- by Daniel Fernando
Manta and mobula rays, commonly referred to as devil rays due to their two “horns” or cephalic fins, are among the most iconic and charismatic fish in the ocean. There are 2 Manta spp. and 9 Mobula spp., collectively called mobulid rays, in the Mobulidae subfamily and similar to sharks are greatly sought after by avid scuba-divers intending to check them off their bucket-list!
Mobulid rays are however highly threatened due to the fact that over the past decade, well manipulated marketing, coupled with declines of more desirable fish stocks, has driven a growing international trade for their gill plates – the tough cartilaginous structures that enable mobulid rays to filter plankton from the water column – resulting in the development of a target fishery for these species. The gill plates are dried and exported to China, where they are used as a pseudo-remedy in Chinese Medicine, claimed to purify blood and cure ailments such as the common cold, chicken pox and asthma. Meat of mobulid rays is rarely consumed fresh and mostly sold as low-quality dried fish for human consumption or as animal fodder.
In order to introduce necessary management strategies to effectively protect these species from over-exploitation and eventual extinction, detailed information on their birth rate, growth rate, and mortality are required to effectively determine stock size and other key variables. However, given that these species are incredibly large and pelagic, very little data is currently available. Manta rays for example can grow up to 7 meters from wing-tip to wing-tip and weigh as much as 2 tons, while the smaller mobula rays grow up to 3 meters in width; making them almost impossible to study in captivity.
My research focuses on studying these animals in the Indian Ocean, where 7 of the 11 species can be found. Not as glorious as it may sound since most of my fieldwork does not involve swimming with these graceful animals – in fact almost the opposite! I spend many (early) mornings visiting fish markets in Sri Lanka, which has one of the world’s largest non-discard bycatch fisheries for these species. Local boats target species such as tuna and billfish but their use of fishing gear such as gillnets results in large numbers of bycatch, including species such as manta and mobula rays, among others.
I collect data from landed specimens to help us better understand population structure, stock size, and seasonality (migration patterns). Tissue samples are collected for population genetics and stable isotope analyses, and to determine the accumulation of persistent organic pollutants (POPs) and heavy metals in the tissues and gill plates of these animals. Preliminary data from the heavy metals study for example have revealed high accumulations of arsenic, cadmium, chromium and lead; highlighting the dangers of consuming large quantities of mobulid meat or gill plates!
All data that I collect is fed back into mechanisms that help better manage the fisheries and trade of mobulid rays. Given that these species are not constrained by political geographical borders, some of this management is achieved through international conventions (such as CMS and CITES) that foster regional protection of species or help control trade of gill plates across international borders.
Managing fisheries is no easy feat especially as its consequences reverberate through a large proportion of coastal communities relying almost exclusively on the oceans, not just for food, but also as their primary source of income. While alternate livelihoods such as ecotourism offers opportunities for certain communities, long-term solutions must rely on sustainable fisheries management. These include: the introduction of no-fishing marine protected areas (MPAs) surrounding critical habitats that provide species the opportunity to breed, grow and “over-flow” into neighbouring fishing grounds; seasonal closures to protect spawning events; and the promotion of sustainable fishing techniques such as pole-and-line fishing, that not just ensures tuna is of higher quality, but also prevents bycatch of vulnerable species like mobulid rays.
In order to better understand the importance of MPAs for manta rays, I also carry out a study within the Chagos MPA in the Indian Ocean; the world’s largest enforced no-take MPA. Together with colleagues, we place satellite tags on manta rays to identify their ranges and critical habitats, collect information on their planktonic prey, and determine factors that affect their migration. This information is vital to determine if MPAs can effectively protect large megafauna such as manta rays and help identify other critical habitats in the Indian Ocean that require protection.
Even MPAs such as Chagos are sometimes threatened by illegal fishing activities of neighbouring countries and throughout all oceans, if global fishing pressures are allowed to continue and expand at present levels, many species including the manta and mobula rays will be pushed towards extinction, forever altering the marine ecosystem as we know it, and with that the livelihoods of millions depending upon the oceans. Change is needed now and data from projects such as this are critical to influence positive actions.
- By Daniel Fernando
This work would of course not be possible without Linnaeus University and support from various grants including the Save Our Seas Foundation (http://saveourseas.com/project/which-mobulid-is-which/), the Bertarelli Foundation, the Marine Conservation and Action Fund, and WildAid.
Want more details on my project? Check out this short documentary created by Al Jazeera: https://www.youtube.com/watch?v=moYsYktJB-8
To learn more about manta and mobula rays in general, and to follow our global conservation efforts, “like” us on Facebook: www.Facebook.com/MantaTrust