In power "emergencies, " however, those fish passage measures can be suspended to allow for additional hydropower generation.

CNN: Energy crisis affects Snake River dam proposal

They say that the federal Fish Passage Center, created by the Northwest Power Planning Council to monitor fish counts and water flows on the Columbia and Snake, has documented a direct correlation between higher flows and salmon survival.

CNN: River watch: Is Columbia tapped out?

The Environment Agency said while the passage of tagged fish over the barrage had been low, it had found the numbers of young salmon upstream had increased, which underlined the complexity of the issue.

BBC: Fears over Tees Barrage impact on fish

As approximately 66% of the Mekong fish species are migratory their passage will be blocked by the dam in its current design which will seriously reduce the reproduction rate and overall fish populations downstream will be reduced.

CNN: STORY HIGHLIGHTS