Two Chinook salmon swimming in the water
Chinook salmon swimming in the water

Probable Chlorine Exposure Kills 21,000 Fish at UC Davis

Threatened and endangered species were among the dead, likely poisoned overnight by a chlorination system used to decontaminate the animals’ tank water.

Catherine Offord
Catherine Offord

Catherine is a senior editor at The Scientist.

View full profile.


Learn about our editorial policies.

Aug 16, 2022

ABOVE: Chinook salmon (Oncorhynchus tshawytscha) were among the fish killed. © ISTOCK.COM, DAVEALAN

Roughly 21,000 fish have died at a University of California, Davis, research center after apparently being exposed to toxic levels of chlorine released by the tanks’ decontamination system. The fish, which were discovered dead in their tanks at the Center for Aquatic Biology and Aquaculture (CABA) last Tuesday (August 9), included Chinook salmon (Oncorhynchus tshawytscha) and green and white sturgeon (Acipenser medirostris and A. transmontanus), species that are threatened or endangered in some parts of their ranges. Their deaths, experts say, could set back research at the center by years.

“We are devastated to report that a catastrophic failure has resulted in the loss of about 21,000 fish” at CABA, UC Davis said in a statement last Thursday. “The loss appears to be due to chlorine exposure, to which fish are especially sensitive,” it added, noting that an internal investigation and external review were underway to figure out exactly what went wrong.

The center keeps fish for research on sustainable aquaculture and conservation as well as the impacts of disease and environmental change. The tanks had been checked on Monday night, and no problems were identified, Laurie Brignolo, executive director of the Research and Teaching Animal Care Program at UC Davis, tells The New York Times. By morning, all but around 100 of the 21,000 fish kept in the tanks had perished.

The cause of the deaths seems to be an excess of chlorine, which is used to decontaminate the water of pathogens but can damage fishes’ gills and skin, Brignolo tells the Times, although it’s not yet clear how they might have been exposed. One possible explanation is that the system used to cycle water through the tanks became backed up, triggering the system to release dangerous levels of chlorine into tanks, she adds.

Nann Fangue, a physiological ecology professor at UC Davis whose lab was working with the fish, tells The Washington Post that the loss “was a very traumatizing and devastating event for all of us.” She adds: “It’s very, very important for the future of conservation science that we learn from this event and that it never happens again.”

This marks the first time CABA has experienced such a disaster. However, something similar happened at the University of Alberta in 2017, when around 9,000 fish were killed by chlorine exposure, The Washington Post reports. In that instance, the cause turned out to be a power failure that halted two dechlorination pumps, allowing a subsequent influx of chlorinated tap water into fish tanks.

In its statement, UC Davis notes that other aquatic facilities were not affected by the failure, but that it would “evaluate risk” at those facilities in response to what happened at CABA. It adds: “We know that many researchers, regulatory agencies, Native American tribes and other partners trust us to care for their aquatic species. We will work hard to earn that trust by conducting a thorough review of our facilities, holding ourselves accountable for what happened, and taking steps to prevent it from happening ever again.”