Anglers in Port Talbot say salmon and sea trout stocks in the Afan river are at risk due to falling water levels.
The Environment Agency said the problem centred on a weir
They say dry weather and water extract for nearby industries, including the Corus steel plant, make it difficult for fish to migrate.
The Environment Agency admits there is a problem and said it was working with all parties to try and solve the issue.
Corus said it had complied with any regulations set down and had met with members of the angling community.
John Phillips, of the Afan Valley Angling Club, said its members had worked hard in recent years to try to boost the numbers of fish in the river.
"It's a fabulous littler river - which over the last 40 years has been reborn after the demise of the coal mines in the valley," he said.
"Salmon and sea trout have returned after much effort by the angling club.
"However, it now seems that all of the good work is lost."
He said the recent dry spell was partly to blame but local industry, including Corus and Port Talbot docks, also drew water from the river.
"If only part of this water could be allowed to leave the river naturally through the estuary, maybe the Afan's salmon would continue to return," added Mr Phillips.
The Environment Agency Wales said it understood the fishermen's concerns.
"We are currently working with industry and the local community to resolve this historic problem," said a spokesperson.
It said the problem centre on a weir.
"Our aim is to maximise the flow of water going over the Green Park Weir and to allow the unimpeded migration of salmon and sea trout - this will improve the natural fish population."
A spokesman for Corus said it worked hard to balance the need of industry with the environment.
He said it worked closely with the Environment Agency and complied with any regulations set down.