Marine biologists are baffled by the huge numbers of dead leopard sharks being washed ashore San Francisco Bay, from San Mateo to Bolinas.
Since the second week of March, hundreds of dead leopard sharks have been spotted at the shorelines of Oakland, Berkeley, Hayward, Foster City, Redwood City, Alameda and San Francisco. The huge die-off of the stripped fish is considered to be largest leopard shark mortality event since 2011.
“My estimate is that several hundred sharks have already died,” said Mark Okihiro, the senior fish pathologist for the California Department of Fish and Wildlife, as per SF Gate. “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.”
At the peak of the mortality event, investigators reported finding at least 12 dead leopard sharks daily.
During spring and summer, leopard sharks come into shallower waterways to mate and reproduce. Tidal gates inside the bay area that opened during low tide allowed the shark inside the area. However, these tidal gates were closed during high tide and storms to prevent flooding of the coastal communities. As a result, the leopard sharks were trapped in the stagnant, toxic waters of the bay.
Stagnant, saltwater marshes could turn into a breeding ground for fungal blooms. High levels of suspended fungi in the water can suck out the oxygen and infect fishes and other marine mammals living in the area, which could lead to their death.
Aside from fungal blooms, experts noted that the recent rainy season is somewhat responsible for the leopard shark die-off. The rainwater carries the toxic accumulated in the ground during the drought into the bay area.
Leopard sharks, or Triakis semifasciata, are one of the most common near-shore sharks along the Pacific Coast of North America, ranging from Washington to Mazatlan, Mexico, including the Gulf of California. It is listed as Least Concern under the IUCN Red List of Threatened Species.
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