Post-release behaviour, physiological stress and survival of longline-caught Greenland sharks

Incidental capture of sharks by fisheries is common, making it critical to assess species' physiological and behavioural stress responses and post-release survival. Previous studies have examined blood chemistry, post-release behaviour or long-term survival in various bycatch species; however, few have combined these approaches within a single species. Here, we integrated hook timers, blood chemistry, accelerometry and acoustic and satellite telemetry to assess the impacts of longline capture on Greenland sharks Somniosus microcephalus, a deep-sea species encountered in Arctic fisheries. Due to extremely slow growth and late maturity, this species is considered vulnerable to anthropogenic impacts. Lactate and glucose concentrations were within normal ranges for related species and lower than most species. Following release, individuals exhibited reduced vertical movement with elevated swim speed and tailbeat frequency, returning to baseline behaviour after ~ 10 h. Recovery duration was positively correlated with lactate concentration, but not with time hooked. Acoustic and satellite tracking confirmed survival for > 30 days in 9 of 10 tagged individuals, with evidence of multiyear survival up to 6.7 years. The remaining individual moved offshore over 4 days, but long-term survival could not be confirmed. In addition, one shark that was tail-wrapped during capture, for which tracking data were unavailable, may have experienced immediate post-release mortality. Overall, our findings indicate that Greenland sharks are resilient to longline capture stress when properly handled. This resilience likely reflects their slow swim speed and low metabolic rate associated with cold environments, their limited fight response when hooked, and the narrow vertical temperature range of Arctic waters, which minimizes thermal stress during capture. We emphasize the importance of appropriate handling practices and recommend further monitoring of Greenland shark post-release survival from (i) longline fisheries by capture mode (i.e. mouth-hooked, body-tangled or tail-wrapped) and (ii) other fishing gears such as trawl and gillnet.