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Scientists Yet to Pinpoint Specific Cause of Shark Die-Off in SF Bay

Strandings can be stopped if efforts are made to improve Bay water quality and marine habitat, says conservationist

You have probably heard of whale strandings, where whales beach themselves for unknown, but possibly human-caused reasons. But in the San Francisco Bay Area right now, shark strandings are the talk of the town. Hundreds of leopard sharks (Triakis semifasciata) have been washing up on Bay Area beaches and marshes over the past three months. Sharks have been found dead on beaches in Foster City, Hayward, Berkeley, Oakland, San Francisco and other areas around the Bay.

photo of leopard sharkPhoto by Eric Heupel Hundreds of leopard sharks have been stranding on Bay Area beaches over the past several months, but researchers have yet to determine the exact cause of the die-off.

Similar, massive unexplained die-offs occurred in 2006 and 2011. However, the root cause of such die-offs continues to elude researchers. 

Leopard sharks are the most common shark in the San Francisco Bay Area and are not listed as threatened or endangered under the Endangered Species Act. However, these predatory fish, — which eats crabs, other small fish and mollusks and can grow to up to 5 feet in length — are an important part of the Bay marine food web. Researchers suspect that they are being exposed to pathogens when they congregate in shallow waters in the springtime to give birth.

Dr. Mark Okihiro, a fish pathologist with the California Department of Fish and Wildlife, suspects that a fungal pathogen could be infecting the meninges – the tissue covering the brain — disorienting the sharks and causing them to strand themselves on land. But he says that the fungus is most likely not only pathogen that’s affecting the sharks. Okihiro, who has been working tirelessly to investigate why leopard sharks are stranding, released his preliminary findings last week.

So far, Okihiro has performed 10 necropsies on recovered sharks and was able to successfully isolate a fungal pathogen from the tissues of three stranded sharks. When these sharks were found on local beaches, they were still alive, but were so close to death they would not have survived even if they had been returned to the water. “Necropsies...have shown the infection is in their brain and inner ears,” which could explain their confusion and how they end up on shore, John Traverso, a spokesperson for the fish and wildlife department said in response to emailed inquiries. Researchers are yet to detect the specific species of fungus causing the infection. 

The brain tissue of two of the sharks that had fungal pathogens, as well as one additional shark, also tested positive for a protozoan known as Miamiensis avidus. Outbreaks involving this pathogen have occurred at White Sea Bass hatcheries in southern California. But Okihiro suspects that other pathogens are involved in the shark strandings because testing of the cerebrospinal fluid of seven of the sharks revealed other invasive, foreign cells. These likely are fungi, but it is proving difficult to distinguish fungal structures from nerve tissue and to get consistent results from brain tissue samples and genetic testing of spinal fluid.

Researchers are looking to the general health and ecology of the San Francisco Bay to understand what might be making sharks vulnerable to such pathogens.

Since the mid-1800’s, the San Francisco Bay has experienced rapid changes to its marine ecosystem, including widespread habitat loss and the installation of tide gates that help protect low-lying areas from flooding. When gates close to protect neighborhoods from flooding, typically during the winter, they can inadvertently trap fish inside, not allowing them to move back into the Bay as quickly as they may desire. Lack of water flow behind the gates can result in stagnant conditions that promote the growth of fungal blooms.

“‘Fungal’ blooms, which occur in stagnant marshes, lagoons, and sloughs where leopard sharks are trapped could kill them,” Traverso says. When it rains, fungal-laden plumes make their way into the Bay, further exposing aquatic life to pathogens.

Some entities that control tide gates, such as the Don Edwards San Francisco Bay Wildlife Refuge, have implemented a variety of protocols to help protect fish from getting trapped for prolonged periods in regulated lagoons and marshes. In one area where they are working to open historic salt harvesting ponds to tidal influence, they have installed a screen on one water intake pipe to prevent fish, particularly salmon, from entering ponds, says refuge manager Jared Underwood. Though they were not installed specifically with sharks in mind, this type of screen helps protect sharks and keep all but the smallest fish from entering the ponds. 

Underwood noted that oftentimes the refuge leaves the screen open so fish can come and go as they please year-round. But sometimes when conditions deteriorate, often during the summer, fish (including sharks) can die.

David McGuire, founder of the conservation group Shark Stewards, says his organization is investigating these die offs through two main channels. Shark Stewards — which is a project Earth Island Institute, the publisher of Earth Island Journal — has contacted the public works departments in Redwood City and Foster City to request water quality data from some lagoons in the Bay Area. They have also contacted the State of California’s Water Quality Control board, which is investigating the process by which lagoons are sampled for the presence of pathogens and contaminants prior to releasing water from them. Shark Stewards has requested additional monitoring of tidal basin water in coming years.  

However, identifying the source of contaminants might not be so cut and dried since it’s not coming from a single source, such as a factory. Non-point source pollution occurs when there are many sources of pollution that affect waterways via storm drains, such as people washing their cars in driveways, and pet owners not picking up pet waste. This kind of pollution, which might contribute to the proliferation of pathogens in lagoons, is more difficult detect and control. 

Looking at the bigger picture, McGuire points out that “this localized disease is not threatening the leopard shark and bat ray population at large.” Leopard sharks are considered a species of “least concern” by the International Union for Conservation of Nature, he says. “The major problem in the Bay is water quality and habitat loss,” he says, pointing out that the San Francisco Bay has been heavily impacted by human activity, with as much as 99 percent of habitat being lost or impacted. “We are trying to prevent another die off, but the south bay is so unnatural and impacted it is likely these will continue until we can restore the ecosystem,” he says.

McGuire says planting eel grass beds, the preferred habitat for leopard sharks and bat rays as well as other fish species, would help. “Healthy eelgrass beds and wetlands act as filters and mediators to potential toxins,” McGuire says. If more work is done in the Bay Area to promote the conservation and restoration of healthy habitat and protect water quality by preventing urban pollution from entering waterways, it will have a lasting, positive impact on the health of the Bay Area’s marine ecosystem.

“This die off will die off just like any localized disease if we can address proper water quality and habitat,” says McGuire. “We need to focus on the health of the Bay... the entire ecosystem from shrimp to sharks.”

Erin Banks Rusby
Erin Banks Rusby is an intern at Earth Island Journal. A transplant from the greater Seattle area to the San Francisco peninsula, Erin works with volunteers on projects to help restore native south Bay Area ecosystems. She is exploring pursuing a career in environmental journalism.

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