TIBURON (CBS SF) -- Bay Area Fish and Wildlife officials are trying to determine the reason behind a spike in leopard sharks dying off around San Francisco Bay.
Just Thursday, a large leopard shark was found after it beached itself in Burlingame near the Coyote Point Recreation Area.
The concentration of dead leopard sharks is around the Foster City and Redwood Shores area, although numerous others have washed up in Alameda and Marin counties as well as San Francisco.
Sean Van Sommeran and the Pelagic Shark Research Foundation have been following the seven week die-off.
"From a conservation perspective, it's very serious if not critical. This is timed with their annual breeding cycle," explained Van Sommeran.
This is the third die-off in the past six years.
Carrey Wilcox with the Richardson Bay Audobon Society remembers the one in 2011.
"Occasionally, you'll get a dead animal on the beach. But then we started seeing more and more, and many of them looked healthy," said Wilcox. "Some of them were even alive then died within the day."
Back then, no one knew the cause. But now, U.S. Fish and Wildlife is on the case.
With the help of Van Sommeran and his team, they've collected dozens of the dead predators and sent them off for testing.
Tests have found a viral and fungal infection that somehow makes its way into the shark's brain, causing it to swim ashore.
So it's killing a huge number of leopard sharks. The sharks were declared unsafe to eat going back to our 2011 investigation," explained Van Sommeran. "The healthy specimens were loaded with toxins."
The Pelagic Shark Research Foundation feels those toxins may be coming from dirty runoff from all the rain this winter may be contributing to the problem, but Fish and Wildlife has yet to confirm that.
Scientists are asking that people who encounter beached sharks not try to push them back into the water, since it won't save them.
Instead, call Fish and Wildlife so experts can do further study on the animals.
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