Similar to many fish species and marine mammals, manta rays are threatened worldwide by human actions. A new study has been published that placed the focus on manta rays in Brazil, where they are threatened by human actions.
About Manta Rays
Manta rays are large marine animals, belonging to the Elasmobranchii sub-class, a sub-class of Chondreichthyes or cartilaginous fish, which includes sharks, rays, skates and sawfish. Manta rays are peaceful animals, feeding on plankton, and are distributed worldwide. Unfortunately, manta rays have fallen victim to overexploitation by humans. Manta rays are being targeted and overfished due to the increasing trade in manta ray gill plates, and their meat is also being used as a filler for shark fin soup. Their meat is also used as bait and their skin can be served dried and deep-fried. Manta rays are pelagic filter-feeders and require constant movement to breathe through their gills, and if they become entangled in ghost fishing gear or gillnets, they suffocate and die. When they feed close to the surface, their large sizes make them prone to boat strikes, and many manta rays have been observed missing body parts or having injuries. Their life history characteristics, such as a long gestation period of a single offspring, and only breeding every 3-5 years, combined with these threats, makes them vulnerable to extinction.
Manta Rays in Brazil
The largest member of the family Mobulidae, the Giant Oceanic Manta Ray (Mobula birostris), has been listed as Endangered on the IUCN Red List of Threatened Species. In Brazil, the Oceanic Manta Ray is classified as vulnerable according to the regional Red List of Brazilian Fauna Threatened with Extinction. In Brazilian waters, the entire family Mobulidae is protected by law, which “prohibits direct fishing, retention on board, transhipment, landing, storage, transport and any kind of commercialization of the products or sub-products. If incidentally caught, individuals must be returned to sea, alive or dead at the time of their capture.” There are no commercial fisheries in Brazil that target manta rays, however many rays are still captured as bycatch or targeted by illegal fishing activities.
Brazil represents a major gap in knowledge for manta ray research, with only three scientific publications focused on manta ray population structure and ecology in Brazil. Threats facing manta rays in Brazil have never been studied and published, until now. A new paper published in the journal Global Ecology and Conservation studies the possible threats to manta rays in Brazil by compiling and analyzing a national dataset. The aim of the study was to provide information on threats faced by manta rays in Brazil, as well as to evaluate the frequency and geographic extent of fishing and entanglement of manta rays with fishing gear in Brazil. The researchers believe this information is critical to raise awareness on the need for more efficient conservation and management of manta rays in Brazil. The researchers worked with volunteers from Projeto Mantas do Brasil, an initiative of the non-governmental organization (NGO) Instituto Laje Viva, an NGO committed to promote manta ray conservation. Information on manta ray sightings was also obtained using citizen science through the Programa Cidadao Cientista, an initiative by Projeto Mantas do Brasil aimed at raising awareness and training for members of the public to contribute to research and conservation of marine species.
The researchers compiled a photographic dataset, as well as information from a literature review, and a total of 270 records of manta rays in Brazil was obtained between 2000 and 2020, with 54.1% of the records identified as the Giant Oceanic Manta Ray. Of the 270 records, 25.9% of the manta rays were wounded to some degree. 41% of the wounded rays had fatal wounds. The main cause of death was intentional captures, while the remaining wounds were attributed to accidental catch. The study also revealed that ghost fishing gear – discarded fishing gear like nets and lines that float out at sea – caused about 34% of the observed injuries, with images of animals dragging fishing gear from their pectoral and cephalic fins. The researchers were luckily able to rescue some of these animals and release them from the gear.
Threats to Manta Rays in Brazil
It cannot be disputed that manta rays are facing many threats in the waters around Brazil, but the exact significance and impact of these threats are still underestimated due to a lack of data. Despite laws in place to protect manta rays in Brazil, the results of the study indicated that manta rays are still facing many threats from different sources, including anthropogenic causes (fishing, bycatch or ghost gear), as well as boat strikes.
Natural mortality of manta rays does not seem to be a big issue, and is mostly opportunistic shark predation. Although shark-inflicted bite wounds have been observed, they are infrequent in Brazil. The lack of shark-inflicted injuries in the 20-year dataset compiled by the study might be due to a low density of sharks along the Brazilian coast (due to overexploitation for many years). The researchers also found that of the manta rays that were accidentally caught as bycatch, many were not returned to the ocean as required by law, but were rather taken to the beach and dismembered.
An interesting result of the study was the finding that none of the observed rays in the north and northeast region of Brazil were larger than 3 meters. This indicates immature individuals, and corroborates the occurrence of a manta ray juvenile population in the waters of northeast Brazil.
The large length of the Brazilian coastline, and the increased fishing pressure from illegal fishing activities due to the growing demand for manta ray products suggest that the threats identified and observed in the study are most likely underestimated. Despite having strong historical ties to fishing, Brazil is still deficient regarding information about enforcement, surveillance, and direct and indirect impacts of fishing on manta rays.
The study highlighted the multitude of threats facing manta rays in Brazil, and also the presence of different types of injuries affecting manta rays. There is a need for future studies that investigate the long term impacts of ghost gear related injuries on the mobility of manta rays, and also a need for monitoring of fishing impacts and more effective enforcement of regulations aimed at protecting manta rays in Brazilian waters,