Beyond Post-release Mortality: Inferences on Recovery Periods and Natural Mortality From Electronic Tagging Data for Discarded Lamnid Sharks

Abstract

Accurately characterizing the biology of a pelagic shark species is critical when assessing its status and resilience to fishing pressure. Natural mortality (M) is well known to be a key parameter determining productivity and resilience, but also one for which estimates are most uncertain. While M can be inferred from life history, validated direct estimates are extremely rare for sharks. Porbeagle (Lamna nasus) and shortfin mako (Isurus oxyrinchus) are presently overfished in the North Atlantic, but there are no directed fisheries and successful live release of bycatch is believed to have increased. Understanding M, post-release mortality (PRM), and variables that affect mortality are necessary for management and effective bycatch mitigation. From 177 deployments of archival satellite tags, we inferred mortality events, characterized physiological recovery periods following release, and applied survival mixture models to assess M and PRM. We also evaluated covariate effects on the duration of any recovery period and PRM to inform mitigation. Although large sample sizes involving extended monitoring periods (>90 days) would be optimal to directly estimate M from survival data, it was possible to constrain estimates and infer probable values for both species. Furthermore, the consistency of M estimates with values derived from longevity information suggests that age determination is relatively accurate for these species. Regarding bycatch mitigation, our analyses suggest that juvenile porbeagle are more susceptible to harm during capture and handling, that keeping lamnid sharks in the water during release is optimal, and that circle hooks are associated with longer recovery periods for shortfin mako.

Integrating Biologging and Behavioral State Modeling to Identify Cryptic Behaviors and Post-capture Recovery Processes: New Insights From a Threatened Marine Apex Predator

“…Despite a low sample size, we also revealed that recovery times increased as shark length decreased. Although physiological stress responses to capture (e.g., lactate accumulation) are known to be magnified in smaller individuals for some shark species, which can influence postrelease outcomes (Gallagher et al, 2014;Talwar et al, 2017;Bowlby et al, 2021), white sharks across a range of sizes are quite physiologically resilient to short captures (<75 min, as per our study; Gallagher et al, 2019;Tate et al, 2019). Therefore, other factors (e.g., size-specific personalities) may underlie the 10-fold variance in behavioral recovery (∼3-30 h) we observed across sizes.…”

“…So far, daily diary tags have been used to reveal many aspects of aquatic animals' natural behaviors including foraging activities, cost-efficient movement strategies, and habitat use (Shepard et al, 2011;Benoit-Bird et al, 2013;Andrzejaczek et al, 2019a). By contrast, applied behavioral research on disturbances following capture in marine and terrestrial systems has mostly relied on either traditional biotelemetry, pop-up time-depth archival tags, or accelerometers (Afonso and Hazin, 2014;Rode et al, 2014;Barnes et al, 2016;Becciolini et al, 2019;Bowlby et al, 2021;Shuert et al, 2021). However, biotelemetryderived movements or depth profiles are often insufficient for resolving cryptic behavioral processes (e.g., foraging and resting) relevant to recovery, especially in aquatic systems where telemetry data are particularly intermittent and coarse (Andrzejaczek et al, 2018(Andrzejaczek et al, , 2019a.…”

Variable post-release mortality in common shark species captured in Texas shore-based recreational fisheries

“…For sharks that survive capture and tagging, electronic tags can evaluate swimming performance metrics that may be related to behavioral changes following release. Irregular dive behavior consists of inconsistent or restricted depth use [ 33 – 35 ], increased tailbeat frequencies and elevated overall dynamic body acceleration (ODBA) [ 28 ], and reduced frequency and amplitude of dives resulting in a diminished dive variance [ 36 , 37 ]. Quantifying these changes over time allows for a recovery period to be estimated as the amount of time it takes for an individual to restore normal values for these swimming performance metrics.…”

Conservation implications of white shark (Carcharodon carcharias) behaviour at the northern extent of their range in the Northwest Atlantic

“…The noncapture methodology was preferred to minimize tagging effects and ensure that movement data were more likely to represent natural behaviour. Tagging onboard following capture is known to alter movement behaviour during a prolonged recovery period for lamnid sharks (Hoolihan et al 2011;Bowlby et al 2021) and carcharhinid species (Vaudo et al 2014). Also, our non-capture tagging methodology was unaffected by gear selectivity, unlike capture-based sampling that is known to be size-selective (Maunder et al 2006;Rotherham et al 2007;Christiansen et al 2020).…”

“…Water temperature becomes a key environmental variable because it is both recorded by PSAT tags and is one of the most readily available oceanographic covariates to evaluate (Fourcade et al 2018). Typically, PSAT data collected throughout ocean basins are aggregated to increase sample size (e.g., Bowlby et al 2021) and longer-term deployments are considered optimal (Sippel et al 2015). However, our comparisons show that the distribution of temperatures experienced by white sharks during seasonal movements to Canadian waters is significantly narrower than those experienced at the same times of year in the US and differs markedly from other times of the year.…”