Whale Shark Numbers Boomed Before They Crashed

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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.

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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.

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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.

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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