Effects of acclimation temperature on the thermal tolerance, hypoxia tolerance and swimming performance of two endangered fish species in China

Temperature is one of the most important environmental factors affecting the physiological activities and, thus, the fitness of fish, and physiological studies can help predict the effects of climate change on fish species in the field. The aim of this study was to investigate the effect of acclimation temperature on the thermal tolerance, hypoxia tolerance and swimming ability of two endangered fish species in the upper reach of the Yangtze River, namely, the Chinese sucker (Myxocyprinus asiaticus) and rock carp (Procypris rabaudi). The fish were acclimated at either 15 °C or 25 °C for a 3-week period. Then, thermal tolerance as indicated by the critical thermal maximum (CTmax) and critical thermal minimum (CTmin), hypoxia tolerance as indicated by the aquatic surface respiration (ASR50) and loss of equilibrium (LOE50), swimming performance as indicated by the critical swimming speed (Ucrit), aerobic capacity as indicated by the maximum metabolic rate and aerobic scope were measured. As expected, the thermal indicators of both species increased with temperature, and their values at both acclimation temperatures were similar to those of fish living in the Yangtze River. However, both species showed poor hypoxia tolerance compared to most fish species in the Yangtze River, according to previous studies. In particular, Chinese sucker acclimated at a low temperature exhibited an unusually strong decrease in hypoxia tolerance with decreasing temperature (fish usually showed high hypoxia tolerance due to decreased oxygen demand and high environmental oxygen tension at low temperature). Furthermore, Chinese sucker exhibited poorer swimming performance than rock carp (which is also a relatively poor swimmer among the fish species in the Yangtze River) when maintained at a high temperature due to low swimming efficiency, possibly as a consequence of its deep body shape. The difference in Ucrit was magnified at low temperature due to the more profound decrease in metabolic scope in Chinese sucker than in rock carp (55% vs 20%), but Chinese sucker showed a higher resting metabolic rate than rock carp at a low temperature, which is difficult to explain. This result suggested that low hypoxia tolerance and poor swimming performance due to the low cardiorespiratory capacity and (or) non-streamlined body shape of both fish species, especially Chinese sucker, reared at low temperature might be two of the reasons why they are not well adjusted to the change in their natural habitat and have thus declined in recent decades. The underlying physiological and biochemical mechanisms involved in the unusual adjustment of the physiological function of Chinese sucker and its ecological relevance must be investigated further. The present study provides a good example of a physiological investigation yielding very interesting and useful data for species conservation in a changing world.