Dictionary of Ichthyology  

ladder = a series of steps with flowing water and pools enabling a fish to circumvent an obstruction such as a dam by leaping from step to step. Also called fishway, fish pass.

Barrage de Verbois on the Rhône River, Switzerland and Nishiotaki Dam, Japan from Wikimedia Commons: http://commons.wikimedia.org/wiki/File:Echelle-poisson-verbois.JPG and http://commons.wikimedia.org/wiki/File:Nishiotaki_Dam_fish_ladder.jpg

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