Fish kill events across the Bay likely related to persistent, widespread “red tide” algal bloom
Responding to reports to Baykeeper’s pollution hotline of large numbers of dead or struggling fish, our science team investigated beaches along the eastern San Mateo shoreline earlier last week (Tuesday and Wednesday). They found dozens of dead striped bass, the carcass of an adult white sturgeon (picture available, Courtesy: San Francisco Baykeeper), and the carcass of an unidentified species of shark (Picture available, Courtesy San Francisco Baykeeper). On Thursday (8/25) we received a report from researchers with the US Geological Survey who encountered dozens of dead fish floating in the Bay south of the Hayward-San Mateo bridge.
Over the weekend, evidence that San Francisco Bay experienced a substantial fish mortality event (commonly known as a “fish kill”) increased exponentially. We received numerous reports of dead fish – large sturgeon, sharks, and striped bass, as well as masses of smaller fish – in the water and on shorelines around the Bay, including Pt. Molate, Keller Beach, Alameda, and Hayward in the East Bay, and Sausalito and Foster City on the west side of the Bay. On Sunday, uncountable numbers of dead fish were observed in Oakland’s Lake Merritt. The fish kill appears to be ongoing at different locations around the Bay; there is no way to know when it will end.
The number of large dead fish reported to Baykeeper in such a short time is unprecedented. The fish kill is likely caused by a “red tide” algal bloom that we’ve been tracking for over a month now. This harmful algal bloom, caused by an organism known as Heterosigma akashiwo was first detected in the Alameda estuary, but has since expanded to discolor vast expanses of South, Central, and San Pablo Bays. Though it is not known to be acutely toxic to humans, H. akashiwo can release toxins that kill fish. Blooms of this algae have also been known to deplete dissolved oxygen in the water and this also causes fish kills.
Baykeeper Senior Scientist, Jon Rosenfield said: “Fish kills of this size are unheard of in San Francisco Bay. But they’re not unexpected following on the heels of a red tide algal bloom that is unprecedented in its extent and duration. Based on decades of agency and academic research, Baykeeper has long warned that the Bay was susceptible to a bloom like this because of high loads of nitrogen and phosphorous that are dumped into the Bay every day by the 40 wastewater treatment plants that surround it.
“Prevention is the only cure for a harmful algal bloom. Now we’re seeing the unfortunate outcome of the Bay Regional Water Quality Control Board’s inaction. The Regional Water Board needs to develop more stringent requirements for wastewater treatment plants, to limit excessive nutrient inputs to San Francisco Bay.” Rosenfield added.
While not generally considered an acute risk to people, a bloom of this magnitude can cause irritation of skin or airways – people and pets are advised to avoid prolonged contact with affected water. H. akashiwo is always present in the Bay at background levels, but we have never observed a bloom of this magnitude in the Bay before. The algae will die off eventually, as the days get shorter, temperatures cool off, and/or the bloom exhausts its supply of nutrients.
Pictured: a dead sturgeon along the Bay. Photo by Baykeeper scientist Ian Wren.