"Officers found a bag containing oversized barbel which had been removed from the water which is an offence under our national byelaws."
The agency said it was now being investigated by its enforcement officers.
Dave Throup, Environment Agency manager for Herefordshire and Worcestershire, said: "As a result of intelligence received fishery enforcement officers found and reported illegal fishing and fish theft on the Severn at Bridgnorth.
"These operations are funded by money we receive from rod licence sales."
Those caught fishing illegally can be given hefty fines up to £2,500.
Anyone with information about illegal fishing activities can contact the Environment Agency Incident Hotline 24/7 on 0800 80 70 60.
Alternatively, contact Crimestoppers anonymously on 0800 555111 or at crimestoppers-uk.org