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San Francisco Baykeeper: Dying Algal Bloom Leaves Dead Zones in Its Wake [1]

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Date: 2022-09-08

From the San Francisco Baykeeper, September 7, 2022:

SF Baykeeper continues to advise caution against contacting the murky, discolored waters or eating fish and shellfish from affected areas; the current heat wave may promote new harmful algal blooms.

An algal bloom, or “red tide”, that emerged in July and grew to affect wide stretches of the Bay has receded, likely because the bloom temporarily exhausted its nutrient supply. As the bloom died off, other microorganisms feasting on the dead algae rapidly proliferated, drawing down dissolved oxygen in areas of the Bay to levels that are lethal to most fish and shellfish, as well as other invertebrates. We don’t know yet how extensive these “dead zones” are or how long they will last.

Starting in late August, we received increasingly frequent reports of unprecedented numbers of dead fish – including large sturgeon, sharks, rays, and striped bass, as well as masses of smaller fish – in the water and on shorelines, including Point Molate, Keller Beach, Alameda, and Hayward in the East Bay, and the Marin and San Mateo coastlines on the west side of the Bay. Reports of dead fish, and particularly newly killed fish, tapered off significantly by September 2.

A glimmer of hope came on Tuesday, September 6, as the California Department of Fish and Wildlife reported catching healthy anchovies in San Pablo Bay during a regularly scheduled fish survey. Anchovies are more tolerant of low dissolved oxygen than other Bay fishes, so this does not change our initial assessment that the initial fish kill and subsequent formation of dead zones was widespread and catastrophic.

Fish kills may continue to occur while dissolved oxygen remains extremely low, as the bloom decomposes. If you notice an unusually large number of dead or struggling fish near the shoreline, please report it with this observation form from our partners at the San Francisco Estuary Institute (SFEI) and/or Baykeeper’s Pollution Hotline. Crowdsourced reports are our best indicator of when new fish kill events occur. Our scientists have been monitoring the situation and are in communication with relevant agencies and researchers. Both the California Department of Public Health and the National Oceanic and Atmospheric Administration first identified the dominant algal species forming the bloom as Heterosigma akashiwo – a species that, when it forms a bloom has been associated with fish kills in other places. The US Geological Service and SFEI confirmed high densities of H. akashiwo, from around south and central San Francisco Bay and San Pablo Bay. While not generally considered an acute risk to people, Heterosigma akashiwo is a lesser-studied algae that can emit toxins and lead to low dissolved oxygen, harming fish and wildlife. Heterosigma akashiwo is always present in the Bay at background levels, but has not formed a noteworthy bloom since the early 2000’s. Given uncertainty over health impacts to people and pets, we advise caution when considering recreating in the Bay or consuming fish, and particularly shellfish, caught in affected areas. The algae bloom has nearly disappeared from the Bay’s waters at this point. But new blooms are possible until the days get shorter and temperatures cool.

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