Species for Sale: Manta Ray
by Thomas Gomersall
Manta rays (Genus: Manta) are giant cartilaginous fish found in tropical and subtropical seas worldwide. They are planktivores that migrate along well-established routes and gather in large numbers at predictable zooplankton hotspots to feed, ingesting large volumes of water and filtering plankton from it using rigid sieving pads called gill plates. They are slow breeding, naturally uncommon animals, with females normally producing one pup per year and the largest populations numbering in the low thousands (Couturier et al, 2012; Cornish, 2020).
Item on Sale:
Along with the closely related devil rays, manta rays are targeted for their gill plates, which are sold dried for traditional Chinese medicine (TCM) under the trade name pengyusai (O’Malley et al, 2017). Although gill plates are rarely prescribed by TCM practitioners (who have even acknowledged the lack of evidence for their effectiveness), they became highly sought after over a decade ago thanks to traders in Guangdong aggressively marketing them as a health tonic ingredient. As a result, some fishermen who had once not targeted manta rays and would even release ones they accidentally caught switched to actively hunting them (Hilton, 2012; Cornish, 2020, O’Malley et al, 2017).
Hong Kong is one of the five biggest Asian importers of manta gill plates, which are often sold in dried seafood shops or on online shopping platforms like Taobao and can reach over US$430 per 500g (Hau et al, 2016). Sheung Wan has by far the highest concentration of shops selling gill plates and the availability of gill plates here is even higher than in Guangzhou; the biggest gill plate consumer (O’Malley et al, 2017; Hau et al, 2016). Interestingly, despite these large volumes, demand and consumption amongst Hong Kongers is not that high (Cornish, 2020) and a 2016 study found evidence suggesting that the main buyers of gill plates here may actually be mainland Chinese, with e-commerce sales mainly targeting this demographic and three out of five gill plate sellers in Hong Kong likely to be advertising to mainland Chinese customers (Wu, 2016).
Manta ray habitats and migration routes often overlap with fisheries, which combined with their tendency to congregate at predictable sites, makes it easy to catch large numbers of them (Couturier et al, 2012; Germanov and Marshall, 2014). Given their very slow reproductive rate and natural uncommonness, an annual global catch of 3,400 individuals — not accounting for unreported and subsistence catches -– is unsustainable and some of the largest fisheries, like Indonesia, have seen major declines (Hilton, 2012; Lewis et al, 2015).
Since not much is known about the ecology of manta rays, it is unclear what the wider environmental effects of their decline are. But one possible consequence is a disruption of marine nutrient cycling. Studies have found that mantas feed in deeper, nutrient-rich waters by night then return to the nutrient-poor shallows and surface waters during the day (Braun et al, 2014; Peel et al, 2019). As water usually doesn’t mix much between different ocean layers on its own, in areas where they are more common, mantas could play important roles in transferring nutrients between them through defecation or simply by their large mass shifting water as they move.
“When [manta rays] travel vertically, because they’re such large animals, they are pulling water behind them and that is pulling some of the nutrient-rich water from deeper waters into shallow waters,” says Andy Cornish, leader of WWF’s global shark and ray programme in Hong Kong.
How can Hong Kong help?
Despite Hong Kong’s historical role in the gill plate trade, recent developments have made it easier than ever for the city to combat it. In 2014, manta rays were listed on Appendix II of the Convention on International Trade of Endangered Species (CITES), meaning that export permits for their parts can only be granted if the exporting country can prove that fishing is sustainable. But as manta fisheries are inherently unsustainable, no country is known to have attempted to legalise a sustainable trade, the implication being that any international gill plate trade today is illegal. This means that manta gill plate imports into Hong Kong are much more tightly regulated than before the CITES listing (Cornish, 2020).
“While a CITES Appendix II listing is designed to allow for a sustainable legal trade, manta rays have such low rates of reproduction that they should really be fully protected,” says Cornish. “I would be very doubtful that any country could undertake the CITES sustainability assessment necessary to show that [manta ray fisheries] could be sustainable.”
Authorities also need to monitor the Hong Kong stockpile of manta gill plates that were imported prior to the CITES listing in 2014. Although these can be sold legally, as with elephant ivory, there is still the risk that the gill plates of recently killed manta rays could be laundered with the old gill plates. If the stockpile does not significantly decrease over time, this might suggest that it is being illegally supplemented with new gill plates, in which case authorities will need to increase their efforts to prevent their entry into Hong Kong and enhance the monitoring of stockpiles (Cornish, 2020).
There’s more to learn about rays and their relatives, the sharks, which will be the focus of an upcoming WWF-Hong Kong Ocean Celebration event at Hoi Ha Wan Marine Life Centre on 23–24 May. Bookings available here.
· Braun, C.D., Skomal, G.B., Thorrold, S.R. and M.L. Berumen. 2014. Diving behavior of the reef manta ray links coral reefs with adjacent deep pelagic habitats. PLoS ONE, vol. 9(2): e88170
· Cornish, A.S. (PhD), interviewed by Thomas Gomersall, 2020, WWF-Hong Kong.
· Couturier, L.I.E., Marshall, A.D., Jaine F.R.A., Kashiwagi, T., Pierce, S.J., Townsend, K.A., Weeks, S.J., Bennett, M.B. and A.J. Richardson. 2012. Biology, ecology and conservation of the Mobulidae. Journal of Fish Biology, vol. 80: 1075pp.–1119pp.
· Germanov, E.S. and Marshall, A.D. 2014. Running the gauntlet: Regional movement patterns of Manta alfredi through a complex of parks and fisheries. PLoS ONE, vol. 9(12): e115660.
· Hau, C.Y.L., Ho, K.Y.K. and S.K.H. Shea. 2016. Rapid survey of mobuild gill plate trade and retail patterns in Hong Kong and Guangzhou markets. BLOOM Association Hong Kong: 1pp.–20pp.
· Hilton, P., ‘There is a catch’. South China Morning Post, 7 October 2012, https://www.scmp.com/magazines/post-magazine/article/1053711/there-catch (Accessed: 5 March 2020).
· Lewis, S., Setiasih, N., O’Malley, M.P., Campbell, S., Yusuf, M. and A. Sianipar. 2015. Assessing Indonesian manta and devil ray populations through historical landings and fishing community interviews. PeerJ Pre-Prints, vol. 3: e1642.
· O’Malley, M.P., Townsend, K.A., Hilton, P., Heinrichs, S. and J.D. Stewart. 2017. Characterization of the trade in manta and devil ray gill plates in China and South-east Asia through trader surveys. Aquatic Conservation: Marine and Freshwater Ecosystems, vol. 27: 394pp.–413pp.
· Peel, L.R., Daly, R., Keating Daly, C.A., Stevens, G.M.W., Collin, S.P. and M.G. Meekan. 2019. Stable isotope analyses reveal unique trophic role of reef manta rays (Mobula alfredi) at a remote coral reef. Royal Society Open Science, vol. 6: 190599.
· Wu, J. 2016. Shark fin and mobulid ray gill plate trade in mainland China, Hong Kong and Taiwan. Traffic Report: 1pp.–78pp.