Behavioural indicators of post-release survival in a deep-sea skate.

Deep-sea skates are among the most frequently bycaught species in Southern Ocean demersal fisheries. They face heightened susceptibility to fishing pressure due to their life-history characteristics. In longline fisheries targeting Patagonian toothfish, skates caught in good condition are released; however, their post-release survival remains uncertain but is expected to be low, given the extreme capture depths (>1000 m). Post-release survival rates are essential to determine an acceptable fishery mortality for sustainable management. During a 2023 Kerguelen Plateau fishing voyage, 24 satellite tags were deployed on Bathyraja irrasa >1056 mm in total length caught at 1200-1600 m depth, for 30 day investigations into post-release survival. Vertical migrations within depth time series indicated that at least six skates survived. Hidden Markov models applied to tag mobility data and summary values of other activity metrics, in the context of topography and currents, were used to determine the fate of skates with no detectable vertical movements, revealing a 26% (95% CI 13-46%) survival rate. The probability of survival decreased with capture depth. Surviving skates underwent extensive non-diel vertical movements. The survival rate is lower than that of other deep-sea skate species with estimated survival rates; which prompts a review of skate bycatch management strategies in deep-set demersal fisheries with high release rates.