CAPITOLA (KPIX) – Great white shark sightings are on the rise in the Monterey Bay, but scientists want to know why are so many dead ones washing ashore.

At least fifteen salmon sharks from Monterey Bay have mysteriously washed up dead on Santa Cruz County beaches in the past month.

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Another startling sighting on shore these days is great white sharks. Three of them have washed up in the last year. Last month, a juvenile was found on an Aptos Beach. Marine life experts said the shark weighed 500 pounds.

The Department of Fish and Wildlife has opened a criminal investigation into the shark’s death, which suggests possible foul play.

Sean Van Sommeran is Executive Director and Founder of the Pelagic Shark Research Foundation, a conservation group based in Santa Cruz.

“Regardless of any discussion of the cause, we’ve got three dead white sharks. We haven’t had that,” said Van Sommeran.

Pelagic Shark Research Foundation responds to stranded sharks on Bay Area shores, and assists the state in sending them to labs for testing. It is seeing more unusual die-offs this year.

“One’s a clear indication of a pathogen infection that caused the stranding,” says Van Sommeran. “A second shark in August of last year was apparently struck by a boat. It was a mortal propeller strike to the head.”

Van Sommeran believes people may pose more of a threat to great whites than the predators pose to us.

“That includes everything from runoff to plastics, pollutions, chemical toxic spills.”

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Van Sommeran says debris and trash in Bay Area storm drains that gets washed into the bay may be killing the creatures.

But at the same time sharks are dying, young great whites appear to be thriving in Monterey Bay.

For the first time Van Sommeran is taking a media crew along with him on his latest project: identifying individual sharks using GoPro video near Capitola and Aptos. On previous trips, he’s also taken biopsies and tagged the sharks. He notes the sex, age and peculiar marks.

After being on this shark survey for about two hours, there have been three shark sightings. The boat is pretty close to shore, less than a thousand feet. Van Sommeran said the last shark was less than a year old.

“The area’s been famous for very large sharks for a very long time. What’s unusual is the presence and gathering of very young sharks which are typically born in Baja, Southern California,” said Van Sommeran.

As for why Monterey Bay become a new home for so many young great whites, Van Sommeran says they were first spotted in the bay in 2015, and his studies have found that they’ve returned every spring and summer since.

“I think it’s a combination of the current influences, as well as the gathering of food sources,” said Van Sommeran.

A healthy Monterey Bay may be attracting the young great whites. But there is so much more we don’t know about the feared fish.

“There’s not a lot known about the juveniles in particular. White sharks have never been documented mating or pupping.”

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The Pelagic Shark Research Foundation hopes to better understand their behavior and will continue to fish for more information, both on and off shore.