Feeding, Growth, and Fat Deposition by Brown Trout in Constant and Fluctuating Temperatures

Hatchery brown trout Salmo trutta intended for stocking into Lake Michigan were exposed for 57 days to three temperature regimes: (1) a diel temperature cycle (9–18 C; mean 12.5 C; amplitude 4.6 C) simulating that selected by brown trout residing in thermal effluent areas in spring; (2) a constant 13 C, the optimum constant temperature for brown trout growth; and (3) naturally fluctuating, arrhythmic temperatures of inshore Lake Michigan waters (4–11 C; mean 7.7 C). Fish in each treatment were fed to satiation twice a day with alewife Alosa pseudoharengus. Food consumption and growth rates were nearly identical in the constant 13 C and the ambient-temperature regimes; they were significantly greater in the thermocyclic regime than in the other two regimes. Gross conversion efficiencies were not significantly different among treatments, indicating little, if any, energetic penalty for exposure to large or small diel temperature fluctuations. Brown trout exposed to a constant 13 C accumulated less total lipid (whole body) and deposited a lower proportion of lipids in muscle than fish in the fluctuating temperature regimes. Apparent acclimation temperatures of brown trout in the cyclic and the ambient regimes, based on expected food consumption and growth at constant temperatures, were at or above the maximum temperature of each exposure range.