This study explored the effects of a sublethal 96-h dehydroabietic acid (DHAA) exposure on aspects of the immune system of juvenile coho salmon under varying temperature conditions. Coho were exposed to DHAA concentrations below the determined LC50 value of 0.94 mg/L (95% confidence limits of 0.81 to 1.24 mg/L) for 96 h at either their acclimation temperature (8 or 18°C), or during an acute warm-shock (8 to 18°C) or cold-shock (18 to 8°C). Acclimation temperature alone significantly affected hematocrit (Hct), neutrophil respiratory burst activity (RBA) and leucocyte proportions. With temperature-shock, leucocrit (Lct), RBA and leucocyte proportions were altered. All parameters were affected by DHAA exposure, but not always in a dose-dependent manner. Across groups, DHAA caused Hct, lysozyme, thrombocyte, neutrophil and monocyte proportions to increase, and Lct, RBA and lymphocyte proportions to decrease. DHAA-temperature interactions resulted in the exacerbation of DHAA-induced effects. Exposure temperature had the most significant effect on the susceptibility of coho to Aeromonas salmonicida; fish were more susceptible at cold temperatures and when subjected to a temperature-shock compared to their respective controls. DHAA exposure modulated the response of temperature-shocked fish to this pathogen.

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