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

Griffiths S, Wallace B, Swimmer Y, et al (2021) Vulnerability status and efficacy of potential conservation measures for the east Pacific leatherback turtle (Dermochelys coriacea) stock using the EASI-fish approach. In: IATTC - 10th Meeting of the Working Group on Bycatch. IATTC BYC-10 INF-B, Electronic Meeting, p 41

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 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) will enter into force that will subject EPO tuna fisheries to 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 statistically using conventional approaches of assessing the status of sea turtle populations. Consequently, alternative means to assess vulnerability status and better understand the potential efficacy of different conservation and management measures (CMMs) for effective fisheries management. 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 CMMs scenarios that may mitigate fishery-imposed risks to the species.

This paper describes a collaborative research project between 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 stock under 39 different hypothetical CMM scenarios simulated for EPO industrial (purse-seine and longline) and artisanal (longline and gillnet) fisheries for 2018. CMMs involved decreasing post-capture mortality (PCM), implementing the use of circle hooks in longline fisheries, and various spatial and temporal closures adjacent to important nesting beaches of the EPO. The “status quo” scenario revealed a proxy for fishing mortality (? 2018 ) and the spawning stock biomass per recruit (BSR 2018) exceeded precautionary biological reference points (F 80% and BSR 80%), classifying the EP leatherback turtle stock as “most vulnerable”.

Of the 39 scenarios, only 14 resulted in the species being classified as “least vulnerable”. Only one scenario involved a single CMM—that achieved an estimated PCM of 20% or less— changed the status the “least vulnerable”. This involved imposing an EPO closure period of at least 270 days per year for industrial fisheries; a measure that is unlikely to be feasible. The remaining effective CMMs involved using multiple strategies in concert, with the most effective being closure of coastal fishing grounds adjacent to nesting areas coupled with the use of circle hooks in all longline fisheries and exercising best handling and release practices in all fisheries.

This modelling exercise provided an important first step towards assessing the potential effects of CMMs described in the recently approved IATTC Resolution C-19-04. The results of the EASI-Fish models can inform strategies to implement these CMMs within the IATTC Convention Area to reduce bycatch impacts on EP leatherbacks.