Six foreign fishing vessels were caught trespassing in Indonesian waters without a proper license and detained by local authorities on Tuesday.

Two patrol boats belonging to the Maritime Affairs Ministry seized two fishing vessels from Malaysia in the Malacca Strait and four vessels flying the Vietnamese flag in the North Natuna Sea on April 9, The Jakarta Post reported.

Agus Suherman, a leading official from the Ministry, said more than 30 crew members from the ships were detained for not having a proper license while in Indonesian waters.

They were also caught using a trawl for fishing, which is an outlawed form of fishing in the country and considered unsustainable.

The boats from Vietnam were caught at around 8am by the boat KP Hiu 011, commanded by Captain Samson. Some 24 Vietnamese crew members were arrested from those vessels. They were then taken to Pontianak Marine and Fisheries Resources Monitoring Base in West Kalimantan for legal procedures.

The Malaysian boats were caught in the Malacca Strait at around 3pm on the same day, with nine crew members being arrested by the boat KM His Macan Tutul 002, led by Captain Ilman Rustam. The nine crewmen were found to be Myanmar citizens.

The Malaysian vessels and crews were taken to the Batam Marine and Fisheries Resources Monitoring Base in the Riau Islands.

The crewmen caught now face the threat of jail terms of up to six years, plus a fine of up to 20 billion rupiah (US$1.4 million) if found guilty of violating Law 31/2004 for illegal fishing.