Thousands of fish carcasses float to the edges of the San Francisco Bay, and the grimy top of Oakland’s Lake Merritt – simmering in the sun and giving off a putrid stench in nearby neighborhoods.
Dead bat rays, striped bass, sturgeon, anchovies and clams are likely the mass victims of an algal bloom that scientists are racing to figure out. In the meantime, citizen scientists, local photographers, joggers and naturalists captured spectacular photos of the death.
“The diversity of life in Lake Merritt is just amazing,” said Damon Tighe, a naturalist who documents the lake’s wildlife, an ecosystem unique to Oakland. California which contains both fresh and salt water. A range of fish, crustaceans and molluscs inhabit the lake, as do large breeding populations of herons, egrets, geese and ducks. Salmon, sturgeon, jellyfish and leopard sharks have also cruised the lake in recent years.
The mass mortality in the lake is probably due to an algal bloom of We say heterosigma, which was first spotted at various locations in San Francisco Bay and estuaries in late July. Algae likely suck up all the dissolved oxygen in the water, leaving fish to suffocate, said Jon Rosenfield, a scientist with the San Francisco Baykeeper conservation group who has been tracking the phenomenon. We say heterosigma also produced a toxin which may have killed the fish.
The result is reddish, brown-tinged water – flowing over the banks with thousands of swollen fish bodies. According to the San Francisco Bay Area Water Quality Monitoring Board, the algae density measured on August 10 was the highest observed in more than 40 years.
The sturgeon deaths in parts of San Francisco Bay have been particularly striking, given that the fish are so large and protected from many environmental threats. Fish can live for decades.
“It’s like losing giant sequoias,” Tighe said. “I don’t think people quite understand the meaning – it’s really huge.”
Researchers are also conducting tests to determine what might have caused the bloom. Effluent from sewage treatment plants is likely to blame, according to Rosenfield.
“Researchers have known for decades that the levels in this estuary are really high,” he said. “In fact, many have wondered why it hasn’t happened yet.” Existing sewage treatment systems were not designed to filter out the phosphorus and nitrogen that lead to such red tide algae blooms, he added. “It’s an issue that society as a whole has ignored for too long.”
The officials have suggested this decrease in water flow from estuaries to the ocean, due to statewide drought, may also have contributed.
As the Bay Area braces for a brutal heat wave over the coming week, Rosenfield said warm temperatures could exacerbate bloom, depending on whether or not it’s already past its peak. The heat could also cook the carcasses further and make the stench worse. Crews in Oakland began cleaning up dead fish in anticipation of warmer temperatures over the holiday weekend.
“I’m concerned about how long this will last and the longer-term impacts,” Tighe said, noting that many birds feed on the fish in the lake. “It could cause big ripple effects.”
In 1870 Lake Merritt became the first protected wildlife refuge in America. In recent years, local authorities have worked to restore the quality of its water and rid it of accumulated waste.
The Associated Press contributed to this story