As Aboriginal sea ranger Dion Cooper's patrol boat cuts through Australia's northern waters, he maintains a running commentary about what seems a featureless blue expanse.

Off the port bow is a breeding ground for dugongs, the slow-moving sea cows still hunted by Aborigines in the Northern Territory, further on, a spot where traps have been laid to snare a five metre (16.4 foot) crocodile.

"He's a crafty one," Cooper says with a laugh, his voice barely audible over the roar of the outboard motor. "We haven't got him yet, keeps giving us the slip."

But Cooper turns serious as his open-topped aluminium craft approaches a submerged reef where an Indonesian fishing vessel ran aground about three years ago.

"It was stuck, they couldn't get away that time," he says.

Cooper is a sea ranger off the Northern Territory's Arnhem Land, part of an effort by authorities to use indigenous know-how to curb illegal fishing in the remote area.

Under a pilot programme, Australian Customs and Northern Territory Fisheries have contracted the Djelk Aboriginal people to patrol a 2,000 square kilometre (772 square mile) area of water off their traditional coastal lands.

About 30 rangers using six boats conduct daily patrols looking for illegal foreign fishing vessels and Australian commercial fishing boats not authorised to be in the area.

"A few years ago we would see them everywhere," Cooper said of the foreign vessels, which mainly seek shark fins for Asian markets.

Djelk ranger coordinator Shaun Ansell said the poachers used "incredibly destructive" fishing methods, including gill nets which indiscriminately scour the ocean and long lines that ensnare bird life.

"It was very distressing for our rangers to come across this massive waste of the resources that we all depend on around here," Ansell said.

He said that the Djelk rangers' patrols for Customs had been an outstanding success, with the number of foreign fishing vessels sighted in the area falling to zero shortly after they began three years ago.

"It's been a great partnership between the Djelk and Customs," he said.

"Customs get local guys who know the area intimately, they're out there day to day keeping a really close eye on what's going on -- they can detect things in an area where the Australian government can't put resources on a large scale."

While large Customs ships still conduct weekly sweeps on the high seas, Ansell said his rangers came into their own in the shallower coastal waters most often targeted by maritime poachers.

"We've got the local knowledge and the boats and the skill to get out there into the coastal environment, into all the small creeks where the (illegal) boats go and hide," he said.

The Djelk rangers have also proved successful against Australian commercial fishing vessels exceeding their quotas or operating in sacred Aboriginal sites, assisting in three successful prosecutions in the past year.

The rangers do not have the power to arrest or apprehend illegal fishers but use high-tech equipment on their patrol boats to immediately report poachers to authorities and then shadow their movements to ensure they do not escape.

They also monitor the coast for crocodiles, relocationg them to uninhabited areas where possible, as well as collecting debris such as nets and driftwood discarded by foreign ships that could harbour exotic pests.

Ansell said if the pilot programme was expanded to other indigenous communities it would give Customs a cost-effective way of protecting Australia's vast northern coastline.

"There's the potential there to create a web of surveillance right across the remote north," he said.

It also gives local Aborigines such as Dion Cooper full-time work in an area where jobs are scarce and allows them to protect waters their ancestors have fished for thousands of years.

"I get to go out on patrol and after we do that I sometimes have time for fishing," he said.

"It's the best job in the world and I get to do it every day."