River pollution: Pearl mussels sign of healthy river - expert

The success of a critically endangered fresh water mussel is an indicator of the health of river, according to an expert.

Katie Fincken-Roberts, of Natural Resources Wales (NRW), said one of the rivers where fresh water mussels were still found was in Gwynedd.

"If they're in your river, there's not much wrong with the water quality," she said.

NRW is preparing to release pearl mussels into Welsh rivers in an attempt to save them from extinction.