Indonesia has detained three traders for attempting to illegally sell manta rays, the first such arrests since the world's biggest archipelago introduced legislation protecting the huge winged fish, conservationists said.

In February, the biggest manta ray sanctuary on Earth was established in Indonesia in a bid to put a stop to the creatures being hunted for use in traditional medicine, and attract more tourists.

Manta rays are now protected within the millions of square kilometres of ocean surrounding Southeast Asia's biggest nation, with fishing and export of the creatures banned.

US-based group the Wildlife Conservation Society, which operates a wildlife crimes unit in Indonesia, said that three manta ray traders were arrested in August and September on the main island of Java.

The arrests send a "clear message that Indonesia is serious about protecting its natural heritage against illegal wildlife traders," Joe Walston, the society's vice president for field conservation said in a statement released this week.

Indonesia's Marine Affairs Minister Sharif C. Sutardjo hailed the arrests, and was cited as saying: "Illegal trading of protected species threatens the sustainability of marine and fisheries resources in Indonesia."

The first arrest on August 22 involved a trader in Surabaya city in the east of Java attempting to sell a shipment that included manta ray gills, which are sought after in China for their use in traditional medicine.

A second trader was arrested in Sidoarjo, near Surabaya, on September 1, while the third was detained on September 26 in Indramayu in west Java, attempting to sell an entire manta ray.

Trafficking manta rays, whose numbers in Indonesia have fallen sharply in recent years, is punishable by a maximum fine of $25,000, the conservation group said.

The rays have wingspans up to 25 feet (7.5 metres), which they flap to propel themselves through the water, and are a favourite sight for foreign tourists, many of whom come to Indonesia to dive.

Protection group the International Union for Conservation of Nature classifies both species of manta ray -- the oceanic and reef varieties -- as vulnerable.