An alarming study three years ago reported that great white sharks in the northern Pacific were under threat, but new research refutes that claim. In fact, scientists say their numbers likely are growing.
The findings are good news for shark conservation, says George Burgess, director of the Florida Program for Shark Research, and they indicate that measures to protect the ocean’s apex predator are working.
“White sharks are the largest and most charismatic of the predator sharks, and the poster child for sharks and the oceans in general,” says Burgess, whose research program is based at the Florida Museum of Natural History at the University of Florida.
“If something is wrong with the largest, most powerful group in the sea, then something is wrong with the sea, so it’s a relief to find they’re in good shape.”
Difficult to count
The National Marine Fisheries Service (NMFS) was considering a petition to add white sharks to the endangered species list but declined, based on its own research and bolstered by a preview copy of the latests study, says Heidi Dewar, a fisheries research biologist.
NMFS estimated the Eastern North Pacific population at about 3,000 sharks.
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“We determined there were enough animals that there was a low to very low risk of extinction, and in fact, most developments suggest an increasing population.”
White sharks can be notoriously difficult to count. They are highly mobile and migratory and group themselves by age, sex, and size. Unlike marine mammals, they do not surface to breathe.
Some gather at aggregation sites to dine on seals, others stay at sea, dining on fish. Most tagging studies use photographic tags—pictures of unique markings, such as nicks on fins or scars—and those markings can change over time.
Population estimates, however, are important to conservation. Sharks are sensitive to overfishing, both as bycatch for fisherman seeking other fish and as targets for sport or in areas where shark meat is a delicacy.
White sharks are protected in many areas internationally, including the west coast of the United States, but because they swim in and out of jurisdictions, they are still vulnerable, and the older study raised concerns.
Low extinction risk
For their re-analysis, Burgess and colleagues examined the two aggregation sites where the earlier count was obtained, the Farallon Islands and Tomales Point, which attract seals and the sharks that feed on them. They found that the sub-populations at both sites were so fluid, with both resident and transient sharks, that it would not be possible to extrapolate a total population number.
To get a better picture of the white shark population in the eastern north Pacific, the team examined several other known aggregation sites, from Mexico into British Columbia and Alaska.
They also conducted a demographic analysis to account for all life stages for the sharks at Farallon Islands and Tomales Point and found that the total population is most likely at least an order of magnitude higher—rather than just over 200 sharks there likely were well over 2,000.
“The listing of a species as ‘endangered’ places substantial demands on governments,” Burgess says. “Listing species that are not under the threat of biological extinction diverts resources away from species genuinely at risk. We want to use our resources for the neediest species.”
The earlier study also compared shark population numbers with other apex predators, such as polar bears and killer whales. That study, however, ignored the differences in the community structures for those three species and the fact that polar bears and whales, as mammals, are easier to count.
“That we found these sharks are doing okay, better than okay, is a real positive in light of the fact that other shark populations are not necessarily doing as well,” says Burgess, a cofounder of the Shark Specialist Group of the International Union for the Conservation of Nature.
“We hope others can take our results and use them as a positive starting point for additional investigation.”
The team published its findings in the journal PLOS ONE.
Source: University of Florida
