A group of people damaged wildlife and disturbed protected species after driving 4×4 vehicles through a river, it has been claimed.
David Lewis said he called Natural Resources Wales (NRW) when he saw the vehicles enter the River Usk in Monmouthshire, on Sunday.
Landowner Bev Baker said he was not present at the event and will prevent it from happening in the future.
NRW said its officers were not required to attend but is investigating.
Angling journalist Mr Lewis often fishes on the stretch of the river, which is a Site of Special Scientific Interest (SSSI) and Special Area of Conservation (SAC).
He said sandpiper birds return to the area every May to nest on a small beach.
“Where are they going to nest now? The vehicles were driving back and forth over their nest site,” he said.
Mr Lewis said anglers are also concerned because the shad – a protected species of fish – spawns in the river at this time of year.
The BBC has learned the group were taking part in a regular, marshalled event in woodland running alongside the river, but had no permission to enter the water.
“I was gutted to hear this, we have warned the club that this can’t go on and we won’t let this happen again,” said landowner Mr Baker.
Mr Lewis said he reported the incident to NRW but was told no officers were available to attend.
Responding to Mr Lewis’ claims, NRW said it is investigating the incident, adding it did not require “immediate attendance” because it did not believe any fish were killed and there was no pollution.
Gwent Police was informed of the incident.