RESUMO
In recent years, Arctic char populations in Iceland have declined and the objective of this experiment was to throw further light on these changes by examining the effect of temperature (5, 9, 13, 17, and 21 °C) on the survival, growth rate, metabolism, and physiological indices of juvenile Arctic charr (initial mean body mass 4.02 ± 0.8 g). Mortality was 60% at 21 °C while at lower temperatures it was below 5%. However, Arctic charr populations in Iceland are declining in locations where the ambient temperature is lower, suggesting that other factors may be more important in determining the abundance of the species. The optimum temperature for growth was near 14 °C. The growth rate was progressively reduced at supra-optimum temperatures with almost no growth at 21 °C. Indicators of energy reserves: condition factor, relative intestinal mass, and hepatosomatic index are all consistent with reduced feed intake at supra-optimum temperatures. The standard and maximum metabolic rate (SMR; MMR), as well as the aerobic scope for activity (AS), were maximum at 13 °C. The routine metabolic rate (RMR) increased exponentially with temperature and, at T21, it was equal to the MMR suggesting, that the RMR was limited by the MMR. Moreover, increased heart- and gill mass at 21 °C are consistent with increased stress on the cardiovascular system. These findings are in keeping with the OCLTT hypothesis that the thermal tolerance of fish is limited by the capacity of the cardiovascular system to deliver oxygen and support metabolism. Taken together, the results of this experiment suggest, that growth rate is reduced at supra-optimum temperatures because of reduced energy intake, increased metabolic demand, and limitations in the capacity of the cardiovascular system to support metabolic rate at high temperatures. At lower temperatures, growth does not appear to be limited by the AS.
