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Impacts of Hypersaline Conditions on the Biotransformation and Toxicity of the Pesticide Bifenthrin in Salmonid Species of the San Francisco Bay Delta


Pyrethroids are synthetic derivatives of pyrethrin insecticides whose urban usage and continuous municipal wastewater discharge results in "pseudo-persistent" levels in Northern California waterways. Climate change causes warmer global temperatures which diminishes snowfall and reduces freshwater input into the San Francisco Bay Delta (SFBD) resulting in increased salinity over time. The SFBD and its drainage serve as spawning and rearing habitat for salmonid species and is under threat by multiple stressors including pesticide pollution.

Endocrine disrupting effects of bifenthrin at low-level exposures were investigated in laboratory and field experiments. 96-Hour laboratory exposures to bifenthrin (1 ng/L to 10 ug/L) showed no changes in plasma levels of the estrogenic biomarker, vitellogenin (VTG) protein, nor changes of the sex steroids testosterone (T), 11-ketotestosterone (11-KT), and 17beta-estradiol (E2) levels in two salmonid species: Oncorhynchus mykiss and Oncorhynchus tshawytscha. Field exposures using storm water containing bifenthrin, also showed no significant change in VTG or sex steroid levels in both fish species, suggesting short-term low dose exposure to bifenthrin does not result in estrogenic activity in salmonids.

To evaluate the impacts of hypersaline conditions on bifenthrin toxicity, a 14-day laboratory exposure was performed using two populations of O. mykiss acclimated to freshwater, 8 g/L, and 17g/L salinity and then exposed to 0, 0.1, and 1.5 ug/L bifenthrin. The first population (Jess Ranch) was previously treated as embryos with high levels of E2 to obtain uni-gender populations, while the second population (Nimbus Hatchery) was cultured without hormone treatment. Only Jess Ranch fish exhibited significant mortality following exposure to 1.5 ug/L bifenthrin in freshwater. No significant difference was observed in VTG levels in either population. Jess Ranch Fish showed no significant difference in T, 11-KT, or E2 levels. Nimbus fish only showed a significant increase in E2 following exposure to 1.5 ug/L bifenthrin in freshwater. In vitro biotransformation studies using livers were also performed. Saltwater acclimation significantly reduced conversion of bifenthrin to polar metabolites in Jess Ranch fish. This study suggests that embryonic exposure to estrogens can increase susceptibility to bifenthrin in freshwater fish and saline acclimation protects against acute lethality through an unknown mechanism.

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