Assessing Indonesian manta and devil ray populations through historical landings and fishing community interviews

Abstract

International concern is growing with regard to the sustainability of manta and devil ray (collectively mobulids) fisheries as demand for mobulid products has increased in international markets over the last decade. While Indonesia has been reported to be one of the worlds’ top three catchers of mobulid rays, detailed information on these fisheries and the status of Indonesian mobulid populations are lacking. Through collection of historical and recent mobuild fisheries data from published and unpublished sources, this study aimed to identify trends in abundance of Indonesian manta and devil rays and explore socio-economic factors and incentives associated with mobulid fisheries. Comparison of catches from 2001-5 to the most recent data from 2013-14 revealed dramatic declines in mobulid landings over the study period of 64% at Cilacap, 75% at Lamakera, and 94% at Tanjung Luar. The largest declines were observed for Manta spp. and the two large devil rays, Mobula tarapacana and Mobula japanica. Anecdotal reports indicated that catches had declined substantially at three additional sites and local extirpations are strongly suspected to have occurred at three locations. A lack of data on the population ecology of Indonesia’s mobulids makes it difficult to determine whether natural fluctuations may be playing a part in the declining catch rates. However, mobulid life history traits, including low reproductive rates and late age of sexual maturation, indicate that fishing pressure is likely the primary driver in these declines. Interviews in Lamakera, a community which depends on income from its targeted mobulid fishery, suggest that programs focused on education, training and infrastructure development to enable shifts to sustainable livelihood alternatives are likely to offer the most successful path to long-term conservation and management of manta and devil rays, while simultaneously yielding economic and social benefits to fishing communities.

Characterization of the trade in manta and devil ray gill plates in China and South‐east Asia through trader surveys

“…Gill plate exports from these locations, however, have yet to be verified and some may refer to illegal catches within the waters of these countries. For example, Indonesian fishers from Tanjung Luar, Lombok have admitted to illegally targeting mobulids in northern Australian waters (Lewis et al ., ), a Hong Kong trader reported in 2015 to be receiving manta ray gills that were illegally transhipped from Indonesia (P. Hilton, pers. comm.…”

“…Consultation with these researchers provided further confirmation of average dried gill plate yields. Lamakera fishermen reported typical dried gill plate yields of 5 kg per M. birostris (and up to 7 kg from very large mantas) and 2 to 3 kg for M. tarapacana (Lewis et al ., ). The Puqi plant manager reported yields of 2.5 kg dried gills per 500 kg of whole animals.…”

“…Data on dried gill plate yields per animal were obtained from interviews with the manager of a shark and mobulid processing plant in Puqi, Zhejiang Province, the city known as the shark processing centre of southern China (Li et al ., ), investigation of a directed mobulid fishery in Lamakera, East Flores, Indonesia (Lewis et al ., ), and review of the Fernando and Stevens () report on Sri Lanka's mobulid fisheries and trade. Consultation with these researchers provided further confirmation of average dried gill plate yields.…”

“…These characteristics, in addition to extremely low reproductive rates (Couturier et al ., ), leave mobulid populations exceptionally vulnerable to mortality from directed fisheries and bycatch with limited capacity to recover from overfishing (Couturier et al ., ; Dulvy et al , ). Fisheries supplying the Asian dried gill plate market have been documented in India, Sri Lanka, Indonesia, Malaysia, the Philippines, Mozambique, Palestine, and China, and have heavily impacted many mobulid subpopulations (Alava et al ., ; Rajapackiam et al , ; Fernando and Stevens, ; Heinrichs et al , ; Abudaya et al , ; Dent and Clarke, ; Lewis et al ., ). This escalating threat has led to IUCN Red List classification of Manta alfredi and Manta birostris as globally vulnerable to extinction (Marshall et al , 2011a, b), and Mobula tarapacana , Mobula japanica , and Mobula thurstoni as vulnerable in South‐east Asia (Clark et al , 2006a, b; White et al , ).…”

Feeding – Cleaning Trade-Off: Manta Ray “Decision-Making” as a Conservation Tool

“…The existing conservation measures in regard to manta rays are mainly regulatory and legislative, such as the listing of the two species, Mobula alfredi and Mobula birostris, as "Vulnerable" and "Endangered, " respectively, under IUCN; their inclusion in Appendix-II in the CITES and CMS treaties; and local protection (Marshall et al, 2011). However, additional fisheries are continually being established (Croll et al, 2016) and manta ray populations are declining (Lewis et al, 2015;O'Malley et al, 2017), suggesting that these measures fall short of effective management plans, mainly due to lack of enforcement and an absence of alternative livelihood sources for the fisherfolk (Acebes and Tull, 2016).…”