Big Fish, Global Fish

Whale sharks are anything but camera shy. Between 1995 and 2006, scientists, tourists, divers, and tour guides snapped more than 5100 underwater photographs of these gentle giants at Ningaloo Marine Park, off the coast of Western Australia. The photographs weren't random portraits of fish. They were all captured as part of a long-term survey of the region's whale sharks.

While swimming astride the left side of a shark, photographers carefully aimed their viewfinders at the patch of skin behind the shark's gill slits, just above the pectoral fin. The goal: to record the pattern of white spots and streaks on the shark's flank. This pattern, unique to each shark and retained throughout its lifetime, is like a human fingerprint. It can be used to identify the shark if it's sighted again.

The study, led by marine scientist Brad Norman of ECOCEAN and Murdoch University, used photo recognition technology to analyze the images and establish a database of shark "photo IDs".

Brad Norman joined forces with computer programmer Jason Holmberg of ECOCEAN and astronomer Zaven Arzoumanina of USRA and, NASA. Together, they adapted software that was originally designed for the Hubble space telescope to analyze the shark photos and to create a database of shark identities. That database now provides a foundation for monitoring the region's whale sharks.

Each year between April and June, large numbers of whale sharks congregate in Ningaloo Marine Park, a protected area that encompasses Ningaloo Reef, Australia's longest fringing reef. The sharks feed on plankton and other small prey and their arrival coral reef erupts in a mass spawning. This gathering of whale sharks supports a thriving ecotourism trade. Brad Norman and his colleagues tapped into the tourism industry to find people that could help them collect their shark images.

"To study whale sharks in a meaningful way, we really had to rethink how we collect data and how we analyze it," said Jason Holmberg. "The results surpassed our expectations, allowing hundreds of individuals to contribute and providing the necessary data to obtain a closer look at the population's health."

The results of the study indicate that more whale sharks are visiting Ningaloo Marine Park each year. Some of the sharks return again and again, but others seem to be one-time visitors. This suggests that whale sharks at Ningaloo Reef are not an isolated population.

View photo gallery of whale sharks →

Refs:
Holmberg J, Norman B, & Arzoumanian Z (2008). Robust, comparable population metrics through collaborative photo-monitoring of whale sharks Rhincodon typus. Ecological applications : a publication of the Ecological Society of America, 18 (1), 222-33 PMID: 18372568

Photos © Kurt Amsler / Rolex Awards.