Aerobic scope does not predict the performance of a tropical eurythermal fish at elevated temperatures

Climate warming is predicted to negatively impact fish populations through impairment of oxygen transport systems when temperatures exceed those which are optimal for aerobic scope (AS). This concept of oxygen-and capacity-limited thermal tolerance (OCLTT) is rapidly gaining popularity within climate change research and has been applied to several fish species. Here, we evaluated the relevance of aerobic performance of juvenile barramundi (Lates calcarifer) in the context of thermal preference and tolerance by (1) measuring standard and maximum metabolic rates (SMR and MMR, respectively) and AS of fish acclimated to 29 degrees C and acutely exposed to temperatures from 23 to 38 degrees C, (2) allowing the fish to behaviourally select a preferred temperature between 29 and 38 degrees C, and (3) quantifying alterations to AS after 5 weeks of acclimation to 29 and 38 degrees C. SMR and MMR both increased continuously with temperature in acutely exposed fish, but the increase was greater for MMR such that AS was highest at 38 degrees C, a temperature approaching the upper lethal limit (40-41 degrees C). Despite 38 degrees C eliciting maximum AS, when given the opportunity the fish selected a median temperature of 31.7 +/- 0.5 degrees C and spent only 10 +/- 3% of their time at temperatures > 36 degrees C. Following acclimation to 38 degrees C, AS measured at 38 degrees C was decreased to the same level as 29 degrees C-acclimated fish measured at 29 degrees C, suggesting that AS may be dynamically modulated independent of temperature to accommodate the requirements of daily life. Together, these results reveal limited power of the OCLTT hypothesis in predicting optimal temperatures and effects of climate warming on juvenile barramundi.