Data review and potential assessment approaches for mobulids in the Western and Central Pacific Ocean

Mobulid rays (manta and devil rays) are pelagic elasmobranchs that are widely distributed throughout the world’s oceans. Throughout their range, manta and devil rays experience substantial fishery-related mortality, both in target fisheries and also as bycatch. In view of the conservative life history of mobulid rays, there have been growing concerns about the impact of fishery mortalities on the sustainability of mobulid populations. At the same time, their extensive distribution and large-scale movement, combined with a general lack of fishery catch and landings data make population and stock assessments difficult.

Mobulid rays were designated as key shark species for assessment purposes by the Western and Central Pacific Fisheries Commission (WCPFC) in 2016. The Fifteenth Meeting of the Scientific Committee (SC15) requested further research into the stock and ecological risk status of mobulid rays, with updated information to be provided to the Scientific Committee in 2020. Specifically, the focus of the additional research was a review of available data to allow the Scientific Committee to determine the feasibility of assessing the status of mobulid rays, and the potential types of assessment approaches that may be suitable. The present project presents the outcomes of this research by providing a summary of available data from 1995 to 2018 in the context of potential assessment approaches.

In the Western and Central Pacific Ocean, there are two species of manta ray and five species of devil ray: giant manta ray Mobula [previously Manta] birostris, reef manta ray M. [previously Manta] alfredi, spinetail devil ray M. mobular (including M. japanica), longhorned pygmy devil ray M. [eregoodootenkee] eregoodoo, shortfin devil ray M. kuhlii, sicklefin devil ray M. tarapacana and bentfin devil ray M. thurstoni. Most of these species have been recorded as bycatch in purse-seine and longline fisheries in this region, with observer records including manta and devil ray captures at different taxonomic resolutions.

The analysis of observer data for the period between 1995 and 2018 highlighted distinct differences between purse-seine and longline sets, with markedly higher numbers of observed mobulid captures in purse-seine than in longline gear. The total number of observed captures was 38,034 individuals in purse seine compared with 2205 captures in longline. This difference reflects in part the considerably higher rate of observer coverage on purse-seine vessels, but also the tropical distribution of a number of mobulid species.

For both fishing gears, the taxonomic resolution of bycatch records was low until the mid-2000s, with early observer records aggregating mobulid captures as all rays or as manta and devil rays combined. Capture records at higher taxonomic resolutions were almost exclusively at the species level for manta ray in both gear types, compared with the predominantly generic reporting of devil rays that persisted including in recent years. Amongst the devil rays, spinetail devil ray, the largest in this group, was the species with the highest frequency of species-level classification. The scarcity of species-level records for smaller species of devil ray indicates the lack of observer training for distinguishing features other than size.

Observed capture rates of both manta and devil rays showed some fluctuations over time, and a clear signal of increased reporting. This signal occurred in the later part of the study period as the taxonomic resolution of the captures increased: observers first started reporting mobulid bycatch as rays, then classified it as manta and devil rays, before identifying manta rays, and subsequently some of the devil ray captures at the species level. The average capture rate of manta rays in purse seine in the last three years of this study (2016–2018) was 45.2 individuals per thousand observed sets. For devil rays, observed captures rates in purse seine were highly variable, with an average capture rate of 35.8 individuals per thousand observed sets between 2016 and 2018. In longlines, observed capture rates for both manta rays and devil rays were variable, with an overall decrease in recent years. The average capture rate for manta and devil rays combined in the last three years was 4.2 individuals per million observed hooks.

Post-release mortality is a key parameter for quantifying the impact of fishing on mobulid populations in the Western and Central Pacific Ocean, as observer records indicated that most mobulid captures were discarded. Condition-at-release is an important variable to account for when estimating post-release mortality. There were some distinct differences between purse-seine and longline observer data regarding the collection of individual condition. In purse seine, the condition at capture was not recorded for most individuals. In longline sets, this variable was recorded for most individuals; however, in recent years, there was still a considerable proportion of individuals that were recorded as alive, but without a health classification.

Among the mobulid species captured, giant manta ray (M. birostris) had the most data to inform an assessment in the Western and Central Pacific Ocean. In a recent analysis of observer data from the Inter-American Tropical Tuna Commission, this species was classified as the most vulnerable to fishing in the Eastern Pacific Ocean (Duffy et al. 2019). The comparative analysis also included two species of devil ray (spinetail devil ray and bentfin devil ray), highlighting that M. birostris should be prioritised for an assessment within the mobulid group based on data availability and also from a management perspective. The lack of a clear trend evident in the nominal catch-per-unit-effort for the giant manta ray, together with trends in reporting, prevents the implementation of a medium-data assessment in the short term. A spatial risk assessment based on a recent catch history is likely to be most suited for an assessment of this species and could be implemented immediately. Alternatives include eSAFE (Sustainability Assessment for Fishing Effects) and EASI-fish (Ecological Assessment of the Sustainable Impacts by Fisheries). Ideally, more than one approach would be trialled to increase the confidence in the resulting reference points to inform management based on fishing mortality. The assessment approach could then be upgraded to a medium-data assessment once a more extensive catch history of at least 15 years beyond the start of widespread recording is available from reconstruction.

The ongoing lack of consistent classifications of devil rays at the species level impedes the application of any assessment approach that requires catch histories. An alternative framework for assessing the vulnerability of species lacking a recent catch history is a PSA (Productivity-Susceptibility Analysis). This semi-quantitative framework ranks vulnerability to overfishing within designated species assemblages. Although it would not provide a population status for mobulids as a function of a reference point, it could assist in the prioritisation of research and observer training for the smaller species of devil ray, for which identifications at the species level remain limited. In the near future, a quantitative risk assessment such as eSAFE and EASI-fish could also be applied to spinetail devil ray M. mobular, as this species shows the highest frequency of reporting at the species level amongst devil rays.

While there is a range of data-poor assessment approaches that can be applied to mobulids based on the specific data availability for each species, the true status of the stock for this group will remain uncertain on the medium to long term. A management metric that could be collected and monitored for improvement is the combined implementation of safe release guidelines and the resulting condition of individuals at release. Improving these inter-related variables will have an immediate, positive impact on the survival of bycaught individuals and lower the overall fishing mortality on populations of mobulids in the Western and Central Pacific Ocean.

Finally, life-history data for mobulid species occurring in the Western and Central Pacific Ocean remains generally scarce, particularly for devil rays. It might be possible to expand the scope of sampling of mobulids by observers so that key samples are collected from any dead individuals, such as the vertebrae which do not require specialized storage facilities in the short-term, and could provide valuable information on growth. Such expanded sampling would require initial consultation with observers and trainers within the Regional Observer Programme to assess feasibility across different tissue types. The Pacific Specimen Tissue Bank could serve as a repository for new mobulid samples and a point of coordination between biologists studying mobulids in the Pacific.