The power struggle: assessing interacting global change stressors via experimental studies on sharks.

Ocean warming and acidification act concurrently on marine ectotherms with the potential for detrimental, synergistic effects; yet, effects of these stressors remain understudied in large predatory fishes, including sharks. We tested for behavioural and physiological responses of blacktip reef shark (Carcharhinus melanopterus) neonates to climate change relevant changes in temperature (28 and 31 °C) and carbon dioxide partial pressures (pCO; 650 and 1050 µatm) using a fully factorial design. Behavioural assays (lateralisation, activity level) were conducted upon 7-13 days of acclimation, and physiological assays (hypoxia tolerance, oxygen uptake rates, acid-base and haematological status) were conducted upon 14-17 days of acclimation. Haematocrit was higher in sharks acclimated to 31 °C than to 28 °C. Significant treatment effects were also detected for blood lactate and minimum oxygen uptake rate; although, these observations were not supported by adequate statistical power. Inter-individual variability was considerable for all measured traits, except for haematocrit. Moving forward, studies on similarly 'hard-to-study' species may account for large inter-individual variability by increasing replication, testing larger, yet ecologically relevant, differences in temperature and pCO, and reducing measurement error. Robust experimental studies on elasmobranchs are critical to meaningfully assess the threat of global change stressors in these data-deficient species.