Factors influencing mortality estimates in post-release survival studies

Campana et al. (2009; Mar Ecol Prog Ser 387:241-253) explored the survival of blue sharks Prionace glauca captured and released from the North Atlantic commercial longline fishery. We think that their comments and comparisons do not accurately reflect a previous survival study of blue sharks in Hawaii (Moyes et al. 2006; Trans Am Fish Soc 135:1389-1397). The differences in mortality between the studies, about 5% in the Hawaii-based fishery and about 35% in the North Atlantic fishery, were suggested to be due to failure of Moyes et al. (2006) to accurately reflect commercial fishing conditions. Careful examination of the data, however, suggests that the mortality depends on fishery-specific features - hook type, soak time and handling of the bycatch during release - rather than the respective tagging protocols. Survival studies based on pop-up satellite archival tags (PSATs) are cost-prohibitive; alternative (e.g. biochemical) approaches to estimate stress and morbidity are needed to supplant PSAT studies and to increase sample sizes. Standardization of fishing methods and sampling protocol is needed for future survival studies, to reduce experimental bias and improve the cost:benefit relationship.