The fish died when pollution contaminated the water supply to the EA fish farm at Maerdy, in Denbighshire. The pollution is believed to have originated upstream of the hatchery in the Afon Ceirw, a tributary of the River Dee.

EA investigators believe they have identified the cause of the contamination but are waiting for laboratory confirmation. “Once we have confirmation of the cause of the pollution we will consider pursuing formal enforcement action, including the recovery of costs, against those responsible,” a spokesman said.

“It is essential that all those who keep potentially polluting materials in this catchment do so in such a way that they cannot cause pollution. The Dee and its tributaries are an extremely sensitive and important environment and we will not hesitate to take action against those whose actions or inaction result in its pollution.” He added.

The Dee is still recovering from the death of more than 100,000 coarse fish last July. Salmon bred at Maerdy are used to replenish fish stocks in the Dee catchment.