The rivers Mourne, Finn and Foyle are considered to be three of the best salmon fishing rivers in Europe - but they have now become Ireland's poaching hotspot.

It is an illegal activity and the cross-border Loughs Agency is determined to catch those responsible, by mounting daily patrols on the water and on the ground.

The operation is led by John McCartney.

"We are trying our best to apprehend people who are illegally fishing for salmon with nets," he told UTV, as he began his evening inspection of the river.

It is a fairly big operation with a number of covert surveillance teams supported by response vehicles.

John McCartney

"Our hope is that, well, nothing happens and that nobody is netting illegally for salmon in the Foyle system, but our expectation is that we will find people who are and our jobs will be to apprehend them and seize their nets."

The majority of poaching takes place between the Lifford Bridge and Claudy, where salmon returning from the Atlantic are concentrated.

It is a lucrative business - just one salmon can fetch up to £100 on the black market.

Up to seven gangs are involved in this form of criminal activity across both sides of the border and they have recently become increasingly violent.

Bricks, stones and knives have been used to intimidate and attack patrol staff at work in the Foyle fishery.

"Some of the poachers in the area have stepped this thing up quite a bit - I've had two staff recently quite badly assaulted," Mr McCartney continued.

"We have run into booby traps that have been set for us with spikes for vehicle tyres and we have come under regular stone throwing attacks."

As night falls the risks increase, with a constant game of cat and mouse playing out between the authorities and the poachers.

Teams keep watch from hiding places, while the operation is monitored and co-ordinated by a communications team, parked on a Donegal hillside overlooking the area.

The reason such a huge effort is being made is because salmon stocks are falling.

The fish are a unique, migratory species which breed in fresh water before heading out to the oceans to feed and return to breed again.

Sea trawling and climate change have impacted on stocks and fewer and fewer salmon are returning to rivers in Ireland.

"In the late 80s, for every 100 young salmon that leave our rivers to go to sea 30 of those will make it back to the coast," explained Loughs Agency biologist Paddy Boylan.

"Today we are down to about five in 100."

During UTV's night on patrol with the teams, a major poaching suspect was detained and a number of nets found, one by a canoe team on the Donegal side of the River Finn.

At the end of the patrol, the evidence is bagged and stored to be used in prosecutions.

The Loughs Agency says each net found represents a success in what they describe as the long and constant battle to save local salmon from the poachers.