CAUSES | Infected Predators

What’s behind the Bay Area’s shocking shark death toll?

By Mat Weir


photo rozalyn erickson


Killers. Predators. Monsters of the deep. Misunderstood. These are just some of the many ways to describe one of the most feared creatures: sharks. The prehistoric fish fascinate us humans. From suspense-thriller movies like Jaws to the Discovery Channel’s popular Shark Week, the powerful beasts mesmerize us. Although these captivating animals have very few predators in the wild, it seems they can’t escape nature’s most invasive species: man.

Earlier this year, the San Francisco Bay Area saw thousands of dead fish, rays and sharks—mostly leopard sharks—wash up on the shores of Redwood City, San Mateo, Alameda, Lake Merritt and more. Even Santa Cruz wasn’t spared: a great white washed up in Pleasure Point last April.

“This year was brutal,” admits Sean Van Sommeran, executive director and founder of the Santa Cruz-based Pelagic Shark Research Foundation. “I knew it would be, too.”

Since 1990, the nonprofit has worked tirelessly to research and conserve the 11 different shark species that call the Bay Area home. While he openly admits he does not have a PhD in the field (he was an anthropology major), Van Sommeran comes from a rich background of fisherman. After witnessing the environmental impact fisheries were having on the ecosystem, he began studying sharks, seeing them as the nexus point of ocean activity. In 1995, the Pelagic Shark Research Foundation began tagging sharks in conjunction with UC Santa Cruz.

Since then, the foundation has broken a number of world and state records in tagging sharks. In 2012, it was the first organization to use satellite transmissions to track the migration of basking sharks. Van Sommeran theorized the migration pattern was much larger than previously thought and his suspicions were proved right when movements were documented from the Bay Area all the way to Baja California, Mexico.

He claims the most recent die-off was not surprising, as he is seeing a pattern develop. “It also happened last year,” he says. “There was a scattering of over 100 dead sharks and rays in Berkeley Park, although there was no press.”

So what exactly is killing one of the ocean’s top predators? Initial assumptions were that the leopard sharks were swimming too far inland into fresh waters, following their instinctual mating habits. The sharks become trapped in tide gates, which are used to stop flooding in inland areas, and when rain falls the waters become less salinated. However, Van Sommeran disputed that claim.

“It might’ve been plausible in the first five weeks [of spring], but when the rain stopped, the deaths persisted,” he says. “Also, leopard sharks are the most able to cope with low and high salinity.”

As more carcasses continued to wash ashore, researchers scrambled to find the cause, and they did. Sharks instinctively swim to estuaries to pup and become trapped in the man-made tide gates. There, they are exposed to Miamiensis avidus, a parasite found in the tide gate waters that creeps through the animal’s nostrils and plants itself inside the brain. Once infected, the sharks cannot be treated and death is guaranteed. To make matters worse, when the tide gates reopen, infected sharks and corpses go back into the ocean, allowing the parasite to infect the greater, open-water population. Experts believe the carcasses they see are only the tip of iceberg.

“It’s only a fraction of the losses that are washing up on the beaches,” Van Sommeran states. “At least four sevengill sharks—who live in the deep water—washed up, bolstering the concern that this is being swept to the deep ocean.”

As for the white shark that washed up in Pleasure Point, Van Sommeran says it had an entirely different bacteria that scientists are still studying.

This isn’t the first time shark populations have faced sizable deaths in recent history. From 2002 to 2006, dozens of sharks and other marine animals washed up on Bay Area shores. As with the recent die-off, bacteria played a role then, too. However, these deaths also coincided with four major toxic brine spills from the agribusiness Cargill. The company ultimately paid more $600,000 for the spills—a diminutive fine for a multi-billion dollar corporation.

“I have no doubt that was the cause,” Van Sommeran says. “They were fined for the spills, but weren’t reprimanded at all for the wildlife kills.”

While shark deaths from Miamiensis avidus have appeared to slow since July 2017, Van Sommeran believes we still need to be concerned about other shark killers. Bacteria, protozoa (single-celled organisms) and fungal pathogens were found in many of the first carcasses from 2017, which he attributes to stagnant water in the tide gates along with human pollution dumped into the ocean from storm drains, sewage and liter. In 2014 the Pelagic Shark Research Foundation collected a number of thresher sharks and determined they were dying from the bacteria Carnobacterium.

“It’s a myriad of health issues stemming not only from stagnant water, but many of the carcasses tested also had low levels of pesticides,” he says. “It’s a toxic broth in the San Francisco Bay because these are large cities with a lot of people.”

What can be done, if anything, to save the Bay Area’s shark species? Van Sommerman believes the first step is redesign local tide gates. Not only are they built without considering the local wildlife, he says, but many in the Bay Area are old and in a state of disrepair.

However, there is hope. Van Sommeran says there is usually a three-day gap from when the sharks are trapped in the tide gates to when they are infected. Small crews of three to five people with nets and a pick-up truck can save many of creatures—and the greater population—from being infected, but more funding is needed. “Thankfully, it’s San Francisco,” he says. “There are a lot of conservationists, environmentalists and tech-savvy individuals who could work on the engineering.”



2 comments on “CAUSES | Infected Predators

  1. Monterey bay has nearly two dozen species of sharks that either live in the bay year around or highly migratory species that travel clear across the Pacific seasonally.

    There were actually 2 white shark strandings and a handful of other species that stranded in Monterey Bay, first shark that stranded back in April was confirmed to have a brain infection (TBT), the second stranded white shark (another pup close to 10ft long) had been hit by a boat, the head was badly damaged and so we were unable to determine if it was infected.

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