Image: Researchers photograph a great white shark
Stanford University
Researchers photograph great white sharks that were lured to their boat by a seal-shaped decoy.
By
updated 3/9/2011 12:11:23 PM ET 2011-03-09T17:11:23

The world's first detailed count of great white sharks has found these marine predators may be even more endangered than previously thought.

The count, which focused on great white sharks off Central California, determined that only 219 adults and juveniles exist in waters there, according to a new Royal Society Biology Letters paper. Since this region is thought to support one of the largest populations of these sharks, the outlook for great whites elsewhere is now also grim.

Image: Great white shark with seal decoy
University of California, Davis
A great white shark investigates a fake seal decoy used by UC Davis researchers.
"These findings should be used to inform further assessment of the IUCN Red List categorization of white sharks," lead author Taylor Chapple, a University of California at Davis researcher, told Discovery News.

The Red List, created by the International Union for Conservation of Nature and Natural Resources, currently classifies great white sharks as being "vulnerable." In the future, the sharks could be classified as "endangered" or "critically endangered" if other counts support Chapple and his team's findings.

The present study was conducted from 2006 to 2008 at two known shark aggregation sites in Central California: Tomales Point and the Farallon Islands. The researchers attracted sharks to vessels using a seal-shaped decoy and a small piece of bait. Once a great white shark approached, digital images of its dorsal fin were snapped from either above or below the water.

Image: Shark fin
University of California, Davis
Unique markings on white sharks' fins can be used to identify individuals.
Great white shark dorsal fins are analogous to a human fingerprint. Each one is unique for a particular shark.

"Differences can be present in the general shape (i.e., triangular fin, rounded fin), but the most telling of the differences is along the trailing edge of the fin," Chapple said. "Each fin has a unique arrangement of notches, cuts, and/or flat sections. We have found that these characteristics are conserved over very long periods of time, at least in mature and sub-adult white sharks."

Once shark numbers were estimated based on the images, the researchers used additional information from a database, along with math models, to come up with the 219 figure.

Although no one knows precisely how many white sharks existed in the region years ago, population counts for other top marine predators are much higher. The scientists point out that killer whales in a smaller range number 1,145, while polar bears from the Southern Beaufort Sea have been estimated at 1,526.

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"If we assume white sharks and killer whales fill a similar niche (i.e., prey base) and compare their metabolic rates (a physiological measure that is sometimes correlated with population size), we would predict that abundance levels of white sharks should be at least similar to killer whales, if not greater."

It's unclear how the suspected loss of great white sharks has affected the California marine ecosystem, but he said "it's likely that loss of white sharks would have significant negative effects on the health and functioning of these systems. Even the loss of a single individual could have serious consequences."

Chris Lowe, an associate professor at California State University at Long Beach and director of the university's SharkLab, told Discovery News that he thinks "this study is a great starting point."

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But Lowe suspects that more great white sharks could be in Central California waters.

"Part of the challenge here is using traditional marine mammal survey methods on a non-air breathing species," he said. "So obviously it is easier to count orcas than it is sharks, because there is a much higher chance of seeing them on the surface."

Most experts agree, though, that white sharks are in need of protection. Shark finning, bycatch due to fishing, habitat loss, pollution and other problems continue to threaten white sharks and other shark species.

© 2012 Discovery Channel

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