New studies examining the great white shark population off the Atlantic and Pacific coasts of the United States have found that the number of white sharks is larger than previously estimated.
George Burgess, director of the Florida Program for Shark Research, lead author of the Pacific study and co-author of the Atlantic study, said the gradual increase in population is due to less killing of the white sharks as well as increased food availability.
Burgess' study, which included nine other researchers and was published in the journal PLOS ONE in June, specifically reexamined the white shark population off the coast of California.
In 2011, a group of researchers from Standford University and UC Davis, published a study which used research methods that included taking pictures of dorsal fins to look for certain scars or unique markings. They also monitored the sharks over several years in certain aggregation areas where there was a greater concentration of prey.
The group eventually concluded that the number of mature and sub-mature great white sharks off of California was 219 and that number comprised "approximately half the total abundance of mature and sub-adult white sharks" in the northeastern Pacific Ocean.
This alarmingly low number elicited calls from conservationists for the species to be placed on the U.S. and State of California Endangered Species lists.
The study from Burgess’s group puts the number of white sharks at closer to 2,400 off the California coast. In an interview with AccuWeather.com, Burgess said there could be as many as 7,000 to 8,000 throughout the entire northeastern Pacific, which stretches from Alaska to Mexico and out to Hawaii.
Burgess said his group took the findings from the study done in 2011 and used models with different assumptions. In their report, the group said that one of the problems with the Stanford study was that they only looked at two aggregation areas off central California, the Farallon Islands and Tomales Point, but didn't sample other sites such as Año Nuevo Island.
In an interview with Reuters, Stanford researcher Barbara Block stood behind her group’s findings.
“We stand firmly behind the findings of our study, and our ongoing research only increases our confidence in its accuracy,” Block told Reuters.
Burgess said after reviewing his group’s paper, the California and federal governments denied the request to make the shark an endangered species.
“This is a success story, one of the few times we as scientists and conservationists these days can say something really good is happening to sharks,” Burgess said.
“Hopefully we can have this kind of conversation 20 years or 30 years down the line with some of the other species of shark."
Part of the success for the increase in white sharks goes back to October 1972, when the Marine Mammal Protection Act prohibited the taking of marine mammals, such as seals and sea lions. This has led to an increase in the amount of these mammals, which happen to be one of the primary prey items for white sharks.
On the Atlantic side, Burgess said they examined encounter data, some of which went back as many as 100 years ago, as well as fishery and catch data. The study focused on the western north Atlantic and indicated that starting in the 1990s, there was a gradual upturn in the population.
Protections for the white shark went into effect in the 1990s and in an interview with the National Oceanic and Atmospheric Administration’s (NOAA) National Marines Fisheries Service, lead author of the Atlantic study, Tobey Curtis, said those measures have helped.
“The signs of recovery are coincident with the fisheries regulations, so we think that the conservation measures have really benefited the white sharks and helped them rebuild,” he said.
Under the protections, great whites, whether intentionally or inadvertently caught, would have to be released.
Unlike the Pacific study, Curtis's group focused on population increases and decreases over time, and did not provide a historical or current population estimate, according to NOAA Fisheries Writer Rich Smith.
The white shark population began to decline in the 1970s, in large part due to unregulated over-fishing according to the NOAA interview.
Curtis also said the seal population has recovered dramatically in the last 10 years.
"That’s probably helped give the white sharks a little extra boost with reliable high quality prey source," Curtis said. “So during the summertime, the grey seals have become very abundant off Cape Cod in particular, and it’s become the new sort of white shark hot spot for the Atlantic."
While the increase in great whites is promising, many other shark populations are not doing as well.
Burgess said most of the catches and deaths that occur are related to “bycatch” or when fishermen target one species and end up catching another.
“Wherever you go in the world, anywhere there’s sharks, there’s a problem, because fisheries are catching too many of them and the sharks aren’t able to recover their populations in the same period of time as other species,” Burgess said.
Burgess said white sharks are long-lived animals and it takes them a while to reach the stage where they can begin to reproduce. Even when that happens, the sharks will only produce a limited amount of young.
Burgess cautioned that there is still a ways to go before the recovery of the population is complete.
“The return of these animals is measured in decades, not in years,” he said.
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