Illegal dumping of sheep dip is being blamed for a fall in the numbers of brown trout in a mid Wales river.
The river Teifi used to teem with brown trout
Samples taken from the river Teifi between Tregaron and Lampeter show levels of the sheep dip Cypermethrin which could be damaging to invertebrate life such as fly hatch.
These insects are the food for brown trout and a decline in fly hatch is thought to have contributed to the fish's decline.
The Environment Agency is worried that farmers are not being careful enough when they get rid of sheep dip.
The feeling amongst farmers is that there isn't a huge problem of illegal dumping of dip on the upper Teifi,
But the Farmers' Union of Wales claims that the entry of chemicals into watercourses could only have happened by accident.
"The feeling amongst farmers is that there isn't a huge problem of illegal dumping of dip on the upper Teifi, said Dai Jones, the FUW's area officer, based at Pontrhydfendigaid.
Since 1999, agency authorisations have been needed to dispose of dip onto farmland.
But only 30 farms along the upper Teifi have these.
"Not many farmers use Cypermethrin-based dips any more, and because licences are needed the agency should be able to trace whoever is using it," added Mr Jones.
"But I am sure that whatever has happened has taken place by accident."
Visits to local farms during the next few weeks will now form part of the Environment Agency' investigation into the problem.
"The upper part of the Teifi has historically supported a widely acclaimed fishery for brown trout," said Paul Varallo, the agency's environment manager.
"If this fishery can be returned to its former status there will be considerable economic benefits for the local community."
Sheep dips are designed to kill unwanted insects on sheep and they are particularly toxic to the insects that live in rivers.
A project to re-stock the river started in 2000
Dips can enter watercourses from several sources: directly through drain holes; from dipped sheep being allowed access to streams before they are dry; or from land where the dip is spread.
Alan Williams, a fisherman who sells river permits at Lampeter, said he believed it was not just farmyard pollutants which were responsible.
"I think that people in houses tend to disinfect everything these days and this stuff is going through drains and into the river."
In 2000 the upper Teifi was restocked with young brown trout in an effort to boost dwindling stocks.
The five-year project, aimed to release 3,000 young fish there.