Progress in addressing key research to inform Mobulid ray conservation in the Pacific Ocean [2024]

Manta and devil rays (mobulids) are globally threatened species. Their preference for productive tropical and subtropical habitats where tropical tunas also aggregate increases their vulnerability to tuna purse seine fishing. However, interaction rates between the tropical tuna purse seine fishery and different mobulid species have not been quantified in detail, especially in the western and central Pacific Ocean. One of the challenges to quantifying the impact of the fishery on mobulids is unreliable species identification by crew and observers onboard the vessels, potentially due to poor training and/or lack of time for identification. Additionally, to evaluate the impact on mobulids and test possible interventions, it is necessary to assess mobulid post-release mortality using proper handling and release methods. This project aims to address key research to inform mobulid conservation. Bycatch reduction devices in the form of modified sorting grids were constructed and deployed to perform rapid release for the mobulids captured by 12 purse seine vessels from the U.S. fleet. Since June 2022, 29 mobulid captures were documented, and seven mobulids were released using these devices. Preliminary evidence suggests that sorting grids did not increase the duration of captures, even though the grids are used for larger individuals that would otherwise take longer to release. Satellite tags were deployed on three mobulids to estimate survival after release, but further research is necessary to examine the impact of the devices on mortality. In addition to testing the grids, eleven mobulid tissue samples were collected to contribute to knowledge of population genetic structure for mobulids. Lessons learned from applying current protocols and practices will allow developing improved best practices to be implemented by the U.S. tropical tuna purse seine fleet, which can be scaled up to other purse seine fisheries around the world.