Vulnerability status and efficacy of potential conservation measures for the east pacific leatherback turtle (Dermochelys coriacea) stock using the EASI-Fish approach

Citation
IATTC (2022) Vulnerability status and efficacy of potential conservation measures for the east pacific leatherback turtle (Dermochelys coriacea) stock using the EASI-Fish approach. In: IATTC - 11th Meeting of the Working Group on Bycatch. IATTC BYC-11-02, Online
Abstract

Industrial and small -scale coastal (i.e., ‘artisanal’) pelagic fisheries in the eastern Pacific Ocean (EPO) interact with one of the most vulnerable fishery bycatch species, the East Pacific (EP) stock of leatherback turtle (Dermochelys coriacea). As a result of the species’ longevity, slow growth, low reproductive output, and critically low population size, it is currently classified as “Critically Endangered” by the International Union for Conservation of Nature (IUCN). EPO tuna fisheries have been mandated since 2008 (Resolution C-07-03) to ensure, by all practical means, the safe handling and release of captured sea turtles. On 1 January 2021 , a revised resolution on sea turtles (C-19-04) entered into force that requires EPO tuna fisheries to implement various measures designed to reduce the bycatch of sea turtles, in particular the use of circle hooks and finfish baits in shallow longline sets. The low encounter rates of sea turtles by fishing vessels make these ‘rare event’ data difficult to analyze using conventional approaches for assessing the status of sea turtle populations . Consequently, alternative means are needed to better assess vulnerability status and better understand the potential efficacy of different conservation and management measures (CMMs) to improve sea turtle conservation . In response, the spatially-explicit ecological risk assessment (ERA) approach —Ecological Assessment for the Sustainable Impacts of Fisheries (EASI-Fish)—was developed by Inter -American Tropical Tuna Commission (IATTC) staff to quantify the vulnerability of bycatch species, such as the EP leatherback stock, to the cumulative impacts of multiple fisheries in the EPO and to simulate hypothetical CMM scenarios that may mitigate fisheryimposed risks to the species. This paper describes a collaborative research project conducted by an ad hoc joint working group of participants from the IATTC, the Inter-American Convention on the Protection and Conservation of Sea Turtles (IAC), and international sea turtle experts where EASI-Fish was used to explore the changes in the vulnerability status of the EP leatherback turtle subpopulation. This working group developed 70 different hypothetical CMM scenarios simulated for EPO industrial (purse-seine and longline) and artisanal (longline and gillnet) fisheries for a recent representative year (2019). CMMs involved decreasing bycatch rates (i.e., “contact selectivity” in EASI -Fish) and post -capture mortality (PCM), implementing the use of circle hooks and/or finfish bait in longline fisheries, illuminated gillnets, best practices for safe handling and release of leatherbacks, and combinations of CMMs. The “status quo” scenario revealed a proxy for fishing mortality (𝐹2019) and the breeding stock biomass per recruit (BSR2019) exceeded precautionary biological reference points (F80% and BSR80%), classifying the EP leatherback turtle stock as “most vulnerable”. Industrial and artisanal longline fisheries had the highest 𝐹2019 values, likely because they also had the highest areal overlap with the modelled EP leatherback species distribution (61% and 34%, respectively). Of the 70 scenarios, 42 resulted in significant improvements in EP leatherback vulnerability status (i.e., less vulnerable). Although use of circle hooks, finfish bait, and to a lesser extent best h andling and release practices were each predicted to decrease vulnerability when examined individually, by far the most effective scenarios involved using these three measures in concert, followed by using circle hooks with either finfish bait or best practices. However, benefits predicted from EASI-Fish for CMM scenarios assume 1) 100% compliance with CMM implementation to the full extent of each applicable fishery, and 2) that CMMs achieve the estimated levels of efficacy reflected in the model inputs. Thus, the results of the model scenarios provide estimates of what may be possible under such conditions in comparison to the status quo under ideal conditions . This modelling exercise provided detailed results that enable evaluation of the potential efficacy of CMMs established in in IATTC Resolution C-19-04 in reducing impacts of fisheries bycatch on EP leatherbacks and can inform development of fisheries-specific strategies to implement CMMs described.