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Project M.2.c - Manta and devil ray post-release survival, movement ecology, and genetic population structure

01 Jun 2021 - 01 Jun 2023

Program(s) in charge: Ecosystem & Bycatch Program
  • Quantify baseline capture and survival probabilities of mobulid species and identify best practices for handling and release
  • Identify vertical and horizontal habitat use of the species to improve selectivity
  • Quantify the accuracy of onboard observer species identification
  • Characterize population genetic structure and effective population size across the Eastern Pacific for four mobulid species.
  • Manta and devil ray populations are impacted globally by targeted fisheries and bycatch, including purse seine fisheries operating in the EPO
  • The IATTC forbids retention of mobulid rays and requires release without the use of gaffs, hooks, or damage to the body or gills.
  • Fishing crews have begun employing a variety of handling and release methods, from release by hand to the use of cargo nets. To date, there is no quantitative data to estimate the effect of these methods on the survivorship of the species
Relevance for management
Contribute to a cleaner fishing, reducing interaction and post-release mortality of sensitive bycatch species, and providing guidelines for best handling and release practices
Workplan and status
  • Train selected observers to deploy satellite tags and collect tissue samples
  • Develop specific complementary data collection forms and protocols for data collection and tagging
  • Analyze satellite tags to investigate animals’ post release survival, ecology, and horizontal and vertical behavior
  • Analyze tissue samples using Restricted Site Associated Sequencing (RAD-Seq) techniques to infer population structure and size from genetic information, as well as assess the accuracy of onboard observer species identifications
  • Conduct skippers’ workshops to discuss potential improvements and help shape best handling and release practices
  • Develop bycatch mitigation and management measures based on scientific evidence
External collaborators
The Manta Trust, The Monterey Bay Aquarium, The Conservation Action Lab at University of California Santa Cruz
  • A peer-reviewed publication on the post-release survivorship of manta and devil rays released alive from tuna purse seine vessels
  • Empirically derived guidelines for the best handling and releasing practices
  • Peer-reviewed publications on the horizontal and vertical distribution of mobulid rays, and their environmental preferences
  • A peer-reviewed publication on the population genetic structure of four mobulid species
  • A peer-reviewed publication on the accuracy of species identification and the effort to improve species identification forms and training for observers
  • Dissemination material for the Bycatch Working Group
Updated date: 01 May 2023
Progress summary for the reporting period
  • 2021: Develop data collection forms and protocols as well as discuss and agree the sampling design.
  • 2021-2022: distribute tagging kits to IATTC and TUNACONS observers for opportunistic tagging. Collect tissue samples at sea, on land, and from collaborators.
  • At least 31 tags were deployed on mobulids to date: 16 M. mobular, 8 M. Thurstoni, 5 M. tarapacana, 2 M. birostris.
  • 398 usable tissue samples were collected and analyzed up to date, belonging to 4 species. About 350 more samples will be potentially analyzed in 2022.
  • A manuscript on mobulids genetic and population structure was prepared and submitted to a peer-reviewed journal.
Challenges and key lessons learnt
  • Preliminary tagging analyses suggest species-specific post-release mortality: 50% for M. birostris, 60% for M. Tarapacana, 8% for M. mobular and 80% for M. thurstoni.
  • Preliminary genetic analyses suggest weak but significant population structure for all the species with good data – M. birostris, M. thurstoni, and M. munkiana. Strong evidence of connectivity exists, but local selection may also be occurring.
  • For M. thurstoni and M. munkiana, very low diversity and high inbreeding has been detected, suggesting potential genetic bottleneck or depletion.
  • There is clear distinction between Indian Ocean/W Pacific and eastern Pacific Oceans, suggesting EPO should likely be managed distinctly. Additionally, there are significant differences from northern and southern EPO, though this varies slightly by species. For some, subregions-subpopulations (north-south) may exist within the ETP.
  • Other regional mobulid mitigation initiatives exist, and active collaboration is being undertaken at the moment (i.e., mobulid bycatch mitigation tools in purse-seiners operating in both WCPO-EPO).
  • Several tags failed to report, and arrangements were made with the tag provider to replace them. New tags will be deployed in 2023.
  • A presentation at the BYC-10 meeting.
  • Several presentations for the skippers’ workshops in 2020, 2021 and 2022.
  • A peer-reviewed publication. Cronin et al. 2022, Harnessing Stakeholder Knowledge for the Collaborative Development of Mobulid Bycatch Mitigation Strategies in Tuna Fisheries, ICES Journal of Marine Science.
  • Other peer-reviewed publications are either in preparation or under review.