Salmon have returned to the upper reaches of the River Wye in mid Wales after acid rain wiped out stocks 20 years ago, say experts.
The Wye and Usk Foundation has worked to improve salmon stocks
Lime has been added to water to help neutralise the chemical's impact and this has encouraged fish to breed again, said the Wye and Usk Foundation.
The "liming process" began in 2003, but salmon only started returning to certain remote locations recently.
Stocks began falling in 1975 and within a few years disappeared in some areas.
The foundation, which is leading the project to reintroduce the salmon, has added nearly 2,000 tonnes of lime to the upper reaches of the Wye.
Fish are now being found in the Tarennig and Bidno, tributaries of the Wye, and in parts of the river itself near Llangurig, Powys, for the first time in 21 years.
It is formed when fossil fuels are burned
They release sulphur dioxide (SO2) that is absorbed by rain
It is also caused by nitrogen oxides (NOx)
It can pollute lakes, reduce fish stocks and damage plant life
Levels are lower than they were a decade ago
Foundation spokesman Stephen Marsh-Smith said acid began subsiding in the 1990s, but the geology of the upper Wye did not have the natural capacity to neutralise the chemical.
"Somehow the fish instinctively knew that certain locations in the upper reaches of the Wye were not suitable for breeding," said Mr Marsh-Smith, who is based in Erwood, near Builth Wells.
"The numbers of salmon started reducing in 1975 and by 1985 had disappeared.
"Acid levels started to naturally reduce in the 1990s, but it wasn't until recently that we discovered salmon again in remote areas."
Mr Marsh-Smith said the foundation was the lead partner in a project to improve water quality in the Wye.
Other partners include the European Union, Welsh Assembly Government, Environment Agency and Countryside Council for Wales.