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article imageLeopard sharks dying in large numbers in San Francisco bay

By Karen Graham     May 7, 2017 in Environment
San Francisco - For the second year in a row, large numbers of leopard sharks, along with some halibut and bat rays, have been found along the shorelines of the San Francisco Bay in California.
Researchers say the dead leopard sharks, the most abundant shark in the bay, have been found along the shorelines of Redwood City, Foster City, Alameda, Hayward, Berkeley, Oakland and San Francisco. Some dead sharks and rays were also found on beaches in Bolinas, in Marin County, according to SFGate.
Marine experts say this is the largest die-off of the striped fish seen in six years. Pelagic Shark Research Foundation executive director Sean Van Sommeran believes there could be thousands of dead and dying sharks. In 2011, over 1,000 dead leopard sharks were counted inside and outside the Redwood Shores Lagoon and along Richardson Bay, in Marin County.
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Wayne Freeman
Mark Okihiro, the senior fish pathologist for the California Department of Fish and Wildlife, in a written synopsis of this year's die-off said: “My estimate is that several hundred sharks have already died. There appears to be no leveling off of shark deaths in the bay. I am still getting reports from locations throughout the South Bay regarding dead or dying leopard sharks.”
Okihiro said sharks do not have lungs and will immediately sink to the bottom when they die. This means there are probably a lot more dead sharks than are being seen. “This is just the tip of the iceberg,” Van Sommeran said. “We’re only seeing a fraction of the actual losses.”
The die-offs started about seven weeks ago, and from a conservation perspective, is very critical. This is because it's the annual breeding cycle when pupping occurs. This is the time of year that leopard sharks come into the shallow waterways to mate and pup.
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Wayne Freeman
Some experts believe the sharks are getting stuck in man-made lagoons in Foster City and Redwood City when the tide gates close during low tides. The tide gates are closed by the cities during a low tide ahead of heavy rains to prevent the added rain water mixing with high tide water and flooding homes.
The fungal blooms in the stagnant water in the lagoons suck up the oxygen, poisoning the sharks. “If leopard sharks are trapped within these stagnant waterways with high levels of suspended fungi, then they could be exposed to an overwhelming number of fungi, become infected and die,” Okihiro said. “Leopard sharks in the bay proper are likely exposed because they aggregate in large numbers, in shallow water, during the spring.”
More about leopard sharks, Dieoff, San francisco bay, spring pupping season, Toxins
 
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