A dead leopard shark spotted in Aquatic Park last week. Credit: Citizen reporter

Dead sharks and rays are washing up along the lagoon in Aquatic Park — a grisly, sad and curious sight for walkers, bikers, runners and others discovering the carcasses.

Numerous people contacted Berkeleyside over the past week saying they’d seen the dead marine life at Aquatic Park, many sending pictures.

Naturalists and marine experts are also tracking and documenting the reports, a whirlwind of investigation that consumed some over the weekend, despite the Superbowl, sources said.

Reports of the small, spotted leopard shark carcasses, as well as those of bat rays, related species in a class of fish called elasmobranchii, have also come in from Redwood Shores, in the South Bay. Dead rays were also sighted in Novato. 

It’s impossible to say how many dead fish people have reported to date, but anecdotally it’s at least a few dozen in Aquatic Park.

Both species spend time in shallow bay tidal waters, including for breeding, such as in lagoons, canals and estuaries — the mouths of creeks and rivers.

“It makes me very sad,” said Mary Skramstad, environmental compliance specialist for Berkeley’s Public Works Department. 

She said city staff are investigating it, and “everyone is perplexed.” 

City testing so far doesn’t explain the deaths, Skramstad said, qualifying that the health department is also working on the issue, and she isn’t up on everything they’re doing.

This isn’t the first time eye-catching numbers of these types of dead fish have appeared. Die-offs of significance have occurred in 2011, 2016, 2017 and 2019, and lesser events have also occurred. 

They correlate to heavy rains, experts say.  

Though they aren’t sure exactly why. 

Autopsies of sharks in similar die-offs in the past have shown that they were infected with a nefarious protozoa or microscopic marine parasite, called Miamiensis avidus, that migrated into brain tissue through the nose, said Mark S. Okihiro, a senior fish pathologist with the California Department of Fish and Wildlife who’s considered a state expert in the die-off phenomenon. 

Okihiro is investigating the recent spate of deaths and suspects this protozoa is to blame, though he has yet to gather any evidence to support that hypothesis.  

“I have not necropsied any leopard sharks this year, so everything is conjecture at this point,” he said. “I am, however, fairly confident that we will find Miamiensis protozoa in SF Bay leopard sharks over the next three months. We also hope to find the same pathogens in some of the other species that strand — e.g., bat rays, guitarfish, smoothhound sharks, striped bass, etc.”

Citizen naturalists as well as experts are sending Okihiro photos and locations of dead fish. Some are also reporting their sightings on I-Naturalist, a crowd-sourcing data tool often tapped by scientists.

One of three dead rays a West Berkeley resident spotted in Aquatic Park on Feb. 7, 2023. Credit: Citizen reporter

The cause of death in the die-offs appears clear, experts agree, based on Okihiro’s lab analysis —

the tissue-invading protozoa. But the reason the SF Bay sharks and rays become so susceptible after rains isn’t yet understood, they say. 

“No one has figured it out,” said Damon Tighe, a curriculum developer at Bio-Rad by day and naturalist much of the rest of the time, who helped start the California Center for Natural History  and is coordinating with Okihiro and others in collecting data of recent fish sightings. “There’s plenty of speculation.”

This speculation — which is the basis of ongoing investigation — focuses on water quality changes in the shallow tidal waters where the wet weather die-offs have primarily occurred.

These are areas usually fed by two sources of water, from land and from sea, as is the lagoon at Aquatic Park. 

It’s this interplay of waters during wet weather that likely holds the key to the protozoa that’s killing sharks and rays, experts agree. 

Leading hypotheses include decreased salinity related to intense flows of run-off into bay waters from streets, gutters, soils and groundwater; decreased oxygen, linked to jumps in algae; and toxins and other pollutants or debris washing down in watersheds. 

“It is possible that low salinity may be resulting in immunosuppression of sharks. It is also possible that heavy spring runoff is also resulting in marked contaminant exposure (e.g., pesticides, herbicides, fungicides etc) to sharks and rays, and also causing immunosuppression,” Okihiro said. 

The protozoa might also proliferate in waters of lower salinity, he said, stressing that “these are all unproven hypotheses.”

