Fisheries officers have been stoned by poachers as they tried to seize illegal salmon nets on the River Foyle.
More than 24 adult salmon caught in the nets were killed as the Loughs Agency officers were forced to drag the nets quickly aboard their boats and leave the scene to ensure their own safety.
Loughs Agency director of conservation and protection, John McCartney, said officers have been patrolling the river day and night during peak migration time between May and August to curb the illegal activities of the poachers.
"We are operating with a small number of very tired and exhausted staff," he said.
"They're out every night, every morning, and working through the night too.
"There are some nights when they are going into the netting hotspots and they are outnumbered four to one."
In previous years poachers on the Foyle system have even gone as far as to drop concrete blocks on to the boats used by the fisheries officers.
"Last Thursday we were on the Upper Lough Foyle and the crew, who were in boats, had identified that the nets were there," Mr McCartney said.
"Once we start to take the nets in, we would pull them in very slowly to release the fish and try to keep them alive. But in this case we came under a sustained stoning attack and we had no choice but to get the nets and get out.
"Nobody was injured, thank goodness, but it wasn't for want of trying on their part.
"There were stones going over the boat, there were stones hitting the boat, and some stones glanced off people."
Mr McCartney said the neighbouring River Finn has been named a Special Area of Conservation at European level for its salmon population, but is currently failing to reach the population size set as its conservation limit.
"The poaching is having such a big effect on the population that we do try to get as many nets as we can," Mr McCartney said.
"Fisheries officers are patrolling the system round the clock during the peak salmon run season between May and August."
He added: "Twenty-four good adult salmon were killed and just one of those salmon would have been a lifetime's catch for an angler.
"But I think we are winning the battle. Every time we take the nets, it makes less financial sense for them.
"All we can do is keep trying and keep trying."
In a letter to angling organisations, Mr McCartney said the large number of fish killed was of significant regret to staff, but they would continue to target poachers.