Where Have All The Manta Rays Gone?

            With an 18-22 foot wingspan, soaring motion, and gentle disposition, manta rays are one of the truly amazing creatures to watch underwater.  They are harmless to humans, and considered curious and friendly to divers.  There are two different species of manta rays, Manta birostris (the giant oceanic manta) and Manta alfredi (the resident reef manta).  The migratory giant oceanic manta is known to be found in tropical, sub tropical and temperate waters, and have a much larger geographic range than the reef manta, who is commonly sighted inshore around coral reefs, tropical island groups, atolls and bays.  If the manta ray is harmless to humans and is such a fascinating creature, why are their population numbers dwindling? Just like most wild animals, manta rays have number of natural predators. 

Their survival is most severely threatened by growing pressure from fisheries. They are in high demand for their gill rakers, which are cartilaginous structures that branch off the gill arch and are used to filter plankton. Since manta rays move in predictable patterns, they are known to be easy targets, causing this kind creature to be netted, trawled, and harpooned for this specific body part.  In recent years, fishermen have begun targeting manta rays with modern fishing gear making fishing an even simpler process.  It has been declared that each year, fishermen catch about 97,400 manta rays.  Unreported catches will mean the true numbers to make the catch rate even higher.  With the manta ray classified as vulnerable by the International Union for Conservation of Nature, this means they are facing a high rate of extinction. Manta rays are also subjected to unsustainable fishing pressures sparked largely by demand for use in Chinese medicine.  Their gill rakers are promoted as a cure for a wide array of sickness’s ranging from smallpox to cancer.  Manta rays also provide a cheap substitute for shark cartilage used in nutritional supplements. 

Bycatch is another major threat to the manta ray, meaning that they are “incidentally” caught in industrial fishing nets throughout the waters in which they roam.  Longlines and gill nets used primarily to capture tuna are most frequently responsible for manta rays becoming entangled, usually resulting in death.  Unfortunately, mantas cannot swim backwards, and so attempts to disentangle themselves often results in becoming more tangled. Mantas, which become entangled in nets quickly, die because they are unable to actively pump oxygen rich water over their gills in order to respire. Fishing line can also cause severe entanglement resulting in deep, and often life threatening lacerations if mantas become entangled in it.

Both species of manta ray frequently visit coral reefs to feed and be cleaned, but because coral reef degradation is occurring rapidly, another threat to the manta ray is habitat destruction.  If reefs aren’t striving, the organisms on which manta rays feed and rely on to thrive, will not be abundant putting the manta ray at risk.  Marine debris is another major threat caused by pollution in the water.  Although manta rays are protected in some countries, they migrate through unprotected waters of others making them very vulnerable to be hunted.  Female manta rays are not able to give birth until 15 years of age, giving birth to one pup every two to three years over a lifespan estimated to be about 40 years, only producing a handful of pups in her lifetime.  Because this rate is so low, only a small percent of manta rays can afford to be fished before there is a dramatic decline in their population.