The fish counter at the Tees Barrage only monitors the upstream migration of salmon and sea trout through the fish pass.
Additional routes exist through the:
main barrage gates
turbine fish pass
To help fish migrate, the Environment Agency and the Canal and River Trust keep the main barrage gate open as much as possible. When it’s open, more fish migrate through this gate than through the fish pass. This may explain the lower fish counts when the main barrage gate is open.