MANTA RAYS

In association with the Hawaii Association for Marine Education and Research, Inc. For more information on this fine organization, visit their web site at www.hamerhawaii.org. Our thanks to Mark Deakos – President, Co-founder, Treasurer, and Chief Scientist.

Food and Foraging

Manta rays are filter feeders, preying on microscopic organisms such as copepods, mysids (small shrimp-like creatures), and the larvae of fish, lobster, and octopus. When they are feeding their cephalic lobes are unfurled, directing passing water and plankton into their mouth.

They sometimes swim in repeated summersaults, remaining in a dense patch of plankton. The excess water exits through five pairs of gill slits on their ventral surface and is termed ram-jet feeding. Finger-like projections on the gill arches, known as gill rakers, strain and capture the food.

Manta rays can be found in large aggregations in areas with limited nutrient-rich waters found in the tropics. An adult manta ray may feed on 60 pounds (27 kg) of microscopic plankton, fish larvae, copepods, and zooplankton in a single day.

Status and Conservation

The number of manta rays that exist worldwide is unknown. They used to be hunted commercially for their liver oil and skin. The hunters would kill them by harpooning them from small boats in eastern Australia and the Sea of Cortez (Gulf of California). Today, they continue to be hunted in Mexico, Indonesia, and the Philippines, where their meat is considered a delicacy.

A new market has emerged in Asia, creating a demand for dried manta gill rakers to be used in traditional medicines. This demand has lead to an exponential increase in the Indonesian fishery in just a few years. This fishery is a great threat to the manta ray populations (Dewer, 2002). Also, as a result of over fishing, fishermen have turned to hunting manta rays as an alternative source of income. This has led to a ten-fold increase in manta ray harvesting. An estimated 1,500 manta rays have been taken over a period of six months in Lamakera. There is an increased fear that this harvesting could spread to the Western Pacific. Due to their very low reproductive rate, local populations of Manta fays in Indonesia and the Philippines have the potential for crashing.

Manta rays have not yet gotten global recognition as a species of concern.

The World Conservation Union categorizes them as “Data Deficient.” Being data deficient means that there is an inadequate amount of information to make a direct, or indirect, assessment of their risk of extinction, based on their distribution and/or population status. Manta rays are listed as vulnerable in the Gulf of California, west coast of Mexico, and South China Seas.

Some commercial dive operations have exploited areas where the manta rays can be seen on a regular basis. On these dives, the Mantas can approach within inches of the divers, who can’t refrain from touching them. When touched, the protective mucus layer over the skin is removed, causing pinkish skin lesions to develop that may remain for months. (note from Steve: As I have matured in this industry, I have taken the opinion that these feeding stations, such as the one in Grand Cayman called Stingray City, create unnatural expectations for the Rays and although I thoruoghly enjoyed my visit to Stingray City, I am against this type of feeding. I support the passive observer philosophy.)

Due to their large size, the only natural predator of the Mantas is the large sharks, primarily the tiger sharks. Manta rays are able to use their size and maneuverability to escape predation.

Increased data collection efforts worldwide are producing a valuable global database, providing information on manta ray populations, movements, habitat use, and behavior. Further information on the migration patterns, levels of abundance among regions and habitat preferences are necessary for global management and conservation strategies. A better picture of worldwide populations is necessary for conservation efforts both within local communities and on an international level.

While whales, dolphins, and sharks have been the focus of scientific study, little research is being conducted on Mantas. The Pacific Manta Research Group has been using direct observation and photo-identification as the main research tools to monitor Manta populations in the Sea of Cortez, and the Revillagigedo Islands south of Baja. The Manta Sight Project in Australia is collecting sighting records of Mantas for the Great Barrier Reef. The Nature Conservancy and Pfleger Institue of Environmental Research (Pier) are doing a three-year study in Komodo Marine Park, Indonesia.

Other work includes Keller and Wendy Laros’ Pacific Manta Research Foundation in Kona, Hawaii. And tourism revenues for the tiny island of Yap in Micronesia have initiated identification efforts by dive operators in Micronesia.

Hawaii

Sighting data along the Kona coast have identified about 85 mantas over the last 12 years of research. About 40 of these are re-sighted on a yearly basis. Tim Clark, a researcher with the University of Hawaii, has tagged and tracked many of these animals. It appears that the population within Kona is residential and does not migrate away from the coast. Therefore, it should be treated as a closed population. Populations along the Maui coast are currently unknown.

Manta ray night dive operations off Kona produce over 2.5 million in revenues annually through eco-tourism. In contrast, a single dead manta ray in Indonesia sells for approximately $160 (Dewar, 2002).

Although they are not protected in Hawaii, manta rays are not hunted commercially. Due to the small populations in Hawaii, a Manta fishery could never be sustained. Hawaii State legislature is considering enacting a law that would protect manta rays and make it illegal to kill, capture, or intentionally injure manta rays in Hawaiian waters.

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