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Introduction and status of mobulids
Manta and devil rays (collectively known as mobulids) are a family of migratory elasmobranchs. Their life history traits mean the largest Mobula species have maximum rates of intrinsic population increase among the lowest of all elasmobranchs (Dulvy et al., 2014). Mobulids are vulnerable to both targeted fisheries and bycatch and are caught in both small-scale and commercial (e.g. tuna) fisheries (Croll et al., 2016). Such fisheries are a major threat to mobulids, with some populations exhibiting declines of over 90% (e.g. Lewis et al., 2015; Rohner et al., 2017; Moazzam, 2018). In the Indian Ocean, all mobulid species are assessed as either Vulnerable or Endangered (IUCN, 2020; Table 3.), with steep population declines due to exploitation in fisheries playing a major role in these assessments. In response to growing concern, in 2019 the IOTC adopted Resolution 19/03 on the conservation of mobulids caught in association with fisheries in the IOTC Area of Competence (Annex 1).
Interactions with the IOTC fisheries
Mobulids are mainly caught as bycatch, primarily in the industrial purse-seine fisheries, and to a lesser extent in longline fisheries (Croll et al., 2016; Shahid et al., 2018). They are also incidentally captured in small-scale gillnet fisheries, usually being retained for their meat and gill plates (White et al., 2006; Ardill et al., 2011; Moazzam, 2018). Mobulids are particularly susceptible to incidental catch in tuna fisheries due to their epipelagic distribution in regions of high productivity, leading to a high level of distributional overlap with target species (Croll et al., 2012). However, observer coverage on the IOTC fishing vessels is limited, and often mobulid landings are not identified to species level, meaning data is poor. Despite this, reports submitted to the IOTC WPEB in 2018 (Shahid et al.; Moazzam; Fernando) all highlighted declines in mobulid populations due to tuna fisheries in the Indian Ocean (Table 4.).
Post-release mortality of mobulids in tuna fisheries is currently high (Poisson et al., 2014; Francis and Jones, 2017; Amandè et al., 2008) due to a lack of available tools to safely manipulate mobulids (Grande et al., 2019), and lack of awareness/compliance with safe handling and release guidelines. Potential onboard tools include “manta ray grids” and modified brailer grids. Pre-capture techniques, which should be emphasised, include dynamic spatio-temporal management of key mobulid habitats, and tools such as LEDs to prevent incidental catch.
Data availability and gaps
Research on mobulids, although increasing, has been limited by the difficulties in observing and investigating Mobula species in their extensive oceanic environment (Couturier et al., 2012; Croll et al., 2016; Lawson et al., 2017; Stevens et al., 2018). A systematic literature review undertaken by Stewart et al., published in September 2018(a), identified research priorities to support effective mobulid ray conservation. The review highlighted the need for taxonomic clarifications, better knowledge of mobulid life history parameters, and more studies on bycatch and fisheries (including post-release mortality, species distributions and fisheries data standardisation). A number of data gaps, in addition to the lack of IOTC fisheries observer coverage and lack of research into bycatch mitigation for mobulids, are of relevance to and could be addressed by tuna RFMOs. These are outlined in Table 4.
Recommendations for management
CMM 19/03 is very positive and in order to ensure implementation and compliance, while achieving the goal of significantly decreasing the mortality of mobulid rays, the following actions are recommended:
