Sublethal exposures to bifenthrin impact stress responses and behavior of juvenile Chinook salmon

Juvenile Chinook salmon (Oncorhynchus tshawytscha) populations have decreased substantially in the Sacramento-San Joaquin (California, USA) Delta (Delta) over the past decades, so considerably that two of the four genetically distinct runs are now listed in the Endangered Species Act. One factor responsible for this decline is the presence of contaminants in the Delta. Insecticides, used globally in agricultural, industrial, and household settings, have the potential to contaminate nearby aquatic systems through spray drift, runoff, and direct wastewater discharge. Chinook salmon are therefore exposed, as they out-migrate through the Delta, to insecticides that have been associated with adverse biological effects in aquatic species, ranging from sublethal impairments to lethality. The goal of this study was to assess whether bifenthrin, a ubiquitous pyrethroid insecticide in the Delta, affects thermal tolerance, hypoxia tolerance, and behavior of juvenile Chinook salmon. Fish were exposed for 10 days to environmentally relevant (125 ng/L, associated with resulting body residues in wild-caught fish) and sublethal bifenthrin concentrations (500 and 1,000 ng/L). Juvenile Chinook salmon exposed to bifenthrin were tolerant to increases in hypoxia but not temperature. Fish exposed to bifenthrin showed dose-dependent behavior changes: hypoactivity at 125 ng/L, hyperactivity at 1,000 ng/L, and reduced anxiety-like behavior, including lower thigmotaxis and decreased social interaction. The results revealed that exposure to sublethal concentrations of bifenthrin, leading to environmentally relevant body burden residues, significantly altered upper thermal tolerance and caused nonlinear behavioral changes. The study suggests the existence of behavioral effect thresholds in wild-caught fish and emphasizes that higher concentrations of contaminants may impair Chinook salmon's ability to avoid predators in natural environments.