The arrested man was detained on suspicion of discharging a firearm within 50ft of a public highway, and was later released on police bail pending further enquiries.
According to the 1986 Highways (Amendment) Act, however, discharging a firearm 50ft from a public highway is not a crime in itself.
It is only grounds for arrest if in consequence, a user of the highway is injured, interrupted or endangered.
Anglers raised the alarm after hearing gunshots and a police helicopter and armed officers scrambled to the scene.
It is believed the group, which had nets with them as well as the gun, were trying to land fish to take them away for the table.
Eastern Europeans taking freshwater fish to eat are considered a serious threat to British fish stocks in some quarters because of the methods they use to catch them.
In recent years, officials have put up ?no poaching? signs in different languages on waterways, while fisheries have begun microchipping fish to deter thieves.
Bruno Broughton, chairman of the Professional Coarse Fisheries Association, said: ?Poaching by foreign nationals is a significant but localised problem. Firing shotguns at fish used to be a thing keepers did to get rid of pike in trout fisheries. It is not a clever or effective thing to do now, though. It makes a lot of noise and draws attention.?
There may have been an extra incentive for the poachers, as freshwater fish are an important part of the Christmas menu in many Eastern European countries.