Due to their large size and manoeuvrability, mantas have few natural predators. Orcas and large, warm-water sharks such as tiger sharks have been known to prey on mantas.
Despite this absence of natural predators, scientists suspect that global populations have declined by as much as 30 percent in the last 75 years. In today’s increasingly exploited oceans, mantas are under threat from habitat degradation, tourism development, fishing net and anti-shark net entanglements, and boat strikes.
But perhaps the biggest threat to mantas is fishing. Their large size, slow speed, and tendency to be found on the surface make mantas an easy target. Manta fisheries exist in several countries, including the Philippines, Mexico, Mozambique, Madagascar, India, Sri Lanka, Brazil, Tanzania and Indonesia. Elsewhere, mantas are often caught as bycatch.
In places like Indonesia, manta fisheries were traditionally subsistence fisheries. Mantas were hunted using harpoons from homemade wooden boats powered by paddles and sails woven from palm fronds. An average community fishery might catch around 200-300 mantas per year.
In recent years these same fisheries have increased exponentially. Today, a village fishery may land in excess of 2,400 mantas per year using motorboats and commercial fishing techniques. This increase is the result of two significant developments.
First, as pressure on local fish stocks have increased, fishermen have been forced to look to the manta as an alternative source of meat. Increasingly, manta cartilage is being used as a low-cost filler in shark fin soup, supplementing dwindling shark catches.
Second, and an unfortunate consequence of the rapidly growing economies throughout Southeast Asia, is the increasing demand for wildlife products in traditional remedies. Mantas are targeted for their brachial gill plates which are dried and used in Chinese traditional medicine for anything from diabetes, to cancer, to the common cold. Medical science has yet to demonstrate the effectiveness of these cures.
In China, manta gill rakers can fetch as much as US$ 220 per kilo. This demand is driving the expansion of target fisheries and, with dwindling populations, fishermen are forced to travel further to find the mantas they seek. Very little of the profits filter down to the fishermen, who only gross about US$ 150 for each manta they catch.
The small size and limited migration of manta populations increases the risk of local extinction, and limits the potential for threatened populations to re-establish. Anthropogenic pressures in critical habitats, such as breeding, birthing, and nursery grounds, present a high risk during vulnerable life stages. Seasonal aggregations, ease of capture, and the lack of regulated fishery management contribute to the unsustainability of manta fisheries.