UPDATE (November 01, 2017): 

A brain-eating fish parasite may be the cause behind the mysterious affliction plaguing California's leopard sharks and coastal ray species. Over 1,000 sharks and some 500 bat rays have washed up dead in the San Francisco Bay Area since the die-off began six months ago – and researchers have been scrambling for answers all this time. 

Initial reports pointed to a fungal pathogen as the culprit, but DNA and RNA samples taken from several collected specimens suggest a protozoan parasite called Miamiensis avidus may be to blame. Hanna Retallack, a PhD student at the University of California San Francisco, detected the single-celled invaders in five shark specimens sent to her by California Fish and Wildlife veterinary fish pathologist Mark Okihiro. After receiving news of the ID, Okihiro doubled back and found M. avidus in many more specimens. 

These microscopic organisms enter their hosts through the nostrils. Once inside, it's a quick wriggle through the olfactory pathways and into the brain, where the parasites settle and feed. The presence of protozoans explains the lesions and swelling observed in the affected animals' brain tissues, but Okihiro and his colleagues are still working to piece together the rest of the puzzle. 

This parasite is well known: it's been tied to mass deaths in commercial hatcheries over the years, and has been studied in smaller bony fish species like olive flounder. Exactly where it originated in this case, however, is still unclear. It's possible that M. avidus "jumped the shark" from other Bay Area wildlife, but scientists don't know this for sure. Various environmental factors are still thought to have played some role in its rapid spread. 

Find out more in the original article below.


Scientists in California are closing in on the cause of a leopard shark die-off that's plagued the San Francisco Bay Area for the past two months. While many commenters online have been quick to blame Fukushima radiation, the real culprit may be closer to home. 

A leopard shark strands in shallow water. Image: Pelagic Shark Research Foundation/Facebook 

Leopard sharks (Triakis semifasciata) are a small species native to North America's Pacific coastline and are often misidentified as zebra sharks (Stegostoma fasciatum).

San Fransisco recently saw such a large number of leopard sharks washing up dead on its beaches that local researchers started running out of coolers to store the bodies in. That said, it is difficult to determine the exact death toll as many of these animals sink before hitting the sand.*

"From a conservation perspective, it’s very serious if not critical," Sean Van Sommeran, executive director of the Pelagic Shark Research Foundation (PSRF), said in an interview with local CBS Channel 5 News

Many commenters online are pointing to radiation from the 2011 Fukushima disaster as a cause for the strandings, but that is not likely to be the case. Along with the numerous reasons mentioned in this post, shark mass standings have been reported in the area since the late 1960s, with several major die-offs between 2002 and 2006, which pre-date the Japanese nuclear disaster. 

In the latest strandings, leopard sharks have been seen swimming in "confused" patterns, bobbing along at the surface before hitting the beach. Interestingly, this oddball behaviour may actually tell us what's going on. 

Over the years, Van Sommeran and his team have helped the California Department of Fish and Wildlife collect dozens of specimens  – and many of them, including some from this event, show inflammation in the brain. 

leopard shark-2-2017-5-4.jpg
Image: Pelagic Shark Research Foundation/Facebook

It's thought that this current bout of fishy "meningitis" is caused by a fungal infection that enters sharks' bodies through their ears or nasal slits before reaching their brains. 

"We are still waiting for culture results from seven sharks, but have recovered fungi from the brains of three leopard sharks that stranded," explains CDFW senior fish pathologist Mark Okihiro, who has led the stranding investigation. "We also have [cellular] evidence of a fungal pathogen in 8 of 10 sharks necropsied."**  

Sharks that carry the problematic skull squatters tend to swim erratically toward shore and beach themselves. For this reason, local residents are urged not to push stranded animals back into the surf. An infected individual will simply beach again, or worse, die offshore where officials won't be able to get to it. 

Knowing what's causing these causalities is only half the battle though; deciphering the reasons why is a more difficult task. Okihiro and his team suspect there are multiple factors at play.    

Several of these die-offs have occurred in spring, when the small sharks spawn in shallow water. This could expose them to higher levels of waste and other pollutants, which accumulate in artificial lagoons and areas of controlled water flow. Recent changes in local weather may be exacerbating that contamination. 

"What we're seeing may be an artifact of storm water runoff," Van Sommeran said in a Facebook Update. "We've had long periods of dry, without any rains, and all of these pollutants tend to congeal and build up. The heavy rainfall we've had [recently] has essentially washed a double overdose of these normal toxins into the watershed."

But unravelling the order of operations here is proving particularly challenging. Are the sharks's immune systems being lowered by exposure to pollutants, putting them at greater risk of infection? Are they simply being overwhelmed by natural fungal blooms? Or is it some combination: could human activity be creating the perfect breeding grounds for the deadly pathogens? 

In an interview with The Daily Journal, University of California Long Beach Shark Lab director Dr Chris Lowe notes that leopard sharks further south aren't being affected in this way, which backs up the hunch that water quality is playing some role in the strandings. 

Meningitis has similarly been reported in shark species that live far offshore, like salmon sharks and thresher sharks,  but a different culprit (carnobacteria) is to blame in those cases. The fact that these fungal outbreaks are only affecting coastal animals also points to a water quality issue. 

"We really have to keep an eye on whether any of these sorts of [die-offs] are attributed to things that we do to the ocean,” says Lowe. 

Van Sommeran and the team at PSRF are also concerned that tidal gates around the Bay may be trapping the sharks in toxic waters, though fish screens have been deployed in attempts to prevent that from happening. 

While leopard sharks aren't considered endangered, these events are still cause for concern – especially when they overlap with critical breeding seasons. Other species, like bat rays, guitarfish and halibut have also turned up dead.

Footage from the bay shows a leopard sharks final hours. Image: CBS/YouTube

We'll be updating you as the story unfolds, so watch this space! 

If you see a dying shark or ray anywhere in California, call or text the California Department of Fish and Wildlife hotline. You can also report strandings directly to the Pelagic Shark Research Foundation.


Top header image: Ian Sanderson/Flickr

* Editor's note: A previous version of this article stated that hundreds to thousands of sharks were thought to have died during the current outbreak. While those numbers have been suggested by some of the parties involved, others suspect this is an overestimationIt is likely that hundreds of leopard sharks have died this year. 

** A quote was added from Mark Okihiro for clarity