Biologists are trying to discover why so many leopard sharks are dying in the San Francisco Bay this year. Dead leopard sharks are appearing by the hundreds in the bay, and biologists noted is the largest die-off of the shark in six years.
Thousands of dead sharks, several bat rays, and a few halibut have been discovered since March along the shorelines of California, including Redwood City, Foster City, Alameda, Hayward, Berkeley, Oakland and San Francisco.
Some dead rays and sharks have also been found in Bolinas, in Marin County.
Leopard sharks are dying in large numbers in San Francisco Bay
Experts said it is the second year in a row that leopard sharks, the most abundant shark species in the area, have been dying in the coast of California. They believe the sharks are picking up deadly toxins in stagnant saltwater marshes, sloughs and behind the gates of artificial lagoons in Redwood City and Foster City.
“My estimate is that several hundred sharks have already died,” said Mark Okihiro, senior fish pathologist for the California Department of Fish and Wildlife, in a written synopsis, according to the San Francisco Chronicle. “There appears to be no leveling off of shark deaths in the bay.”
Okihiro added that he is still receiving reports from locations throughout the South Bay regarding dead or dying leopard sharks. He noted there had been dozens of reports, and that he collected 21 leopard sharks and two bat rays in April in the Foster City shoreline.
Sean Van Sommeran, executive director and founder of the Pelagic Shark Research Foundation, noted that he has responded to multiple reports of dead fish along the shore and believes the number of dead and dying sharks in the San Francisco Bay could be in the thousands.
Experts confirmed it is the largest leopard shark mortality event since 2011, when over 1,000 dead sharks died inside and outside the Redwood Shores Lagoon and Richardson Bay, in Marin County. Okihiro explained that sharks do not have lungs, so they immediately sink to the bottom when they die, which means that the numbers they are currently estimating may be well below the actual number of deaths.
“This is just the tip of the iceberg,” said Van Sommeran. “We’re only seeing a fraction of the actual losses.”
Van Sommeran said that the problem is that leopard sharks come into the shallow waterways to mate and pup throughout spring and summer. Specialists believe that the sharks are getting stuck in artificial lagoons in Foster City and Redwood City when the cities close their tide gates during low tide.
Fungi in the water may be killing the sharks
Cities like Foster City and Redwood City close their gates at low tide as a precaution for heavy rains so that the additional precipitation does not combine with high tides and flood homes. Fungal blooms are formed in the stagnant water and suck out the oxygen as well as poison the fish.
“If leopard sharks are trapped within these stagnant waterways with high levels of suspended fungi, then they could be exposed to an overwhelming number of fungi, become infected and die,” said Okihiro, according to the San Francisco Chronicle.
Okihiro added that leopard sharks in the bay per se are more likely to be exposed because they aggregate in shallow water in large numbers during the spring. Van Sommeran also believes the rainy season worsen the damage by washing out toxins that accumulated in the ground during the most recent drought. He noted that with the hard rain there is extra crud going into the watershed.
Expert says if leopard sharks keep dying every year it will be a disaster for the species
Shark mortality events have often occurred in the past. In 1967 more than 700 dead sharks and rays appeared in the coasts of Alameda. Die-offs also occurred every year between 2002 and 2006, and then in 2016. To the date, the biggest shark die-off happened in 2011.
Leopard sharks were so common in past years that they would be spotted as far away as Sacramento. According to Van Sommeran, “Tiburon,” the Spanish word for shark, is believed to have been named after the leopard shark. He noted that the fish once grew 5 to 5 feet long and that it lived between four or five decades.
The striped sharks are not endangered species, but they were caught by bay fisherman to eat them. However, after the 2011 die-off, regulators warned fishers and people not no eat them because of the accumulated toxins in their blood and meat.
Van Sommeran said that leopard sharks are the signature species in San Francisco Bay. According to him, if they keep dying at the current rate every spring when they are trying to pup, it will be a disaster, as the species can’t sustain the losses.
Source: San Francisco Chronicle