Whale Shark Numbers Boomed Before They Crashed

Whale sharks, which are the world's largest sharks, experienced a dramatic population increase before their recent decline, finds the most extensive ever genetic study of these 20-ton sharks.

The study, published in the journal Molecular Ecology, also found that whale sharks, which are also the world's biggest fish (sharks are fish), exist in at least two distinct populations around the world.

Could a Prehistoric, 60-foot Shark Still Exist?

The two populations "rarely mix," according to lead author Thomas Vignaud of the Laboratoire d'Excellence in French Polynesia and his colleagues.

Vignaud and his team obtained DNA from skin samples taken from 635 whale sharks. Genetic analysis revealed that the genomes for Indo-Pacific whale sharks are distinct from whale sharks in the Atlantic Ocean.

"This suggests whale sharks are not all part of a single global metapopulation," the scientists wrote.

Genetic diversity, which coincides with population rises and diminishes when numbers fall, indicates that whale sharks experienced a population boom and a subsequent expansion of their range during the Holocene. This is the geological period that we're in now. The Holocene began around 11,700 years ago.

The researchers suspect that the warmer climate and sea level rise of the Holocene increased plankton productivity. Plankton are a diverse group of drifting organisms, such as protists, algae and bacteria, which live in the ocean and serve as a significant food source for whale sharks.

Video: Learn About Whale Sharks

Whale sharks are filter feeders, so they basically just open their mouths, filter out seawater, and ingest what's left. It's pretty amazing to see them up close, with their 5-foot-wide mouths sucking in water and food.

The whale shark population boom has gone bust in recent years, however, and humans are to blame.

For example, the researchers report: "Declines in genetic diversity are found for six consecutive years at Ningaloo Reef in Australia. The declines in genetic diversity being seen now in Australia may be due to commercial-scale harvesting of whale sharks and collision with boats in past decades in countries in the Indo-Pacific."

Reduced genetic diversity comes with its own set of problems, since it can leave any species vulnerable to inbreeding, diseases, sudden climate changes and more.

Climate-Challenged Whale Sharks Flocking to Island Chain

Protecting whale sharks, and many other sharks, often proves to be challenging, since their ranges can be vast and may include waters off of numerous different countries. It may "take a village" to save particular human communities, but it takes dedicated international effort to save most sharks.

Better defining shark populations, as this latest study does, is an important step toward improving conservation measures.

Photo: A whale shark. Credit: KAZ2, Flickr and Fotopedia

Sept. 5, 2012

-- Five rays and sharks are receiving priority attention at this week's IUCN World Conservation Congress held in Jeju, Republic of Korea. The congress is the world's largest conservation event. One of the five sharks is the porbeagle, shown here. Porbeagles are vulnerable globally, and are classified as critically endangered in the Northeast Atlantic. Their low reproductive capacity combined with over-fishing has led to severe population declines over several parts of its range. "Sharks and rays have traveled the Earth for more than 400 million years," said Dr. Cristián Samper, president and CEO of the Wildlife Conservation Society and keynote speaker at the Jeju congress. "Yet, in only recent decades, many of these species have become threatened from overfishing and, in some instances, have disappeared entirely from major portions of their range." He added, "The potential loss of one of only two groups of the world's living fishes is a crisis the world community must take decisive action to address. We are calling for governments around the world to vigorously support CITES international trade regulations and strengthen fisheries management and protection measures for shark and ray species. We cannot continue to allow the destruction of these wonders of evolution."

Manta Rays Fate Worse Than Sharks

The oceanic whitetip shark is critically endangered in the Northwest and Central Atlantic Ocean. It's frequently sought after for its fins, used to make shark fin soup. Usually the rest of the dead or dying shark is then tossed back into the sea. Andrew Brierley, a University of St. Andrews marine ecologist, told Discovery News that when sharks like this die off, the deaths can cause a domino effect of other ecosystem losses. Shellfish, for example, may go into decline because they are consumed more by predators that the sharks would normally prey upon. "The trophic cascade brought about by the increasing demand for shark fin soup has not only left once economically valuable bivalve fisheries in crisis, but has precipitated an ecological and culinary bankruptcy," explained Brierley.

Scalloped hammerhead sharks are endangered all around the globe. This tropical species forms large migratory schools at certain times of the year. The aggregating behavior, distinct from that of other sharks, makes the shark particularly vulnerable to fishing. "We estimate that many millions of sharks are killed annually through both legal and illegal, unregulated and unreported fishing for the trade in fins, the prime ingredient in shark fin soup," said Dr. Rachel Graham, director of WCS's Gulf and Caribbean Sharks and Rays Program. "The high price for fins has caused the global shark fishery to expand far beyond what is sustainable. The need for international regulation and enforcement has never been greater."

Shark Paradise Found

Many rays, such as these, are also in peril. The Wildlife Conservation Society and over 35 government agencies and NGO partners at the congress additionally highlighted population loss problems associated with so-called "devil rays," which feed on planktonic crustaceans. Small schooling fishes become trapped on their specialized gill rakers. Rays are frequently captured in target fisheries and as bycatch across much of their range. One reason is that the gill rakers are dried and exported for the Asian medicinal market.

The reef manta ray, shown here, is among the world's largest fishes. Both it and the giant manta ray can grow several feet across. They are slow growing, however, typically giving birth to only one pup every two to three years. They are migratory and occur in small, highly fragmented populations that are sparsely distributed across the world’s tropics. Manta rays are captured in targeted fisheries and incidentally as bycatch. In addition, manta rays are used for human consumption, shark bait, and -- as for devil rays -- are increasingly sought for their gill rakers. "The international trade in shark and ray products, including fins, meat, and other body parts, is driving shark and ray fisheries around the world, and most of these are unmanaged or only minimally managed," said Dr. John Robinson, WCS's executive vice president for Conservation and Science. "Lack of controls on fisheries and international trade puts species at risk, but also jeopardizes sustainable fisheries, ecosystems, and food security." The Convention on International Trade in Endangered Species of Wild Fauna and Flora will meet in Thailand in March 2013.

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