Paul Hopkin, a naturalist who lives close to Sandy Water Park, has urged NRW not to use rotenone because of the knock-on impact he claimed it would have on fish-feeding birds and invertebrates. "The birds will leave if there's nothing for them to eat," he said.

Topmouth gudgeon are small freshwater fish from Asia which have spread into Europe. NRW said they reproduced rapidly, ate the eggs and larvae of other fish, survived in low-oxygen water conditions, carried the threat of new diseases or parasites - and were considered a high-risk invasive species.