The thermal acclimation rate varied among physiological functions and temperature regimes in a common cyprinid fish

The acclimation rate of physiological functions that bear fitness costs under a changing environment is key to making predictions for fish, yet this has seldom been investigated. This study aimed to test whether there were different responses when different physiological functions reached steady states and to determine possible relationships with metabolic constraints during temperature changes in fish. Qingbo (Spinibarbus sinensis), a common cyprinid in southern China, was first pre-acclimated at either high (25 °C) or low (15 °C) temperature for eight weeks and assumed to be at thermal equilibrium. Then, the temperature was reversed, that is, the temperature was decreased from 25 °C to 15 °C in one group, defined as the cooling group, whereas the temperature of the other group was increased from 15 °C to 25 °C, defined as the warming group. The thermal tolerance, swimming performance and feeding metabolism were measured before temperature reversal as well as 1, 2, 4 and 8 weeks. The growth performance was also measured after the temperature reversal. Typical thermal compensation was suggested by a stronger heat tolerance, faster swimming speed and higher postprandial metabolic response in those acclimated at 25 °C than in those acclimated at 15 °C before temperature reversal. Cold tolerance suggested by the critical minimal temperature (CTmin) of the cooling group reached a thermal steady state within one week, whereas the adjustment of heat tolerance to the temperature increase indicated by the critical maximum temperature (CTmax) was much slower, reaching a steady state two weeks after the temperature reversal. The high flexibility of the standard metabolic rate as well as the postprandial metabolism and relatively lower feeding metabolism demonstrated that there were negligible effects at the aerobic scope for other physiological functions during digestion in qingbo, even during the temperature acclimation period. However, there was a slower adjustment of the swimming performance with the temperature decrease, which parallels the change of metabolic capacity. Furthermore, both the cooling and warming treatments had negative effects on growth performance, with the former mainly occurring through extra energy expenditure, whereas the latter mainly occurred through decreased feeding. It suggested that the slow adjustment of heat tolerance after the temperature increase compared to that of cold tolerance might be a challenge for cyprinids distributed in southern China. Furthermore, the decreased metabolic scope and hence swimming performance after the temperature decrease might have a fitness cost in qingbo and possibly other cyprinids in China.