Sharks can get trapped in tidal lagoons like Aquatic Park

The Aquatic Park lagoon overflows its boundaries in the rain on Jan. 4, 2023. Credit: Kelly Sullivan

Sean Van Sommeran, executive director of the Pelagic Shark Research Foundation based in Santa Cruz, said he started getting reports of dead sharks and rays a few weeks ago, initially from the Redwood Shores area. 

The foundation is one of several places where people report Bay Area dead fish.

So far, Van Sommeran doesn’t characterize the current die-off as “major” but said “it’s a perfect demonstration of what I’ve been talking about for 20 years.”

Once nicknamed “Sharkman,” Van Sommeran said one of his biggest concerns is sharks and rays getting trapped in shallow waters by tidal gates, which are often lowered to prevent flooding in inland bodies of water. 

During storms, inland lagoons and estuaries can fill up with runoff from watersheds and groundwater, making them prone to flooding. Tide gates are a common way of trying to minimize this impact.

But for marine life used to circulating in and out of inland waters, they can be a killer, Van Sommeran said. 

When waters recede with the tides, the fish can’t survive, especially if the water is low and muddy. 

“Sharks get trapped behind and they gather and die by the dozens, trapped in the stagnant water,” Van Sommeran said. 

 “Sharks need a memo: This is an estuary,” he said, so they can get out while it’s possible. “They’re doubling as stormwater basins and artificial culverts.”

He thinks people aren’t worried enough about the sharks and rays, and their deaths too quickly become “out of sight, out of mind. There is an annual die-off associated with these fish and it gets ignored or people don’t know about it.” 

David McGuire, founder and executive director of Shark Stewards, a Berkeley-based organization working on preserving healthy shark populations, described it this way:

“Tidal lagoons like Aquatic Park, Lake Merritt in Oakland, and those located off Foster City and Redwood City, are subject to polluted street runoff that includes human and animal waste. Periods of high rainfall, such as occurred in early February this year, can also create a lens of freshwater that can create low oxygen conditions and alter the pH of the lagoon. If the tidal gates are closed, fresh saline and oxygen rich seawater is not allowed to enter and mix with the water in Aquatic Park.” 

McGuire has also been collecting Aquatic Park fish death data over the past few days, and sending it to Okihiro, as a project with UC Berkeley students.

Dead leopard sharks seen at Aquatic Park this weekend. Courtesy: David McGuire

The Aquatic Park lagoon, a manmade lake with a history of pollution problems, has at least five tide gates on large 24-inch culverts (or pipes) to the bay. Culverts also connect the three different sections of the waters. Fish come and go through these culverts, which are prone to blockage from sediment, tubeworms and aging, which the city is tackling.

But the tidal gates are never all shut at the same time, Skramstad said. At least one is left open for water to allow exit to the bay. “Otherwise there would be flooding.”

Berkeleyside has reached out to the city for more detailed information on the dead fish, such as the possible role of tide gates and pollution, including the lagoon’s recurring high bacterial counts — specifically of Enterococcus, an indicator of fecal material. (Enterococcus levels in parts of the lagoon rose well above what the the State Water Resources Control Board considers elevated in late December and early January but have since fallen to safer measurements.)

Tighe points out that he is unaware of any data showing that Enterococcus is directly killing the sharks and rays, including from Okihiro’s necropsies. But it might contribute to proliferations of the deadly protozoa, he said.

Lake Merritt has well-known water quality issues, but isn’t a site of the historic shark and ray die-offs. 

If climate change brings increased bouts of intense rain to the Bay Area, as some climate models predict, these die-offs are bound to escalate, experts say.

“The populations do make a come-back, but the asterisk is if these events are increasing in frequency there’s the potential for population collapse,” Tighe said.

If you see a dead or trapped marine animal in Berkeley, experts recommend reporting it to the city of Berkeley, the state Fish and Wildlife website, on I-Naturalist, and to organizations such as the Pelagic Shark Research Foundation

Freelancer Catherine "Kate" Rauch has been contributing to Berkeleyside for several years. Her work as a journalist has encompassed everything from 10 years as a daily news reporter for the East Bay Times,...