A team of researchers headed by Taylor Chapple, a UC Davis doctoral student, has made the first rigorous scientific estimate of White shark (Carcharodon carcharias) numbers in the northeast Pacific Ocean.
The researchers used small boats to reach spots in the Pacific Ocean where white whales congregate and lured them into photo range with a fake seal attached to a fishing line. Out of all the photographs taken, 321 photos showed dorsal fin edges. The dorsal fin edges of white sharks are jagged and each individual displays its own unique pattern. The photographs could therefore be used to identify individual sharks – 131 in total.
The research team then entered this information into statistical models to estimate the number of sharks in the region. According to their estimate, there are 219 adult* and sub-adult** white sharks in this area.
“This low number was a real surprise,” says Chapple. “It’s lower than we expected, and also substantially smaller than populations of other large marine predators, such as killer whales and polar bears. However, this estimate only represents a single point in time; further research will tell us if this number represents a healthy, viable population, or one critically in danger of collapse, or something in-between.”
The white shark population in the northeast Pacific Ocean is one of three known white shark populations in the world; the other two are found off the coast of South Africa and off Australia/New Zealand, respectively.
Earlier studies using satellite tagging have shown that the white sharks of the northeast Pacific Ocean have an annual migration pattern. Each year, they move from the region off the coast of central California and Mexico’s Guadalupe Island to the Hawaiian Islands or to an area of open ocean located between the Baja Peninsula and Hawaii. The latter destination has even been dubbed “White Shark Café” due to its popularity among white sharks. After spending some time away from the mainland, the sharks journey back to coastal waters.
“We’ve found that these white sharks return to the same regions of the coast year after year,” says Barbara Block, marine biologist at Stanford University and one of the co-authors of the pioneering white shark census. “It is this fact that makes it possible to estimate their numbers. Our goal is to keep track of our ocean predators.”
The paper “A first estimate of white shark, Carcharodon carcharias, abundance off Central California” (http://dx.doi.org/10.1098/rsbl.2011.0124) has been published in the journal Biology Letters (http://rsbl.royalsocietypublishing.org).
* Adult white sharks are the ones that have reached sexual maturity, something which happens when the male is roughly 13 feet and the female is about 15 feet in length.
**Sub-adults are roughly 8 feet or longer (but has not reached sexual maturity); at this size their dietary focus shifts from mostly fish to mostly marine mammals.