Dead tired: evaluating the physiological status and survival of neonatal reef sharks under stress

Abstract

Marine protected areas can protect sharks from targeted fisheries, but may not manage other stressors. We demonstrated species-specific physiological responses of newborn reef sharks upon brief gill-net capture during summer months. Carcharhinus melanopterus and Negaprion acutidens were resilient to stress within a narrow temperature range and under ideal capture conditions.

Abstract

Marine protected areas (MPAs) can protect shark populations from targeted fisheries, but resident shark populations may remain exposed to stressors like capture as bycatch and environmental change. Populations of young sharks that rely on shallow coastal habitats, e.g. as nursery areas, may be at risk of experiencing these stressors. The purpose of this study was to characterize various components of the physiological stress response of neonatal reef sharks following exposure to an exhaustive challenge under relevant environmental conditions. To accomplish this, we monitored markers of the secondary stress response and measured oxygen uptake rates ( $\dot{\mathit{M}}{\mathit{O}}_{\mathbf{2}}$ ) to compare to laboratory-derived baseline values in neonatal blacktip reef ( Carcharhinus melanopterus) and sicklefin lemon sharks ( Negaprion acutidens). Measurements occurred over three hours following exposure to an exhaustive challenge (gill-net capture with air exposure). Blood lactate concentrations and pH deviated from baseline values at the 3-h sample, indicating that both species were still stressed 3 h after capture. Evidence of a temperature effect on physiological status of either species was equivocal over 28–31°C. However, aspects of the physiological response were species-specific; N. acutidens exhibited a larger difference in blood pH relative to baseline values than C. melanopterus, possibly owing to higher minimum $\dot{\mathit{M}}{\mathit{O}}_{\mathbf{2}}$ . Neither species experienced immediate mortality during the exhaustive challenge; although, single instances of delayed mortality were documented for each species. Energetic costs and recovery times could be extrapolated for C. melanopterus via respirometry; sharks were estimated to expend 9.9 kJ kg ⁻¹ (15% of energy expended on daily swimming) for a single challenge and could require 8.4 h to recover. These data suggest that neonatal C. melanopterus and N. acutidens are resilient to brief gill-net capture durations, but this was under a narrow temperature range. Defining species’ vulnerability to stressors is important for understanding the efficacy of shark conservation tools, including MPAs.