In 50 years, sightings of stingrays have decreased by an average of 70%, and even by 90% off the coast of Mozambique (5), but what causes their disappearance?
Manta rays have been targeted by small-scale fisheries for centuries; but since the 1990s, rays have been fished more and more for their fins, skin and especially their gills, which are highly sought after by Chinese medicine for their aphrodisiac properties. The democratization of these traditional remedies to hundreds of millions of potential consumers could lead to the total extinction of manta rays (6). Non-selective professional fishing techniques (net, longline, purse seine, bottom trawl…) are also a scourge for manta ray populations and other large marine animals (sharks, turtles and cetaceans). Many by-catches occur during tuna fishing, a species which is highly desired for its meat and whose habitat is intermingled with that of the manta rays.
Several types of pollution threaten manta rays. The first one, not much talked about in the media and still too little known, is noise pollution. Caused mainly by maritime transport, oil and gas exploration, noise pollution prevents individuals from detecting their prey, recognizing their predators, their partners and their environment in general. It affects many marine species, especially migratory and coastal species. It is very difficult to evaluate the impact of this form of pollution on Manta rays; but it is suspected that it is the cause of their disappearance in some highly anthropized areas (7).
Plastic pollution is also a major problem for manta rays. To collect the plankton they feed on, manta rays filter a large quantity of water (more than 86 m³ per hour!) through their gills. It has been reported (Germanov et al., 2019) that rays in Manta Bay (Indonesia) can directly ingest between 110g and 980g of plastic per kilo of plankton (8). In addition, they also undergo bioaccumulation of microplastics, i.e. they accumulate in their body the plastics previously ingested in the plankton they consume.