papa Shillingi, which means “covered in shillings”. This name comes from a story that says that when god made the whale shark he was so pleased with its beauty that he threw shillings, silver coins, onto its back, giving it a pattern of bright spots.

Migration: The need to find sources of plankton rich enough to sustain their massive size means whale sharks must travel to feed. As plankton sources may be widely dispersed in the ocean, whale sharks are believed to migrate long distances to take advantage of feeding opportunities. When a large food source does occur, the normally solitary whale sharks may show up in huge numbers. Such aggregations occur yearly in the coastal waters of Australia, Mozambique, India, Seychelles, Philippines and Mexico, as well as off the southern US coast. In 2009 one aggregation along the Mexican coast numbered more than 400 animals. Interestingly, many of the food sources that draw whale sharks recur on a yearly basis, and the whale sharks show up reliably to exploit them. How the sharks know to be at a particular place at a particular time remains unclear. Whale sharks may also migrate to breed. Genetic analysis of whale shark populations from the Atlantic, Pacific and Indian Oceans, has shown little difference between these animals. This data indicates an evolutionary history of migration, and interbreeding, among distant groups of whale sharks.

Reproduction: Sexual maturity in male sharks can be determined by the maturation of the claspers, paired sexual organs located between the pelvic fins. Mature claspers are rarely seen on male whale sharks of less than 9 meters in length, and an animal of this size is likely 25-30 years old. The feeding aggregations composed of primarily 5-8 meter sharks therefore cannot be breeding populations, but rather are juvenile and subadult gatherings. The habitat of mature adult whale sharks remains unknown. Female sexual maturity is less well understood than males, but it is likely that breeding capability is reached at about the same age. Beyond these few facts, little is known about where adult whale sharks are found, where males and females meet to breed, or where the females deliver their pups. Whale shark mating has never been observed, nor has a female been seen giving birth. Whale shark embryonic development is ovoviviparous, the young develop in egg cases within the uteri of the female, but prior to birth they hatch and complete their development free-swimming within the mother. Only one pregnant female whale shark has ever been scientifically described, an animal caught in a fishery in Taiwan in 1995. This remarkable female carried more than 300 embryos in her uteri, in a range of ages from small sharks still in egg cases to near-term pups ready to be born.

Jennifer Schmidt is a developmental biologist at the University of Illinois- Chicago who works with creatures as disparate as mice and whale sharks.

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