Four Indonesian farmers and a 13-year-old boy have admitted they stabbed, clubbed and shot a critically endangered orangutan at least 130 times with an air gun to protect their pineapple crop.

Villagers spotted the wounded orangutan in a lake in the East Kutai district of East Kalimantan province on Borneo two weeks ago. It was taken to a clinic at an orangutan protection centre, but died while being treated.

Local police chief detective Yuliansyah said four male members of a family, including the 13-year-old, and their neighbour were arrested last week.

An X-ray showed at least 130 air gun pellets in the great ape’s body, including more than 70 in its head, the Centre for Orangutan Protection said.

Tests found the animal was blinded as a result of the shooting and also had 17 open wounds believed to be caused by sharp objects. Its left thigh, right chest and left hand were bruised from blunt object trauma.

If found guilty of violating the National Resources Conservation Law, the adult suspects face up to five years in jail and fines of £5,200. The boy could face half the adult punishment at a juvenile detention centre.

Mr Yuliansyah said the boy will continue to attend school while the case is under way.

The numbers of orangutans on Borneo and on the Indonesian island of Sumatra, recognised as separate species and both classified as critically endangered, have fallen precipitously since the 1970s. A new study published last week estimated that the population on Borneo has dropped by more than 100,000 since 1999.

Orangutans are a protected species in Indonesia and Malaysia, but deforestation has dramatically reduced their habitat and brought them into contact with farmers and plantation workers who kill them to protect crops, and also for meat.

Indonesia has lost half of its rain forests in the last half century in its rush to supply the world with timber, pulp, paper and, more recently, palm oil.

In mid-January, an orangutan was found decapitated and shot more than a dozen times with an air gun in Central Kalimantan, environmental news website Mongabay reported. Police arrested two rubber farmers suspected in the killing.