“Observers thought that what was killing salmon in hydroelectric dams was the blunt trauma of striking the turbine blades, or maybe concrete. But the Sensor Fish and its armory of sensors discovered that the culprit was an abrupt change in pressure as a smolt falls past the blades.
The pressure drop sometimes causes a fish to “burp” from its internal air bladder so by the time it reaches the tailrace (the bottom pool of the dam) it is negatively buoyant. In other words, it sinks like a stone. In order to restore its internal equilibrium, the smolt immediately surfaces to gulp some air. And at the surface an enemy awaits.
Birds of prey have figured out that the tailrace of a dam is an all-you-can-eat salmon buffet. They swoop down on the traumatized, beleaguered fish and add to salmon mortality.” More