Thousands of dead fish wash up in Oakland lake to create a putrid mess | California

Thousands of fish carcasses have been floating up to the edges of the San Francisco Bay, and the scummy top of Oakland’s Lake Merritt – stewing under the sun and wafting a putrid stench into nearby neighborhoods.

The dead bat rays, striped bass, sturgeon, anchovies and clams, are likely to be mass victims of an algal bloom that scientists are racing to understand. In the citizen scientists, local photographers, joggers and naturalists have been capturing dramatic photos of the die off.

“The diversity of life in Lake Merritt is just incredible,” said Damon Tighe, a naturalist who documents wildlife in the lake, a unique ecosystem in Oakland, California that contains both fresh and saltwater. A range of fish, crustaceans and mollusks call the lake home to the large breeding populations of herons, egrets, geese and ducks. Salmon, sturgeon, jellyfish, and leopard sharks have also navigated into the lake in recent years.

A dead bat ray sits in the muck on the shore of a lake.
Lake Merritt in Oakland, California, has sometimes been home to sharks, rays, sturgeon and jellyfish. Photograph: Nathan Frandino/Reuters

The die-off in the lake is likely due to an algae bloom of akashiwo heterosigma, which was first spotted at various points across the San Francisco Bay and estuaries in late July. The algae is likely sucking up all the dissolved oxygen in the water, leaving the fish to asphyxiate, said Jon Rosenfield, a scientist with the San Francisco Baykeeper conservation group, which has been tracking the phenomenon. akashiwo heterosigma it also produces a toxin that may have killed the fish.

The result is ruddy, brown-tinged water – slopping into shores along with thousands of bloated fish bodies. According to the San Francisco Bay Regional Water Quality Control Board, algae density measured on 10 August was the highest observed in more than 40 years.

The deaths of sturgeon in parts of the San Francisco Bay have been especially striking, given that the fish are so large, and armored against many environmental threats. The fish can live for decades.

“This is like losing giant redwoods,” Tighe said. “I don’t think people quite understand the significance – this is really big.”

Overhead shot of two people walking on a path next to a lake that has dead fish piling up at its edges.
The algae bloom coming in from the San Francisco Bay has resulted in mucky water and thousands of dead fish carcasses. Photograph: Justin Sullivan/Getty Images

Researchers are also conducting tests to investigate what might have caused the bloom. Effluent water from wastewater treatment plants is likely to be blamed, according to Rosenfield.

“Researchers have known for decades that the levels in this estuary are really high,” he said. “In fact, many yet why this hasn’t happened.” Existing wastewater systems were not designed to filter out the phosphorus and nitrogen that leads to such red tide algae blooms, he added. “This is a problem society as a whole has ignored for too long.”

Officials have suggested that decreased water flow from estuaries to the ocean, due to the statewide drought, may also have contributed.

People in safety gear, orange vests and hard hats stand in a lake to scoop out dead fish into red plastic bags.
Cleanup workers use nets to collect dead fish along the shores of Lake Merritt. Photograph: Nathan Frandino/Reuters

As the Bay Area braces for a brutal heatwave in the coming week, Rosenfield said the warm temperatures could exacerbate the bloom, depending on whether or not it has already passed its peak. The heat could also further cook the carcasses and worsen the stench. Crews in Oakland have begun cleaning up the dead fish in anticipation of hotter temperatures through the holiday weekend.

“I worry about how long this will last and what the longer impacts will be,” said Tighe, noting that many birds feed on the lake’s fish. “This could cause big ripple effects.”

A white bird picks at the dead remains of a fish in a lake.
Experts are worried about the ripple effects of the die-off since many birds feed on the lake’s fish. Photograph: Nathan Frandino/Reuters

In 1870, Lake Merritt became the first protected wildlife refuge in America. In recent years, local officials have been working to restore its water quality and clear it of built-up trash.

The Associated Press contributed to this story

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