Area marine biologists remain stumped to explain why 50 dead leopard sharks were discovered in April during the rainy spring season. The incident wasn’t isolated to Redwood Shores. Hundreds of sharks began appearing in places as far north as Marin, and near Coyote Point in San Mateo.
Scientists determined that two specimens suffered from the skin, according to Redwood City spokesperson Malcolm Smith. Yet the cause of death is still to be determined.
Marine life enthusiasts are celebrating Shark Week on the Discovery Channel this week, but it’s not this media hype that has scientists what could be the cause of this aquatic mystery.
“I’ve never heard of anything like this,” said Bill Cox of the state’s Mammal Marine Center. “We’ve had leopard shark mortalities before, but it isn’t a yearly occurrence.”
“This is very serious,” added Sean Van Sommeran of the Pelagic Shark Research Foundation, a volunteer-led nonprofit organization. “Leopard sharks are usually very resistant to infections.”
Scientists from the state and independent organizations agree there isn’t a clear-cut reason that the four-foot long sharks died.
"It’s not like a pregnancy test that can tell you 'Yes, the water was contaminated,'" said Van Sommeran.
Van Sommeran was one of the first on the scene collecting samples. He received a call from Redwood Shores resident Cathy Greer, who had initially spotted the sharks washed ashore. He overnighted samples to the California Department of Fish and Game, hoping to preserve the samples’ freshness for better testing.
But despite abundant samples, pathologists could not travel to the Redwood Shores site due to limited state resources. This same burden has slowed the extensive testing process required to produce answers to such a strange phenomenon.
Samples are in the department’s labs as well as the University of California-Davis, but no tests have proved definitive results. State pathologists do have three hypotheses that they are testing, according to Department of Fish and Game spokesperson Carrie Wilson.
Low salinity levels: Scientists believe the unusually heavy rainfall sent large amounts of fresh water run-off into the Bay, causing the level of salinity in the water to drop dramatically. These animals swim to shallow water to give birth around that time of year, but aren’t meant to live in fresh water. The water’s low salt levels may have caused osmotic shock, resulting in the sharks’ death, according to Cox. However, scientists say a more accurate test would be difficult to conduct because they require live blood samples for comparison.
If the first test doesn’t pan out, pathologists move to another test, Wilson said.
Contaminants in the water: Due to fresh water run-off into the Bay from the heavy rainfall, the sharks could have been swimming in water filled with household rat poison, which is an anti-coagulant. Scientists will analyze if areas in the sharks’ vasculature weren’t clotted when they’re supposed to be, Cox said. The sharks will also be dissected to look for specific infections, like encephalitis, that have been detected in other marine species. These infections that would indicate poor water quality, Van Sommeran said.
Toxins: Toxins that could have come from the microsystems of bluegreen algae could have poisoned the sharks. This is seen in mammals like sea otters and dogs, and can cause serious liver damage, according to Cox.
Van Sommeran believes the first hypothesis is too simplistic.
He said although sharks are not meant to live in fresh water, they are highly adaptable, able to withstand variations in salinity. He likened this adaptability to humans’ ability to withstand high temperatures such as in a sauna. However, when in this environment for too long, it poses a danger to the sharks, just as people can't stay in a sauna for too long. He believes that the city’s floodgates, designed to shut out water to prevent flooding in rainy areas, could have trapped some sharks in the shallow water areas.
Cox agreed that the operation of a floodgate wasn’t entirely natural, but in general it mimicked the water's seasonal oxygen and salinity levels.
He also believes the second hypothesis is very possible. The fresh water run-off could have contained a number of contaminants and been washed into the sharks’ habitat.
Van Sommeran said a simple solution to poor water quality would be to install aerators and other water cleansing mechanisms.
But identifying the cause could take weeks, maybe months, Cox said.
“We’re taking three steps forward, two steps back,” Van Sommeran said, who has been closely following this saga since Day 1.
Leopard shark strandings have stopped for the time being, but answers regarding the sharks’ hemorrhaging still elude the state’s top pathologists.