For whoever it was who wondered why we don't have a load of tidal barrages producing power, here's why the Severn Barrage never got built:
The distribution of estuarine invertibrates is determined mainly by salinity levels, a tidal barrage would change the distribution of salinity, and that would change the distribution of invertibrates, and that would change the distribution of the birds that feed on said invertibrates. That's Spellerberg, 1999, reference likers...