Water samples from the arboretum waterway on Monday (Aug. 11) confirmed oxygen levels in the fatally low range — the most probable explanation for the death of an estimated 100 fish.
The fish were found that morning in the channel at the south edge of the Davis campus. Most were identified as native Sacramento blackfish introduced into the waterway about 15 years ago; non-native carp, sunfish and suckers also died.
The arboretum cleaned up the fish — which went to the campus’s energy-producing biodigester — and called in campus Environmental Health and Safety to test the water.
Out of five samples, the three lowest levels of dissolved oxygen correlated with the locations of the dead fish: 0.17 parts per million at A Street, 1.29 ppm at Mrak Hall Drive and 1.79 ppm at La Rue Road (formerly California Avenue). According to the U.S. Fish and Wildlife Service, most fish do well at 5 ppm, whereas fish become uncomfortable at 4 ppm and will suffocate at 2 ppm.
Sue Fields, environmental manager with Environmental Health and Safety, concluded: “I feel quite confident that the fish kill was due to low O2 levels, not the mosquito pesticide application,” referring to last week’s aerial spraying that targeted mosquitoes.
The spraying, conducted by the Sacramento-Yolo Mosquito and Vector Control District, took place last Wednesday and Thursday nights (Aug. 6 and 7), in the district's battle against West Nile virus.
The campus confirmed the aerial spray constituted 0.0071 parts per million of Trumpet EC, an insecticide said to pose low risk to people and the environment. A lethal concentration for fish would be 2.4 parts per million, according to Andrew Fulks, manager of the UC Davis Putah Creek Riparian Reserve.
The waterway is where Putah Creek once flowed, until it was divered to the south more than 100 years ago to stop it from flooding the town than known as Davisville.
The waterway, dammed at both ends, is a catchment basin for campus runoff. In summer, the slow-moving water is more like a pond — with warm water atop cold water, and with less oxygen at the bottom. The drought we’re now in compounds the problem, lowering the volume of water and making the layers thinner and less habitable.
Then, with warm temperatures and bright sunlight, there is increased algae production, algae die-off and other decaying organic matter — which uses up oxygen in the water, reducing oxygen in the upper layer of water and within the pond overall.
Meanwhile, the campus continues to explore ways to improve the waterway’s health — by aerating the water and aiding its circulation. To do the latter, the campus has tested a system to boost the flow with added water — by pumping treated wastewater into the channel.