Mysterious shark die-offs spread in San Francisco Bay as scientists narrow down cause

Researchers are trying to find out why hundreds of leopard sharks have washed up dead on the shores of the Bay Area for 12 weeks straight.

Biologists with the California Department of Fish and Wildlife have been collecting samples from the stranded animals and testing their tissues in labs. The department's senior fish pathologist Dr. Mark Okihiro released his preliminary findings Wednesday evening, which indicated that a common pathogen was found among the three leopard sharks necropsied so far.

Since SFGATE first reported the mass die-offs, the epidemic has spread to other parts of the San Francisco Bay.

Sean Van Sommeran, executive director and founder of the Pelagic Shark Research Foundation, says reports of stranded marine life have tapered off somewhat in the Foster City and Redwood City areas in recent weeks, but they've spiked near Alameda and Oakland. There have also been several reports of dead sharks, rays and fish washing up on beaches near the Golden Gate Bridge.

Hundreds of sharks have washed up on Bay Area shores over the past 12 weeks. Biologists are performing necropsies to definitively determine the cause. Hundreds of sharks have washed up on Bay Area shores over the past 12 weeks. Biologists are performing necropsies to definitively determine the cause. Photo: Pelagic Shark Research Foundation Photo: Pelagic Shark Research Foundation Image 1 of / 15 Caption Close Mysterious shark die-offs spread in San Francisco Bay as scientists narrow down cause 1 / 15 Back to Gallery

Wednesday evening, Van Sommeran received two reports of leopard shark strandings within ten minutes of each other. One was near Crissy Field in San Francisco, the other near Crab Cove in Alameda.

"What's hitting the beach is likely to be just a fraction of it," Van Sommeran said. "Even the most conservative estimates at this point have to be in the thousands, considering all the parts of the coastline we can't get to."

In addition to spreading geographically, Van Sommeran has seen more species being affected in recent weeks. "Last week a sevengill shark washed up near Oakland, just east of the airport. In addition, sturgeon have washed up, halibut, striped bass and flounder-type fish."

The stranding of a sevengill shark is especially significant, according to Van Sommeran, because the sevengills dwell deeper than leopard sharks. If sevengills are also being sickened and killed, it could indicate the scope of the infection is larger than originally thought.

Van Sommeran believes the source of the infections comes down to the city's use of tide gates in residential areas near inland waterways. To keep from flooding during the rainy season, Redwood City closes its tide gates during low tide. That way, when there are heavy rains, the extra precipitation doesn't combine with high tides to flood homes along the water.

The problem is that leopard sharks come into the shallow waterways to mate and pup during the spring and summer, so they often get trapped when the tide gates close.

The storm runoff that fills the waterways isn't good for sharks' health in any case, but years of drought followed by an extremely rainy season have exacerbated the damage. "During drought, stuff that would usually get washed away congeals and backs up," Van Sommeran explained in an interview. "So with the hard rain there's extra crud going into the watershed."

The stagnant water in these inland waterways quickly goes foul (just like a home aquarium would if not regularly cleaned) and sickens the sharks. "The water becomes toxic and the sharks can't cope with it," Van Sommeran said.

When the tide gates reopen, the rotting and decaying sharks are released back into the bay, where Van Sommeran fears they could be contaminating more animals.

"It's the signature species in the San Francisco Bay," the researcher told the San Francisco Chronicle. "If they keep losing these numbers every spring when they are trying to pup, that's asking for disaster. They can't sustain these losses."

While biologists with the California Department of Fish and Wildlife are making progress, they say more research and testing is needed before they can definitively say what is causing the mass die-offs.