For anyone who has wondered how many large great white sharks lurk off California, researchers for the first time have produced a science-based estimate: 219.
That's the number of adult and sub-adult white sharks -- those about nine feet or longer -- believed to be utilizing California waters after an innovative photo-identification study conducted at seasonal shark haunts off Central California.
The number is surprisingly and perhaps alarmingly low given the key role white sharks, as apex predators, play in maintaining a healthy marine environment.
The study's lead author, UC Davis doctoral student Taylor Chapple, stated in a news release: "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."
One prominent scientist has disputed the number, calling it a significant underestimation, based largely on his theory that not all large white sharks in California utilize those Central California haunts. Chapple said his group was not accounting for every shark in the northeastern Pacific, but acknowledged that the 219 figure was an estimate for California waters.
The collaborative study, published today in the journal Biology Letters, involved some of the world's foremost white shark experts. They collected 321 photographs of dorsal fins from what, based on distinctive markings, turned out to be 130 individual sharks. Data from this study -- and previous studies that determined the same sharks were visiting the same areas year after year -- were extrapolated to determine a population estimate.
Involved in the study were 69 male white sharks, 19 females and 42 sharks of undetermined sex.
White sharks, once they reach 8-9 feet, begin to shift from fishes and rays as food items to seals and sea lions. In California this also marks the point at which they travel from Southern California to rookeries off Central California. (The study area included the Farallon Islands west of San Francisco and Tomales Point to the north on the mainland.)
Late summer and fall are feeding periods at the rookeries. During the winter, these large sharks migrate to a vast offshore area between the mainland and Hawaii, where they spend about five months of each year.
The population study did not take into account juvenile or young-of-the-year white sharks. Also, the 219 figure does not include white sharks that utilize waters around Guadalupe Island, a seal rookery 150 miles west of Ensenada in Baja California.
That's because there's no evidence of Guadalupe white sharks visiting Central or Southern California coastal waters; those sharks, therefore, are believed to belong to a separate sub-population.
If both studies are accurate, that places the entire northeast Pacific population of large white sharks at 352.
The only known mixing of these sub-populations -- based on satellite tagging studies -- occurs within the offshore area.
Chapple, while he was surprised by the low number produced at the Central California study area, stressed that it's simply a baseline number, an important starting point that will aid scientists as they strive to learn more about population fluctuations and trends.
"Even though it's lower than you might expect, we don't know if this is a healthy number or something we should be worried about right now," Chapple said in an interview. "I think that the big statement is that we need to follow up with more studies to try to find out whether the population is increasing or decreasing. Because with the size it's at now, just taking out a few or even one individual may have a significant negative effect on the population."
Perhaps, but Dr. Chris Lowe, a professor of biological sciences at California State University Long Beach, believes that white shark numbers have increased over the last 15 years and that the actual number of large white sharks is much higher than 219.
Lowe, who has studied juvenile white sharks off Southern California, cites factors such as a longstanding ban on fishing for white sharks in state waters; a longstanding ban on commercial gill-net fishing close to shore, and the Marine Mammal Protection Act of 1972, which has led to a population explosion of California sea lions -- a reliable and more spread-out food source for adult white sharks.
"My suspicion is that there's another sub-population of white sharks that hang around the Channel Islands [off Southern California] and that they're there year-round," Lowe said. "They're not showing the same sort of seasonal fidelity [toward the Central California rookeries].
"They may go to the middle of the Pacific too, but very few tagged sharks from Guadalupe or the Farallon Islands have come to Southern California, but we see sharks down here at the same times of year. So I think the population is more spread out because the rookeries are more spread out. People are seeing more sharks down here, particularly around the offshore Channel Islands, and more sea lions are getting hit by white sharks down here."
The population study, entitled, "A First Estimate of White Shark, Carcharodon carcharias, Abundance off Central California," alludes to there being "little evidence for long-term occupation at other coastal sites" besides the historic Central California aggregation sites.
Chapple, in response to Lowe's statement, replied via email: "The idea that there is a year-round sub-population of large white sharks that go, and have gone, undetected for many years in an area with such high human use (i.e., Channel Islands, etc.) seems incredibly unlikely."
In any event, the number of white sharks appears to be lower than most people might have believed, yet it remains unclear how this compares to historic levels because nobody knows what they were. That's why establishing a baseline figure is so important.
Said Barbara Block, a leading shark expert from Stanford University, and study co-author: "We've found that these white sharks return to the same regions of the coast year after year. It is this fact that makes it possible to estimate their numbers. Our goal is to keep track of our ocean predators."
-- Pete Thomas
-- Editor's note: A version of this post also appears on the GrindTv outdoors blog
-- Images: 1) Unique markings on white sharks' fins can be used to identify individuals. Credit: UC Davis; 2) A white shark investigates a fake seal decoy used by UC Davis researchers. Credit: UC Davis; 3) Researchers photograph a white shark. Credit: Stanford University