Heavy rainfall may be linked to sharks' deathsThis winter's heavy rains - beneficial to so many species - may, in fact, be diluting saltwater in San Francisco Bay so dramatically that leopard sharks are dying in the very spots where they prefer to give birth and search for food, scientists said Tuesday.
State biologists investigating a rash of leopard shark casualties around the region over the past month think the torrents of freshwater flowing into shoreline lagoons may be throwing the body chemistry of the fish fatally off balance.
"They might be going into these coves to pup," said Carrie Wilson, a marine biologist in Monterey with the California Department of Fish and Game. "If there's more freshwater intrusion and low salinity, it's very tough on these animals."
The analysis is still in the early stages, and there is no conclusive evidence of a connection, but hundreds of leopard sharks and rays also died in 2006, the last year rainfall was this far above normal.
More than 100 deadMore than 100 adult and juvenile leopard sharks have been found dead since mid-April. Early necropsies of a few sharks showed internal bleeding and brain lesions, problems not usually associated with low salinity conditions. However, several scientists and members of the public have reported seeing the animals gasping and thrashing before death, suggesting the sharks could have injured themselves after falling ill.
This week, state pathologists are expected to release the results of blood and tissue samples. Those should shed more light on the cause, which could be anything from low salinity and low dissolved oxygen levels to bacterial infection, virus or man-made contaminants such as pesticides or fertilizers.
Found around bay
The first wave of sharks washed ashore near a residential neighborhood in Redwood City. Since then, dozens of the strikingly patterned bottom-feeders have been spotted along the shores of Foster City, San Francisco and Marin County, according to Sean Van Sommeran of the Pelagic Shark Research Foundation.
Longtime East Bay resident Corky Schweizer observed a shriveled adult leopard shark carcass just south of San Francisco Airport on Saturday morning. He said the animal's remains caught his attention because they were so far above the waterline.
"It basically looked like it had launched itself out of the water," Schweizer said.
Leopard sharks, which can grow 5 feet long and live 40 years or more, are common in the shallow coastal waters from Oregon to Mexico. They typically eat crabs, small fish and mud-dwelling worms.
Prefer salty water
Though they don't venture out into the deep sea, they do favor water with salinity levels at the higher end of the spectrum, said Wes Dowd, a postdoctoral scholar at Stanford University's Hopkins Marine Station in Pacific Grove.
Dowd studied the effect of low-salinity water on leopard sharks as part of his thesis. In his experiments, Dowd found that when salinity fell to 19 parts per thousand, the sharks thrust their noses and fins out of the water in an attempt to survive.
Earlier this spring, sensors in some parts of San Francisco Bay recorded salinity levels below 15 parts per thousand, Dowd said. Seawater salinity maxes out at about 34 or 35 parts per thousand.
"When (salinity) moves lower ... it becomes harder for the sharks to compensate for that change in their environment," Dowd said. "They don't cope well."
Wait for the data
Though Wilson acknowledges the possible link between the deaths and the rapid influx of freshwater, she declined to use the term "die-off" and cautioned against jumping to conclusions until all the data are in.
"There have been some wild speculations based on only a few samples," she said. "We don't want to give out any information that's premature."
Whatever the roots of the problem, shark specialist Van Sommeran suspects that he and his team of volunteers are collecting only a fraction of the ailing animals.
"We found 50 (sharks) in one canal in Redwood City alone," he said. "Now we're getting calls from Millbrae, San Francisco, Mill Valley, Sausalito - it seems that the affected area is expanding."
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