A survey of the River Ystwyth in 1919 showed that due to mining activity in the area, the
fauna was restricted to nine species, mostly of insects. The lead concentration in the river was found to be 0.4 mg/L. In 1922, another survey was conducted; the number of species had increased to 26, with the lead concentration decreasing to below 0.1 mg/L. The number of species increased to 63 by 1939, but there were still no fish present in the river. The lead concentration fell to below 0.05 mg/L. Fish had returned to most of the river by 1975. However, there were no fish within a 3 km stretch below the Cwm Ystwyth mine. ==Gallery==