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Manta Ray Threats


threats

 1) Fisheries

In the past manta rays were mainly fished by local fishers of small communities, for the purpose of:

  • using the oil from their livers for water proofing their boats
  • using their skin for making small percussion instruments
  •  using their meat for food, where food sources were scarce as they are not considered to taste good.

 At that time they used artisanal techniques, fishing with harpoons in small wooden boats powered by paddles and sails made form palm fronts. However, now a days manta ray fisheries are overfished worldwide using big motor boats and efficient catching techniques such as trawling, nets and radars. The main reason for this is that the cartilagenous structure protecting their gills is in high demand in China as traditional Chinese medicine - which is said to cure everything ranging from diabetes to a common cold however their is no evidence to proof this. The consequence of this is that fisheries that in the past did not target manta rays and only caught them in the by-catch, are now targetting them for their gills which have a high value on chinese markets. While initailly a few hundred mantas were caught by artisanal fisheries, now thousands (primarily M. birostris) are caught each year. The main countries that take part are the Philippines, Indonesia, Tonga,  India, Pakistan, Sri Lanka, Brazil, Peru, Madagascar,  Mozambique, Somalia, Ghana and Tanzania but also American and European countries used to join the hunt for the “Giant Devil Rays”.

This change from traditional to comercial fishing generally results in a collapse of the manta populations. In Mexico (Sea  of Cortez), the manta ray fishery became commercial in the 1980s and in 2007 the fishery collapsed. A similar story occured in the Phillipines and now that population is protected.

 By-Catch

Other than targetted fisheries, by-catch is another big problem causing the incidental deaths of mantas in drum-line, shark-nets (used to protect the beaches against sharks however most sharks are found on the inside of the nets, meaning the shark is caught on its´way out to sea), and comercial nets for other target species.

2) Habitat Destruction

Habitat destruction is a huge problem for most marine life at the moment. The coral reefs, where manta rays come to feed, reproduce and clean at cleaning stations, are degrading rapidly.
 

3) Marine Debris

Pollution: such as oil spills, plastics and microplastics cause serious damage to manta rays as they are filter feeders. This means that they filter plankton and with that they intake various forms of pollution which causes internal injuries, diseases and death.

 
OLD FISHING GEAR (ghost nets, fishing lines, moorings): cause entanglement of the head and cephalic fins of the manta rays which results in:

  •          damaged cephalic fins or amputations of other parts of the body impairing their ability to feed.
  •          death as mantas cannot swim backwards and they must swim constantly to flush oxygen-rich water over their gills. So in a lot of cases they died from drowning. 


 4) Climate Change

They depend on plankton as their primary food source. Changing sea temperatures disrupt the phytoplankton’s natural ecological cycles and change the plankton distribution. So they struggle to find adequate food supplies. This forces them to change their migratory patterns frequently.

5) Marine Traffic

Marine traffic is greatly on the increases where manta rays are present. This results in frequent boat strikes, collisions and entanglement with anchor lines. All of these are generally fatal.

6) Unregulated Tourism

Big aggregations of mantas have become tourist attractions resulting in the presence of many boats in places where mantas are easy to observe and spot, a large volume of snorkelers and divers in the water close to the mantas or touching them. This causes great stress to the animals, disrupting their feeding, reproduction of cleaning patterns.

7) Captivity

Although only few mobula rays live in captivity, the unmonitored removal from the wild for the public aquarium trade may have negative impacts on small and geographically isolated populations.

8) Natural Predation

The natural predators of manta rays are: sharks, orcas and false killer whales. In most of the cases non-fatal injuries have been seen, especially with shark bites, but we do not know how these injuries affect the long term survival of mantas or their reproduction.

 On top of these threats, are not a species which can be fished in a sustainable manner as they have a:

  •          long life-span
  •          low reproduction rates due to their: late maturity, long gestation periods and give birth to only one pup every one to two years

Thus all of these threats but especially overfishing and their biological history have led to a dramatic reduction in their populations worldwide. Both Manta birostris and Manta alfredi are now listed as VULNERABLE in the Red List of Threatened Species (ICUN). However in spite of this, mantas remain unprotected in most part of the world (including Australia).Only in the places where a collapse in the fishery was declared, such as in the Phillipines and in Mexico (sea of Cortez), are the species protected.

Data is missing about manta rays

A lot of information essential to the protection of manta rays and their habitats is missing including: data about their biology, ecology and behavior.

Manta Ray Conservation

 Manta Ray tourisms generates far more income than manta fisheries. It is estimated that 50 million USD are generated by tourism each year while only 250-500 USD are paid for a manta at an Asian market. In the Phillipines a large manta ray gives less than 100 USD to a fisherman but generates 8 million USD a year in tourism (Ray of Hope).

Thus Eco-tourism with sustainable diving practices and proper codes of conduct for boat approaches as well as educational material is important for the protection of manta rays. Along with this, Marine Protected Areas and Sanctuaries are required to protect the manta rays and their habitats. Other than this is important to focus on reducing other human threats (marine debris, boat traffic, pollution. ghost nets etc). If suitable actions are taken it will still take decades for manta ray populations to return to their natural state.


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