| Summary: | The thermal biology of underyearling Lake Inari Arctic Charr can be described by point-estimators of temperatures where minima or maxima of a certain phenomena occur, and by rate-temperature relationships. Thermal effects may be divided into chronic effects, acute effects modulated by acclimation, and the effects of diel temperature fluctuations. The ultimate upper incipient lethal temperature of underyearling Arctic charr was found to be 23-24 °C. Acclimation to steadily increasing temperatures improved the upper thermal tolerance of fish slightly but the scope of tolerance became narrower with increasing acclimation temperature. Maximum feed intake occurred at a temperature below these survival limits (14.4 °C) and the optimum for growth of underyearling Arctic charr was within the range of 13.8-15.1 °C. Growth efficiency in energy and nitrogen reached its maximum at a lower temperature (12.4-13.2 °C) than growth. An acute temperature increase of 3.4-6.7 °C produced an oxygen consumption peak that declined within 3-4 days. Therefore, the energy metabolism of Arctic charr appeared to be adjusted rapidly (within 3-4 days). Similarly, diel thermal fluctuation (±1 °C) increased metabolic rate while ammonia excretion, feed intake, growth and faecal losses were not observed to be influenced by this temperature manipulation. Thermal fluctuation resulted in poorer growth efficiencies than the exposure of fish to a constant temperature.
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