Post-Release Mortality of Deep Sea Bycatch Species

Deep-sea organisms are increasingly subject to bycatch interactions worldwide. Recent studies have shown that discard mortality can lead to significant declines in deep sea fish stocks, and highlight the inherent vulnerability of deep sea organisms to overexploitation due to their shared suite of conservative life history characteristics. Estimating the post-release mortality (PRM) rates of these deep-sea organisms is a necessary step towards responsible fisheries management, particularly as PRM represents a substantial source of uncertainty when estimating total fishery mortality. The deep-sea giant isopod Bathynomus giganteus and its relatives are captured as bycatch in numerous fisheries, although knowledge is limited regarding their population trends or response to capture and release. In order to assess and predict PRM in B. giganteus, we used reflex action mortality predictors (RAMP) whereby the presence or absence of target reflexes was used to create a delayed mortality model, and considered factors affecting mortality. Mortality rates five days post-capture ranged from 50-100% and both RAMP scores and time at the surface were significant predictors of mortality, although our conclusions regarding the effect of surface time are limited. In-cage video documented little movement within the 24 h monitoring period following cage deployment, and it appeared that surviving individuals often fed within the holding period after cage deployment. Our results suggest that PRM in B. giganteus is common and that this unaccounted source of mortality should be quantified and investigated for other deep-sea crustaceans as well. Similarly, bycatch interactions with deep-sea elasmobranchs can lead to dramatic declines in abundance over short time scales. Sharks hooked in the deep sea could face a higher likelihood of severe physiological disturbance, at-vessel mortality, and PRM than their shallower counterparts. Unfortunately, robust PRM rates have not yet been estimated for deep-sea elasmobranchs and as such are not currently incorporated into total fishery mortality estimates or bycatch assessments, limiting the effectiveness of conservation or management initiatives. We empirically estimated PRM for two focal species of deep-sea shark, the Cuban dogfish Squalus cubensis and the gulper shark Centrophorus sp. using post-release cages deployed at-depth. We calculated 24 h PRM rates of 49.7% (± 8.5 SE) for S. cubensis and 83% (± 16 SE) for Centrophorus sp. and identified shark size (total length), blood lactate, blood pH, and vitality scores as predictors of PRM in Squalus cubensis. We also observed all PRM within 11 h post-capture and demonstrated the effects of capture and recovery depth on stress and behavior. Our results suggest that PRM rates of deep-sea sharks are higher than previously assumed, and highlight the need for filling in this gap in fishery mortality estimates for other common deep-sea discards in the future.