Leopard shark deaths in the San Francisco Bay have continued to increase over the past several months, marking the largest species die-off in the region since 2011, according to the California Department of Fish and Wildlife (CDFW).
"We estimate several hundred leopard sharks have died in 2017 and the number of sharks that died in 2011 was likely over a thousand," the department's deputy director of communications Jordan Traverso said in a written statement.
"This estimate comes from dozens of firsthand accounts, photos, and videos, discussions with Oakland East Bay Regional Parks officials and rangers, and by brief field surveys made in both 2011 and 2017."
Sharks, bat rays and fish have been found dead along the shorelines of Redwood City, Foster City, Alameda, Hayward, Berkeley, Oakland, San Francisco and Marin counties since mid-March, according to a report from the San Francisco Chronicle.
"One working hypothesis we are looking at involves mass exposure to leopard sharks that gather in large aggregations during spring months," Traverso said. Fungal blooms, which occur in stagnant marshes, lagoons and sloughs where leopard sharks are trapped, could kill the sharks.
Additional exposure could occur as sharks are exposed via "pulses" of fungi-laden water following rain events, he added. The spring and summer months mark the leopard sharks' mating and pupping seasons, which draws them into the shallower waters in large numbers.
Researchers believe that the sharks are getting trapped in the man-made lagoons in Foster City and Redwood City when the tide gates are closed, according to the San Francisco Chronicle.
"The 2011 mass mortality event started inside the Redwood Shores Lagoon, which is adjacent to the Foster City Lagoon," Traverso said, stating the epicenter in 2017's outbreak is in Foster City Lagoon.
At the moment, the estimates of how many sharks have died are conservative because only a small fraction of the total number dead are seen, he added.
"The shoreline along San Francisco Bay is extensive, with many miles that are inaccessible except by small boats, kayaks or canoes," he said.
In addition, sharks do not have swim bladders or lungs and sink following death. During a shark or ray die-off, researchers only see the sharks and rays that strand in coastal zones easily accessible by the public.
"The vast majority of sharks that do not strand, die in deep water and are never seen, or die in mudflats and disappear into the mud," Traverso added.
Department biologists have also reported halibut dying in the Redwood Shores Lagoon. Fishermen have also reported dead fish near Alameda.
Department researcher Dr. Mark Okihiro conducted a field survey where he recovered 21 leopard sharks and two bat rays from the Foster City shoreline in April.
Sharks strand when they become disoriented because the examination of the dead animal's body have shown the infection is in their brains and inner ears, Traverso said.
Of the 10 leopard sharks examined by Okihiro to date, a fungal species has been isolated. At the moment, all indications currently point toward a fungal pathogen, according to Traverso.
However, he stressed that Dr. Okihiro’s hypotheses concerning the cause and development of the disease killing the sharks is "just that, hypothetical." The department is currently working on acquiring more data.
"The causes, and thus any potential solution, will remain an unknown until CDFW and others understand the cause, being investigated by Dr. Okihiro’s work, and will be largely beyond our ability alone to address if it has to do with low salinity and pollution," Traverso said.