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The Effects of Temperature, Salinity, and Bifenthrin on the Behavior and Neuroendocrinology of Juvenile Salmon and Trout

Creative Commons 'BY-ND' version 4.0 license

The San Francisco Bay-Delta in California has been experiencing increasing surface water temperatures, salt water intrusion, and intense runoff events that wash pyrethroids into waterways. Salmonid populations in the Bay-Delta have been declining, including the endangered Steelhead trout (Oncorhynchus mykiss) and Chinook salmon (Oncorhynchus tshawytscha). Previous studies suggest that estrogenic compounds can inhibit smoltification, and the pyrethroid pesticide bifenthrin has been shown to have estrogenic effects. Thus, the objective of my research was to investigate the effects of bifenthrin and climate change stressors on the neuroendocrine response of juvenile salmonids. Trout alevin and fry were exposed to 110C, 16.40C and 190C temperatures and then immediately challenged to sea water (SW) for 24 hours. Trout parr were slowly acclimated to SW and simultaneously exposed to the same temperature regime. Estradiol-17β, cortisol, triiodothyronine, and thyroxine hormone levels were measured in blood serum or whole animal homogenates using ELISAs. Brain growth hormone 1, brain gonadotropin-releasing hormone receptor 2, and gill Na+/K+ ATPase mRNA levels were measured using qPCR. Alevin were more sensitive to salinity than temperature, and fry were more susceptible to increasing temperatures, thus demonstrating stage-dependent responses to climate change stressors. To evaluate the combined effects of bifenthrin and temperature, Chinook alevin, fry, and parr were exposed to the same temperature stressors for 10 days (alevin) or 14 days (fry and parr) and, in the final 96 hours of rearing, fish were exposed to 0, 0.15, or 1.5µg/L bifenthrin. The same endpoints were measured as in the trout, with the addition of quantifying testosterone levels, the expression of brain dopamine receptor 2α, and measuring brain dopamine levels. In addition, a predator avoidance Y-Maze behavioral assay was conducted on parr. Bifenthrin significantly impacted olfactory-mediated predator avoidance behaviors in parr. These results suggest that temperature may be impacting endocrine pathways differently at each juvenile stage because of the developmental timing of neuroendocrine signaling pathways. Together, these studies provide insight to how multiple stressors and sublethal pesticide exposure may have population-level impacts on Bay-Delta salmonid populations.

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