Correlation between Metabolic Rate and Salinity Tolerance and Metabolic Response to Salinity in Grass Carp (Ctenopharyngodon idella)

SIMPLE SUMMARY: The association between the metabolic rate and salinity tolerance in stenohaline freshwater fish could affect how fish adapt to changes in environmental salinity. In Experiment I, the metabolic rates and upper salinity tolerance limit of the grass carp were determined individually, and we aimed to test whether an association existed between the salinity tolerance capacity and both the resting metabolic rate and maximum metabolic rate. In Experiment II, the effects of increasing salinity on metabolic rates, gill histology, and Na(+)-K(+)-ATPase activities were determined in grass carp. The results suggest that a lower metabolic rate may not necessarily allow for a better salinity tolerance capacity of grass carp. Salinity-induced changes in the gill surface contribute more to ion exchange capacity than to oxygen uptake capacity. ABSTRACT: The metabolic rate could be one of the factors affecting the salinity tolerance capacity of fish. Experiment I tested whether metabolic rates correlate with the upper salinity tolerance limit among individual grass carp by daily increasing salinity (1 g kg(−1) day(−1)). The feeding dropped sharply as the salinity reached 10 g kg(−1) and ceased when salinities exceeded 11 g kg(−1). The ventilation frequency decreased weakly as salinity increased from 0 to 12 g kg(−1) and then increased rapidly as salinity reached 14 g kg(−1). The fish survived at salinities lower than 14 g kg(−1), and all fish died when salinity reached 17 g kg(−1). The upper salinity tolerance limit was not correlated with metabolic rates. Therefore, a lower metabolic rate may not necessarily allow for better salinity tolerance capacity. Experiment II tested how different salinities (0, 0.375, 0.75, 1.5, 3, and 6 g kg(−1) for 2 weeks) affect the metabolic parameters of grass carp. The changes in the resting metabolic rate with increasing salinity could be explained by the relative changes in interlamellar cell mass and protruding lamellae. The maximum metabolic rate remained constant, suggesting that the salinity-induced changes in the gill surface had a minor effect on oxygen uptake capacity.