Moving towards quantitative ecological risk assessment for data-limited tuna fishery bycatch: application of “EASI-FISH” to the spinetail devil ray (Mobula mobular) in the eastern Pacific ocean DRAFT (not to be cited) & Presentation.
The IATTC staff recently developed a new spatially-explicit ecological risk assessment (ERA) approach—Ecological Assessment for the Sustainable Impacts of Fisheries (EASI-Fish)—to quantify the cumulative impacts of multiple fisheries on data-limited bycatch species in the EPO. The method is generally applied to a suite of impacted bycatch species, where a proxy of the instantaneous fishing mortality rate (F) is estimated for each species based on the ‘volumetric overlap’ of each fishery and each stock’s distribution. F is then used in length-structured per-recruit models to assess the vulnerability of each species using conventional biological reference points (e.g. FMSY, SSB40%). EASI-Fish is therefore, primarily used as a quantitative prioritization tool to allow fisheries managers to identify the most vulnerable species to which resources can be directed to either implement mitigation measures to remove the key risk(s) or subject the species to data collection programs to gather sufficient data to facilitate more traditional population assessments. However, EASI-Fish also has the capability of simulating hypothetical conservation and management measure (CMM) scenarios (e.g. spatial and/or temporal closures, gear modifications) that may mitigate fishery risks to a species, without incurring significant investment in costly data collection programs. This paper uses EASI-Fish to explore the changes in the vulnerability status of the spinetail devil ray (Mobula mobular) - a slow-growing species with low reproductive potential and a paucity of information on post-release mortality (PRM) - under 18 hypothetical CMM scenariossimulated for EPO purse-seine and longline tuna fisheries for 2016. CMMs involved various spatial and temporal closures of the EPO and ‘hotspots’, decreasing PRM, increasing the length at first capture and various combinations of these CMMs. The “status quo” scenario revealed that F2016 and the spawning stock biomass per recruit (SSB2016) exceeded precautionary biological reference points (F40% and SSB40%), classifying M. mobular as “most vulnerable”. Increasing the duration and/or number of spatial closures significantly reduced the vulnerability of the species but was insufficient in changing its classification from “most vulnerable”. Only 3 of the 18 scenarios resulted in the species being classified as “least vulnerable”, which primarily involved reductions in PRM. This is fortuitous in that the development of best handling and release practices and the education of fishers is likely to be a far simpler, rapid and cost-effective CMM than the implementation of increased spatial and temporal closures and gear modifications that will likely result in substantial decreases in the catches of target species. However, given the current lack of reliable information on the PRM of M. mobular - and other mobulids - there is an urgent need for a tagging study to quantify the PRM of M. mobular from purse-seine and longline fisheries in the EPO.