Legal action "is almost certain" following a pollution incident in the River Teifi, Natural Resources Wales has confirmed.
More than 1,000 fish died as a result of what is believed to have been a slurry leak in the Tregaron area of Ceredigion in December.
Natural Resources Wales said it had gathered evidence and is investigating.
Anglers and local businesses have been calling for more information.
John Griffiths owns a caravan park near Llanddewi Brefi and has been phoning NRW regularly since the beginning of the year.
"No one ever gives me an answer about what's going on," he said. "I want to know what's happening with the river."
Rivers can get restocked after pollution, but according to angler Hywel Morgan, there would be no point in putting new fish into the Teifi if there is no food to sustain them.
NRW spokesman Deiniol Tegid said spring was the right time to conduct an investigation into the effects of the pollution.
He added it was "impossible to say" what form the legal action will be taken, but it was "almost certain" some form will be taken.