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How the Biggest Fish in the Sea Is Tracked by Tourist Photos

A boom in tourism around whale shark hot spots has helped scientists crowdsource population data for the elusive endangered fish – and could inform its conservation.

Written by Amelia Urry Published on Read time Approx. 5 minutes
Whale sharks roam widely through the tropical oceans, rarely staying in one place for long, which is why they’re difficult to study.Leith Holtzman

Though they are the biggest fish in the sea and can grow to 60ft (18m), whale sharks are surprisingly hard to study. They roam widely through the tropical oceans, rarely staying in one place for long, and they don’t surface to breathe like whales or dolphins.

“It’s a bit hard to work on an animal you can’t see,” said Brad Norman, a marine biologist at Australia’s Murdoch University and program director of the nonprofit ECOCEAN.

Norman started studying whale sharks in the mid-1990s, when scientists knew little about the elusive fish and were just beginning to learn about how they gather in large groups to feed in coastal areas.

Today, with increasing knowledge of these whale shark “hot spots,” there are more eyes on the fish than ever before. Ecotourism is booming in places like Mexico and the Philippines, where the fish congregate yearly. As guide boats take divers and snorkelers to swim with the gentle animals, the resulting amateur photos and videos are proving to be of real value to scientists at a time when whale shark populations are facing significant threats.

“Everybody’s got a camera or smartphone,” Norman said. “And now smartphones are waterproof, so you can actually engage anybody.”

From photos, scientists can identify individual whale sharks based on the pattern of spots just behind their gills. (Aimee Jan)

Using photos Norman had taken of the whale sharks in Australia as an initial database, he and collaborators built up a library of whale shark encounters stretching back to 1992. In 2003, they opened the database, Wildbook for Whale Sharks, to the public, and invited citizen-scientists and researchers alike to submit photos to be sorted by an algorithm. In a study published in BioScience in late November, more than 30 whale shark researchers and conservationists around the world reported on the deeper understanding of the species gleaned through these photos, which document more than 28,000 encounters collected between 1992 and 2014 and include data from 5,000 citizen-scientists. (Since 2014, more identifications have continued to pile up.)

Scientists have long sought a better understanding of whale shark numbers and their distribution. Over the past 75 years, models indicate, the number of whale sharks has fallen by roughly half, according to the International Union for Conservation of Nature (IUCN), due largely to human threats such as fishing and boat collisions. The group cites rough estimates based on limited data that put the possible population of breeding whale sharks today at anywhere from tens of thousands to more than 200,000.

Citizen-science data has and can continue to inform whale shark research. From the database of photos, scientists can identify individual whale sharks based on the pattern of spots just behind their gills, which are as unique as a human fingerprint. That, Norman said, makes photography a less invasive and more reliable and affordable way to track whale sharks than tagging them, which is how scientists often monitor marine wildlife.

The photo identification process has become more efficient since the early 1990s, when sorting through the photos meant dumping a box of photo prints onto the floor and having a roomful of researchers compare spots frame by frame. When software programmer and avid scuba diver Jason Holmberg, a coauthor of the BioScience paper, learned this, he recruited his friend Zaven Arzoumanian, a NASA scientist, to help find a way to identify whale sharks through an algorithm. Working with Norman, they adapted the software used by the Hubble Space Telescope to identify stars against a black background to recognize and match the pattern of light spots against the darker skin of the sharks.

“The most important change was to make [the algorithm] less good,” Arzoumanian said. “The geometry of stars in the sky is always the same, but when you’re looking at a shark the relationship between the spots on the surface of the shark can change from photo to photo.” In different photos, from different angles, the characteristic pattern of each shark might be slightly warped, and the algorithm had to be able to take that uncertainty into account.

Researchers have used the Wildbook data to identify 6,000 individual fish through 2014 and have catalogued more than 8,000 individual whale sharks in total – allowing scientists to analyze the larger patterns of their distribution across the ocean. Following the success of the whale shark program, pattern-matching software is now in use to monitor other wild populations, from zebras to dolphins, Arzoumanian said.

Today, with increasing knowledge of whale shark “hot spots,” there are more eyes on the fish than ever before. (Janine Marx)

Over the course of the two decades of citizen-science research, the number of known whale shark hot spots expanded from 13 to 20, according to the paper. In one area off Mexico’s Yucatan coast, more than 1,000 individual sharks were identified. Other hot spots include Western Australia, Mozambique and the Philippines.

The researchers believe these locations are important feeding grounds, as the observed shark population ebbs and flows seasonally along with the availability of food. They also learned that most sites were overwhelmingly populated with adolescent male sharks, usually 20–35ft (6–11m) long. That’s on the small side for an animal that can grow to double that size and live more than 100 years.

This data could prove valuable as scientists think about how to save whale shark populations from current and future threats. Last year, based partly on an assessment carried out with Wildbook data, the IUCN reclassified whale sharks from vulnerable to endangered, and whale sharks are likely to face more challenges in the future.

Temperature and chemistry changes currently underway as a result of global warming are likely to shift ocean ecosystems in the coming decades, and no one knows what effect this could have on whale sharks. But their size and their exclusive diet makes them especially vulnerable. They are one of only three species of sharks to get their nutrients by filtering plankton and other microorganisms from the water, and because they have to eat a lot of these tiny organisms, they are very dependent on the most productive parts of the ocean, Norman explained.

He hopes he’ll continue to be able to unravel the whale sharks’ mysteries, especially because there’s still a lot to learn, such as where they mate. Their breeding grounds have never been found, he said, and embryos have only been observed once, after a pregnant female was caught by fishermen in Taiwan. “This animal’s been around for millions of years, and we don’t know anything about it,” he said.

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