Overwintering is a critical period for salmonids living in British Columbia, when fish face reduced energy intake. While exposure to current-use pesticides (CUP) may exacerbate the problem of a limited energy budget, increasing over-winter mortalities, there exists little information on the interaction between xenobiotics and overwintering stress in salmonids. To examine this, juvenile rainbow trout (Oncorhynchus mykiss) were exposed to sublethal malathion concentrations in either a fed or fasted treatment group under low water temperatures (10oC). Fed fish displayed enhanced growth (300% mass increase), higher plasma cortisol (4.5±1.1 ng/mL) and elevated energetic indices (liver somatic index (LSI) 0.8±0.01%, muscle lipid 34.4±1.0 mg/g) compared to fasted fish (18% mass decrease, 1.7±1.1 ng/mL plasma cortisol, LSI 0.76±0.02%, muscle lipid 19.6±1.1 mg/g). Sublethal malathion concentrations did not produce significant bioenergetic responses, or elicit a physiological stress response in fed or fasted fish. There appear to be low energy costs associated with malathion defence.
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