Michael Lydy, professor of zoology, and Kara Huff Hartz, senior scientist at SIU’s Center for Fisheries, Aquaculture and Aquatic Sciences, work with salmon in a laboratory. They are cooperating with the California Department of Fish and Wildlife and the University of California-Davis on a project examining the specific sublethal effects that varying amounts and types of pesticides have on the fish’s natural predatory-avoidance behaviors. (Photo by Russell Bailey)
June 23, 2025
SIU scientists examine how pesticides may impair Chinook salmon
CARBONDALE, Ill. – Scientists at Southern Illinois University Carbondale are making an iconic but troubled fish’s life easier, looking at how pesticides may impact Chinook salmon’s ability to avoid predators while migrating to the ocean.
Michael Lydy, professor of zoology, and Kara Huff Hartz, senior scientist at SIU’s Center for Fisheries, Aquaculture and Aquatic Sciences, are cooperating with the California Department of Fish and Wildlife and the University of California-Davis on the project. They are examining the specific sublethal effects that varying amounts and types of pesticides have on the fish’s natural predatory-avoidance behaviors.
The goal is to create a tool called a “response spectrum model” that predicts the various effects associated with exposure to pesticides among the fish. It would give wildlife managers the chance to use residue data to assess the sublethal effects of such exposures in field-caught fish and then take various corrective actions designed to help them survive.
A troubled, charismatic species
The Chinook salmon, a fish of legendary aesthetic and biological significance in the western United States, have led a troubled existence for decades due to various environmental factors including loss of habitat, pollution and hydroelectric dams that interfere with their spawning activity. Pesticides, both current and from the past, exist in the water and sediments in the Sacramento-San Joaquin Delta, which is used by juvenile salmon raised nearby, posing a substantial risk and playing a role in their falling numbers.
“We know pesticides are present in these fish and their invertebrate prey in the delta, but the nature and severity of those effects are unknown,” Lydy said. “We are currently finishing the models from the project, which will help us assess spatial and temporal differences in impacts of pesticide mixtures on juvenile salmon as they out-migrate to the ocean.”
Quantifying the threat
Wildlife managers for decades have tried to address the struggling population, building infrastructure such fish ladders that help it get around dams to spawn and other environmental regulations. California also has an aggressive hatchery effort to breed more and more juvenile salmon before releasing them to the wild.
Once released, the salmon spend up to a year in nearby freshwater rivers before heading out to the ocean for up to four years. Along the way, salmon also face natural predators, such as striped bass, largemouth bass, and sunfishes.
The salmon, now of adult size, eventually make the arduous journey back to their native fresh waters to spawn and then die.
But things have gone off track somewhere.
“One problem is that natural population numbers have been dwindling for years, even as the state has been supplementing population,” Lydy said. “California is raising and releasing millions and millions of salmon, but the number returning to spawn is a fractional amount. We’re trying to help turn that around by developing this tool.”
SIU’s portion of the two-year CDFW-funded $290,000 study specifically is aimed at assessing the detrimental effects of pesticide exposure on growth, olfactory function, behavior and swimming performance, and cardio-respiratory function of Chinook salmon, all of which play important roles in helping them avoid predators. Even low levels of pesticides can therefore be an important driver in the falling populations, Lydy said.
“Pesticide exposure may increase fish mortality risk from direct toxicity, but even at low concentrations, pesticides are known to cause a variety of sublethal impacts on fish,” he said.
Laboratory exposures have shown pesticide exposure negatively impacts overall behavior, Lydy said. Swimming performance and schooling behavior impacts, for instance, would make migrating young salmon more susceptible to being eaten. In addition, olfactory (sense of smell) response effects could raise predation risks while also hurting their ability to imprint odorant cues used later in life to navigate back to their natal location.
“We have found, for instance, that olfactory response can be affected by very low concentrations,” Lydy said.
An accumulating problem
Pesticide residues commonly bioaccumulate in fish. Although state and federal agencies monitor this effect, the potential impacts of that growing burden on a fish’s body remain largely unknown.
Traditionally, scientists have evaluated the risks of such effects based on chemical concentrations in external waters, rather than the potential for bioaccumulation in fish tissues. The internal body concentrations of such chemicals, however, have long been recognized as a more appropriate dose metric for toxicological assessments, Lydy said.
“The relationships between body residues and biological effects, including sublethal effects, are critically important for determining the role pesticides may be having on salmon declines,” Lydy said.
At what point…?
To find the answers, the scientists are performing a series of lab studies exclusively using fish 3-13 inches long that were raised in hatcheries looking at several critical areas of uncertainty. Those include thresholds of pesticide loadings and pesticide mixtures in salmon tissues that result in sublethal impairments.
This information will permit a greater understanding of residue-response relationships, Lydy said, which will be essential in assessing the relative risks of pesticide exposure to the regional salmon population.
“Response spectrum models are needed because they address a critical need related to the impacts of pesticides on Chinook salmon populations, allowing for direct comparisons of data obtained from routine monitoring activities to a residue-effects framework, “Lydy said. “It also will guide future management decisions regarding the effects of pesticides on sensitive salmonid species.”
Armed with such knowledge, state wildlife managers have options in managing the paths of migrating fish through dams and water rerouting to avoid certain types of chemicals.