Environmentally relevant body residues of current-use and legacy pesticide mixtures induce behavioral changes in Juvenile Chinook Salmon.

Pesticides are widely applied in agricultural, industrial, and residential settings, often reaching nearby aquatic ecosystems through runoff and spray drift, particularly during storm events. The Sacramento-San Joaquin Delta (Delta) in California is significantly impacted, with multiple pesticides being frequently detected in surface waters and sediments. These pesticides are associated with various adverse effects on aquatic species, from sublethal impairments to lethal outcomes. Juvenile Chinook Salmon (Oncorhynchus tshawytscha), which migrate downstream through these contaminated waterways, are particularly vulnerable. This study investigated how varying body residue concentrations of two pesticides commonly detected in the Delta (bifenthrin and fipronil) and 4,4'-dichlorodiphenyldichloroethylene (DDE), a ubiquitous degradation product of the legacy pesticide DDT, affect the behavior of juvenile Chinook Salmon. Juvenile Chinook Salmon were exposed for 9 days to four DDE treatments (control, 0.45, 4, and 40 μg/L) or four pesticide-mixture treatments: control; low (0.125 μg/L bifenthrin, 4 μg/L fipronil); high (0.250 μg/L bifenthrin, 8 μg/L fipronil); and high + DDE (0.250 μg/L bifenthrin, 8 μg/L fipronil, 0.45 μg/L DDE). The concentrations were selected to achieve body residues reflective of those measured in wild-caught fish. DDE exposure increased light response and shoaling, whereas pesticide mixtures had the opposite effect, reducing shoaling. At low concentrations, the mixture induced hyperactivity with greater movement and swimming speed, at high concentrations and in high + DDE treatments, fish became hypoactive. Notably, DDE enhanced the toxicity of the bifenthrin-fipronil mixture. These behavioral disruptions likely compromise migratory success in juvenile Chinook Salmon by increasing vulnerability to predation and impairing feeding.