Effects of anaerobic exercise accompanying catch-and-release fishing on blood-oxygen affinity of the sandbar shark (Carcharhinus plumbeus, Nardo)

Recovery from anaerobic exercise is thought to be prolonged in elasmobranchs because they lack several mechanisms for maintaining or increasing oxygen delivery that are present in teleosts. For example, teleosts increase hematocrit and maximal blood-oxygen carrying capacity through red cell ejection from the spleen. Teleosts also counteract the reduction in hemoglobin oxygen affinity resulting from metabolic acidosis through an adrenergic-mediated increase in red cell Na+–H+ exchanger activity. To begin to assess the consequences of anaerobic exercise accompanying catch-and-release fishing occurring within the estuarine nursery habitats of juvenile sandbar sharks (Carcharhinus plumbeus, Nardo), we constructed blood-oxygen equilibrium curves using samples from individuals 1 h after capture by hook and line (exercise-stressed) and samples from fully-recovered animals maintained in a shore-side tank (control sharks). We also compared exercise-stressed and control sharks for hemoglobin concentration, hematocrit, red cell count, intracellular pH, and nucleoside triphosphate concentration ([NTP]). In contrast to results from previous studies on elasmobranchs, we found an elevation in both hematocrit (≈ 21%) and blood hemoglobin concentration (≈ 10%) in exercise-stressed sharks. There was also clear evidence of red cell swelling. Mean red cell volume was ≈ 28% higher and mean cell hemoglobin concentration was ≈ 10% lower in exercise-stressed sharks. Most important, in spite of significant metabolic acidosis (0.3 pH units), blood from exercise-stressed sharks had an oxygen affinity equivalent to that of blood from control sharks. This was a direct consequence of intracellular pH being alkalinized by approximately 0.15 pH units relative to plasma pH in exercise-stressed sharks. Our results using isolated hemoglobin solutions showed that the observed reduction (≈ 15%) in intracellular [NTP] also contributed to the leftward shift in the oxygen equilibrium curves. As expected, we found sandbar shark red cells to be unresponsive to exogenous catecholamines. Regardless, sandbar sharks appear able to prevent the decrease in blood-oxygen affinity resulting from anaerobic exercise (and the concomitant decreases in plasma pH), as has been well-documented in teleosts. Our results suggest, therefore, that oxygen delivery following exhaustive exercise is not necessarily compromised in juvenile sandbar sharks, and that hook and line capture and subsequent release do not increase rates of mortality, although both are yet to be directly confirmed.