Evaluation of the use of alternative biomarkers as indicators of post-release mortality in Blacktip Sharks (Carcharhinus limbatus)

The release of sharks caught in recreational fisheries or as by-catch in non-target commercial fisheries is generally regarded as a sustainable fishing approach. However, post-release mortality can occur in these fish due to physiological damage sustained during capture. It is important to determine the amount of mortality specifically attributed to capture so losses can be accounted for in population management practices. Previous studies have used electronic tagging and/or measurement of secondary stress indicators in plasma (e.g., pH, levels of lactate, glucose, pCO2) to estimate rates of post-release mortality. These methods may not always be the best approaches, as electronic tagging can be costly, and some secondary stress markers may not consistently correlate with survivorship. Consequently, there has been a call for alternative indicators of post-release mortality in elasmobranchs. To address this, the present study evaluated several possible indicators of post-release mortality in plasma from blacktip sharks (Carcharhinus limbatus) captured via commercial and recreational methods, using individuals for which survivorship outcomes were confirmed via electronic tagging. These indicators included biomarkers of oxidative stress-induced macromolecule damage such as lipid peroxidation (malondialdehyde), protein carbonylation (PCs) and DNA oxidation (8-hydroxy-2’-deoxyguanosine); alternative measures of stress-induced energy mobilization (ketone bodies); indicators of rhabdomyolysis (myoglobin and creatine kinase); and general cell damage (cell-free DNA). Data suggested that 8-OHdG may increase with mortality in certain circumstances, but levels of plasma cell-free DNA and MDA did not differ with survivorship. Assays for PCs and indicators of rhabdomyolysis could not be validated. Concentrations of ketone bodies appeared to correlate with secondary stress indicators, but not with survivorship. This study demonstrates that 8-OHdG may be a useful biomarker when assessing mortality in prolonged stressful situations but not in shorter capture encounters. There is still a need for the exploration of additional biomarkers to determine a reliable predictive indicator of post-release mortality