Scientists estimate that the real number of deaths could be up to 1,000.
In a sudden die-off that has researchers and conservationists scrambling, leopard sharks are dying by the dozens in northern California and biologists aren’t certain about why. It started several months ago around the time that much of California experienced heavy rain, and for the Bay Area these showers hadn’t been seen in years and followed a long period of drought for the region.
Dead leopard sharks have been spotted on the beaches in San Francisco, Oakland, Redwood City, Foster City, Alameda, Hayward, and Berkeley. These creatures are harmless to humans and are sometimes found in shallow waters, especially during the spring because of mating and pupping season. At least 80 have been confirmed dead after they were found on shores and brought in by researchers, but scientists think that there are many more that we aren’t seeing.
“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, in a written synopsis. “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.”
“This is just the tip of the iceberg,” added Sean Van Sommeran, executive director and founder of the Pelagic Shark Research Foundation. “We’re only seeing a fraction of the actual losses.”
Since sharks don’t have lungs, once they die they typically fall to the bottom of the ocean, where their bodies would go undiscovered and uncounted in this sudden die-off. That’s why researchers are concerned that there are many more leopard shark casualties that are not accounted for and that the actual number is being grossly underestimated.
Both Okihiro and Van Sommeran have responded to reports of dead sharks and fish for the last several months, with Okihiro himself picking up and performing necropsies for over two dozen leopard sharks. His findings have led him to believe that the sharks are dying of a fatal brain infection linked to a fungus that may have been in abundance in the bay because of the rains.
The heavy rains that replenished some of California’s water supply also caused problems for marine life because of tide gates, reduced salinity in the water, and an increase in fungus and other toxins being pushed out into the oceans. Cities closed tide gates in man-made lagoons to prevent flooding from occurring, which some believe might have left leopard sharks trapped in shallow water that became increasingly toxic. Though the rains reportedly pushed out “old mercury, DDT and PCBs in bay mud out to the ocean,” this means that marine life that lives close to the area suffered as a result from being exposed to these toxins.
The water in the lagoons that has been relatively stagnant ever since the drought started is now teeming with fungi that was produced in the still water. In what’s being called “fungal blooms,” the sharks that are swimming close to shore or trapped inside the tide gates are experiencing the worst of the fungi. Even if they don’t become infected with any fungi, the likelihood that the reduced salinity in the ocean could kill them is also high. The freshwater rains diluted the concentration of saltwater, which poses all kinds of risks to sharks and other marine animals.
“They go from being able to regulate their internal salt content to not being able to,” said Jim Hobbs, a research scientist at UC Davis who has studied leopard sharks in the bay for the past seven years. “It causes a whole variety of impairments. They can’t excrete toxins that build up in their bodies.”
It’s still unclear what exactly is happening to these sharks, although this isn’t the first die-off to happen in this area with leopard sharks. They happened in 2011 and 2006, with reports showing that these incidents occurred as far back as 1967, and there has never been a definitive explanation for the deaths. Scientists suspect that human pollution has caused the fungal blooms and other toxic properties being put in the ocean, but they struggle to pinpoint exactly what would need to change for these incidents to stop completely.
It’s estimated that as many as 1,000 leopard sharks have died since early March in the area, and that the ones that beach themselves are simply the ones that are close to the shore and know that they are going to die. As apex predators, their survival is critical to the surrounding ecosystem and, although they aren’t endangered, biologists are worried about the potential for this continuing to happen.
“It’s the signature species in San Francisco Bay,” Van Sommeran said. “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.”