The Journal of Experimental Biology 244 © 2014. Published by The Company of Biologists Ltd | The Journal of Experimental Biology (2014) 217, 244-251 doi:10.1242/jeb.089755 ABSTRACT 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°C and acutely exposed to temperatures from 23 to 38°C, (2) allowing the fish to behaviourally select a preferred temperature between 29 and 38°C, and (3) quantifying alterations to AS after 5 weeks of acclimation to 29 and 38°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°C, a temperature approaching the upper lethal limit (40–41°C). Despite 38°C eliciting maximum AS, when given the opportunity the fish selected a median temperature of 31.7±0.5°C and spent only 10±3% of their time at temperatures >36°C. Following acclimation to 38°C, AS measured at 38°C was decreased to the same level as 29°C-acclimated fish measured at 29°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. KEY WORDS: Barramundi, Climate change, Lates calcarifer, Metabolic rate, Oxygen- and capacity-limited thermal tolerance (OCLTT), Oxygen consumption rate INTRODUCTION The progressive warming of aquatic ecosystems is of critical conservation concern for fish as it has been associated with shifts in phenology, distribution and abundance (e.g. Perry et al., 2005; Pörtner and Knust, 2007; Martins et al., 2011), yet the temperature- sensitive mechanisms driving these phenomena remain speculative. A leading hypothesis is that these population-level changes result from a decrease in aerobic metabolic performance of fish with increasing temperature, caused by a gradual decline in the capacity of the ventilatory and circulatory systems to deliver oxygen to the respiring tissues (i.e. oxygen- and capacity-limited thermal tolerance, OCLTT) (Pörtner and Knust, 2007; Pörtner and Farrell, RESEARCH ARTICLE 1 Australian Institute of Marine Science, PMB 3, Townsville MC, Queensland 4810, Australia. 2 Zoophysiology, Department of Bioscience, Aarhus University, DK-8000 Aarhus C, Denmark. *Author for correspondence (tommy.norin@biology.au.dk) Received 30 April 2013; Accepted 18 September 2013 2008; Martins et al., 2011). The mismatch between oxygen supply and demand manifests as a decrease in aerobic scope (the degree to which oxygen consumption rate, Ṁ O2 , can be increased above resting levels) at either side of an optimal temperature for aerobic scope (T opt,AS ) set by lower and upper pejus temperatures (T p ) (Pörtner and Farrell, 2008). Consequently, overall animal performance across temperature is proposed to resemble a bell- shaped curve with a peak at T opt,AS , the temperature where critical fitness-related factors such as locomotion, growth and reproduction are thought to be optimised (Frederich and Pörtner, 2000; Pörtner, 2002; Wang and Overgaard, 2007; Pörtner and Farrell, 2008; Pörtner, 2010; Pörtner, 2012). This bell-shaped performance curve suggested for aquatic ectotherms contrasts with thermal performance curves suggested for many terrestrial ectotherms that are highly left- skewed (i.e. right-biased) (Gilchrist, 1995; Martin and Huey, 2008). The foundation for much of the OCLTT concept stems from studies of species inhabiting open oceans (Frederich and Pörtner, 2000; Pörtner and Knust, 2007) where temperature fluctuations are dampened by the water masses and warming is gradual. In contrast, the temperature of coastal and estuarine waters can vary markedly over short temporal scales of hours to days, which is likely to be exacerbated in the event of future increases in storm intensity and daily temperature extremes (Trenberth, 2012). Because these acute environmental changes have the capacity to act rapidly and at the individual level without the opportunity for prior acclimation, coastal species that live in dynamic environments provide interesting models for investigating the effects of acute thermal exposure on animal performance and the OCLTT concept. Even so, the degree to which acute versus chronic thermal exposure influences the aerobic performance of fish in general is not well understood and requires significantly more attention. To shed light on these knowledge gaps, the present study used juvenile barramundi (Lates calcarifer Bloch 1790) from central- eastern Queensland, Australia, as a model organism to examine aerobic performance and thermal preferences in the framework of the OCLTT hypothesis. In tropical Australia, juvenile barramundi inhabit estuaries, coastal swamps and tidal creeks (Pusey et al., 2004) where temperatures can range from 23 to 36°C during summer months (Russell and Garrett, 1985; Loong et al., 2005). Here, fluctuations in water temperature of 10°C or more occur at a rate of up to 2°C h −1 (Loong et al., 2005), which is likely to intensify if predictions of more extreme climatic events hold true (Lima and Wethey, 2012; Trenberth, 2012). Recent studies on juvenile barramundi across a temperature range of 21–39°C suggest that growth performance is optimal around 31°C (Katersky and Carter, 2007; Bermudes et al., 2010; Glencross and Bermudes, 2012) and remains high (≥90% of maximum) over a wide range of temperatures (~27–36°C) before declining precipitously at the extremes (Katersky and Carter, 2007; Bermudes et al., 2010). In addition, locomotor capacity (critical swimming speed, U crit ) in juvenile barramundi from Australia is reported to be thermally Aerobic scope does not predict the performance of a tropical eurythermal fish at elevated temperatures Tommy Norin 1,2, *, Hans Malte 2 and Timothy D. Clark 1