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 Through collection of historical and recent mobuild fisheries data from published and unpublished sources, this study aimed to identify trends in abundance of Indonesian manta and devil rays and explore socio-economic factors and incentives associated with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.

Research Priorities to Support Effective Manta and Devil Ray Conservation

“…Although relatively understudied for most of the Twentieth century, public awareness, and scientific research on these species has increased dramatically in recent years. Much of this attention has been in response to targeted fisheries (White et al, 2006;Croll et al, 2016), international trade in mobulid products (O'Malley et al, 2016), and a growing concern over the fate of exploited populations (Ward-Paige et al, 2013;Lewis et al, 2015;White et al, 2015). Over the past decade, research advances on the biology and ecology of mobulid rays have supported a number of international, national, and local management actions focused on halting or reversing population declines (Croll et al, 2016).…”

Sympathy for the devil: a conservation strategy for devil and manta rays

“…Human populations in many tropical coastal communities are growing rapidly, have low income, and rely heavily on fish for protein and income (Allison et al, 2009). Mobulids provide a source of income and protein in several developing countries, particularly Indonesia and Sri Lanka (Fernando & Stevens, 2011; Lewis et al, 2015). Developed nations can increase the effectiveness of conservation measures by helping to facilitate necessary social and economic transitions (McClanahan et al, 2008).…”

“…Targeted fisheries for mobulids have existed for decades, yet increased demand for mobulid gill plates has fuelled the emergence and expansion of fisheries targeting these species (Alava et al, 2002; Lewis et al, 2015). Devil and manta rays were historically exploited for meat (consumed fresh or dried), and to a lesser extent skin (dried) and cartilage (for shark fin soup filler; White et al, 2006; Acebes, 2013), and there continues to be a market for some of these devil and manta ray products today.…”

High bycatch rates of manta and devil rays in the “small-scale” artisanal fisheries of Sri Lanka

“…The magnitude of these total catch estimates-even the lower estimates from 2017 onwards-are exceptionally high considering the assumed small population sizes for mobulids (Stewart et al, 2018a). To put these catch numbers in context, it was estimated that Indonesia landed between 1,050 and 2,400 M. birostris in their target fishery in 2002 (Dewar, 2002;Lewis et al, 2015), Peru landed 1,985 mobulid rays over one year at their largest mobulid landing site while estimates of the northern Peru region suggest ∼8,000 individuals per year (Croll et al, 2016;Alfaro-Cordova et al, 2017). The estimated global tuna purse seine mobulid bycatch is approximately 13,085 individuals per year (Croll et al, 2016).…”

“…Mobulid rays are highly susceptible to incidental capture in a wide range of fisheries and gear types including gillnets, purse seines, trawl nets, and on occasion even long lines (Croll et al, 2016). Together with their conservative life history characteristics, this has led to a number of suggested population declines around the world (Ward-Paige, Davis & Worm, 2013;Dulvy et al, 2014;White et al, 2015;Lewis et al, 2015;Pardo et al, 2016), including in the western Indian Ocean (Pacoureau et al, 2021). Sri Lanka is considered one of the largest mobulid fisheries in the world (Ward-Paige, Davis & Worm, 2013;CMS Secretariat, 2015;Acebes & Tull, 2016;Marshall et al, 2019) due to the expansive fishing fleet that ventures far beyond the country's EEZ.…”