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Pesticide Impact on Bay Delta Fish Could Be Greater Than Realized

When combined, pesticides at sublethal levels can have deadly synergistic effects on fish – and new monitoring in the Bay Delta shows that the water is a soup of urban and agricultural insecticides, herbicides and fungicides.

Written by Robin Meadows Published on Read time Approx. 3 minutes
A juvenile Delta smelt inside a rearing tank at the U.C. Davis Fish Conservation and Culture Lab. Scientists are studying the impacts of multiple pesticides on at-risk fish like the Delta smelt.Dale Kolke / California Department of Water Resources

At a recent University of California Davis symposium on multiple stressors in the San Francisco Estuary, toxicologist Tracy Collier called for more monitoring in the Delta. “We don’t know what’s in the water,” he explains. And, as if in answer, hydrologist James Orlando presented early results from a new U.S. Geological Survey effort to help find out.

The mix of toxicants in the Delta likely differs from that in the Bay because much of the land is farmed. “I’ve been surprised to hear how little monitoring there is of agricultural water that is pumped back into the Delta from the islands,” says Collier, a member of the Delta Stewardship Council’s Delta Independent Science Board.

Tracking the Delta’s contaminants is key to protecting smelt, salmon and the other at-risk fish that live there or migrate through. Paying attention to multiple stressors is also critical: When combined, pesticides at sub-lethal levels can have deadly synergistic effects on fish. Add viruses and bacteria to the mix and it’s even worse. “When you expose salmon to chemicals at levels encountered in streams, they don’t die,” Collier says. “But when you put pathogens in the water, they die at a faster rate.”

Factor in climate change and it could be worse yet. “Global warming could increase the toxicity of pesticides in the Delta because rising temperatures make the effects of pathogens more pronounced,” he adds. Pesticides and other chemicals weaken the immune systems of salmon, making them more susceptible to pathogens. And when salmon migrate downstream from freshwater to the salty Delta and the Bay waters, they encounter a “whole new set of pathogens,” Collier says.

Salmon may also encounter a whole new set of pesticides. Inputs within the Delta are little known, however. “The Sacramento River and San Joaquin River have been monitored for years, but historically there has been little monitoring in the Delta itself,” says Michelle Hladik, an environmental chemist who leads the USGS Pesticide Fate Research Group. “We need more information on what is actually occurring in the Delta now.”

As part of the recently launched Delta Regional Monitoring Program, the team is in its second year of checking agricultural and suburban runoff at five sites in the Delta. They’re testing for more than 150 pesticides that are in current use. “We’re monitoring a long list of pesticides including the new or understudied, such as those that have increased in use in the recent past but are not currently in other monitoring programs,” she says.

More than half of the pesticides applied in the Delta watershed are not routinely tracked. “It’s a moving target, there’s always something new coming along,” says the USGS’s Orlando, who is also a member of the Pesticide Fate Research Group. “The Delta watershed has had an average of nine new pesticide active ingredients introduced each year since 1995.”

In a series of recent studies, the team has detected a wide variety of insecticides, herbicides and fungicides in the Delta. Examples include pyrethroids, neurotoxicants that have grown more popular as organophosphate use has waned; fipronil, which is used to control fleas on pets; and neonicotinoids, which are implicated in the honeybee decline. “It’s a soup out there,” Hladik says.

Benchmarks for toxicity to aquatic life are based on the effects of individual compounds. But that doesn’t reflect the reality for fish and other creatures that live in Delta waterways. “You can see 30 different pesticides in a sample – how does that mixture affect living organisms?” Orlando says. “There’s not a lot of information on this because it’s so hard to test. Just testing one compound is hard enough.”

New research suggests that the impact of pesticides on the Delta’s fish could be greater than is recognized. In the December 2016 issue of San Francisco Estuary & Watershed Science, Stephanie Fong and colleagues reported that pyrethroid use correlates with declines in several species of fish in the Delta, including Delta smelt, longfin smelt, Sacramento splittail, American shad, threadfin shad and striped bass.

Interestingly, the authors also found that “pyrethroid use was a more important determinant of abundance variability than flow” for all the species tested except longfin smelt. “We hesitate to say that pesticides are causing fish declines, yet we do say that flow causes declines,” the Delta Independent Science Board’s Collier comments. He’s glad to see the issue of contamination in the Delta is getting more attention: “It’s not really flows or any other stressor individually, it’s the combination and that’s hard to manage.”

This story originally appeared in Estuary News. You can read it here.

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