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

Heather D Bowlby,Warren Joyce,Fabio Hazin,Gérard Biais,Hugues P Benoît,Andrés Domingo,Enric Cortés,Rui Coelho,James Sulikowski,Brooke Anderson,Catarina Santos,David Macias

doi:10.3389/fmars.2021.619190

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 (&gt;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.

Highlights

Quantifying fishing-related (F) and natural (M) mortality continues to be one of the main challenges in understanding and managing marine fauna
This study combined data from 177 archival satellite tag deployments during 2001–2019 in the North Atlantic (Figure 1), 73 on porbeagle and 104 on shortfin mako (Supplement 1). Both species were captured during regular commercial fishing activities by pelagic longline fleets (N = 134), scientific cruises using pelagic longline (N = 38) or commercial trawl trips (N = 5) and tagged by fisheries observers, science personnel, or fishermen trained by science personnel
To quantify the duration of the recovery period, we identified the day on which dive variability markedly increased

Summary

Introduction

Quantifying fishing-related (F) and natural (M) mortality continues to be one of the main challenges in understanding and managing marine fauna. Common methods rely on age determination, and are calculated from theoretical longevity, length-at-age and weight-at-age relationships, and/or von Bertalanffy growth function parameters (reviewed in Kenchington, 2014). This means all methods are sensitive to the level of uncertainty in age determination for elasmobranchs, where longevity may be systematically underestimated (Campana et al, 2002; Harry, 2018; Natanson et al, 2018). The substantial cost associated with archival tags still constrains sample sizes (Hazen et al, 2012) and poses a particular challenge for reliable estimation of M for long-lived species

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