California's great white sharks are a distinct population

California's great white sharks are a distinct population

Researchers have long thought that white sharks migrated across oceans, but a new study in the Proceedings of the Royal Society B shows that the population in the northeastern Pacific Ocean, along California, hasn't mixed with other white sharks for tens of thousands of years.

"White sharks are a large, highly mobile species," said Salvador Jorgensen, a postdoctoral scholar at Stanford's Hopkins Marine Station. "They can go just about anywhere they want in the ocean, so it's really surprising that their migratory behaviors lead to the formation of isolated populations."

Employing satellite tapping, passive acoustic monitoring, and genetic tags, researchers with the Tagging of Pacific Predators (TOPP) found that the white shark (Carcharodon carcharias) in northeastern Pacific frequently return to the same spot over and over, and sometimes remain in one hunting ground for over a hundred days.

"The sharks were detected frequently at their chosen site," Jorgensen said, "which means that they are patrolling around there nearly constantly, for long periods of time. They will occasionally visit one of the adjacent sites, but they always come back."

They also never stray far. Genetic studies show that the population of northeastern Pacific white sharks was likely founded by a small number of sharks sometime in the last 200,000 years and hasn't mixed with other shark populations near Australia and South Africa since.

"If you had asked us a few years ago, we would have said white sharks found in California probably migrated throughout the Pacific. Now, even though we know they travel great distances, their paths are surprisingly constrained to specific routes. This explains how a highly migratory marine species becomes a genetically isolated population. This also makes it much easier to appreciate how vulnerable the northeastern Pacific white shark population could become if too many individuals were taken as either catch or bycatch, since these sharks do not appear to interbreed with other shark populations," said Molecular geneticist Carol Reeb, a research associate at Stanford.

The scientists hope to use the new information that the Pacific population is distinct from other to help save the species, which is classified as Vulnerable to extinction by the IUCN Red List.

"Catastrophic loss of large oceanic predators is occurring across many ec osystems. The white sharks' predictable movement patterns in the northeastern Pacific provide us with a super opportunity to establish the census numbers and monitor these unique populations. This can help us ensure their protection for future generations," said Barbara Block, co-author and professor of marine science at Stanford.

The researchers were also surprised to find five white sharks in San Francisco bay. The massive sharks swam directly beneath the Golden Gate Bridge where distinct sounds from their acoustic tags were picked up by an acoustic receiver meant to track salmon. The researchers speculate that the sharks were hunting seals and sea lions in the area.

http://news.mongabay.com/2009/1103-hance_greatwhite.html