Campaigners claim the Tees barrage is damaging fish stocks in the river.
Salmon and sea trout are being held up at the barrage
The Anglers Conservation Association (ACA) is now considering legal action if measures are not taken to protect fish such as salmon and trout.
The organisation says the existing fish pass built into the 10-year-old barrage does not adequately protect fish from foraging seals.
British Waterways, which manages the barrage, has promised to mount a study into ways of improving the situation.
ACA solicitor Guy Lilley Allen, said: "The problem with the barrage is the salmon and sea trout are trying to run up the river, coming up to the barrage and are being delayed because they cannot find the very small opening of the fish pass and consequently seals are having an absolute feast.
"The seal population is increasing just in front of the barrage because they know they're on to a good thing with the fish being held up.
"The simple solution is to build a new fish pass to get them past the barrage more quickly."
A British Waterways spokesman said: "The whole of the barrage is operated as a fish pass by BW staff who operate the locks and the barrage gates to allow migrating trout and salmon to pass upstream.
"Undoubtedly improved water quality has led to an increase in migratory salmon and sea trout travelling back up the Tees estuary and this has led in turn to increases in the sightings of predatory mammals such as seals.
"The activities of these predatory species are almost certainly impacting on the salmon and coarse fish populations and the Environment Agency have recommended detailed and lengthy scientific survey work.
"Only then can we understand how significant these impacts are."