Assessing individual physiological variability and future performance phenotypes is essential for predicting the resilience of fish populations to anthropogenic climate change

Changes in ocean temperature are expected to have a considerable effect on fishes through the impact of temperature on physiological performance, vital energetic processes (i.e. metabolism, foraging and swimming style) and reproductive fitness. To understand the sensitivity of an exploited population of <i>Chrysoblephus laticeps</i> in to temperature variability, intermittent-flow respirometry was used to quantify and compare changes in metabolic rate and aerobic scope under different temperatures (10, 16, 21 and 24°C) mimicking thermal variations experienced in the home range of this species. A total performance score was developed to represent aerobic performance across the range of test temperatures. This score was calculated for each temperature from the lower (25%), mid (50%) and upper (75%) percentiles of the aerobic scope range available for the species. The results of this study identified heterogeneity in physiological performance phenotypes amongst individuals of the exploited population. There was significant variation in the aerobic performance of high, intermediate and low performers at higher temperatures. However, differences in performance were not significant at low temperatures, where several intermediate performers maintained high performance. High performers maintained high rates of physiological performance across a broad range of temperatures, whereas low performers were physiologically limited outside of their optimal thermal range. These results suggest that individuals with a broad aerobic scope (i.e. high aerobic scope (AS) values across a range of temperatures) may likely be the most resilient to short-term thermal variability caused by marine heat waves and upwelling events in temperate coastal environments. Since the shape of thermal performance curves differs between individuals and reflects the range at which individuals can function above specified performance thresholds, individual thermal performance must be measured repeatedly in the same individual over a thermal gradient. An understanding of physiological phenotypic diversity amongst individuals is critical to understand the impacts of thermal variability on fished populations.