Fish passes (often called fishways or fish ladders) are systems designed to create an artificial pathway to fish species in river stretches with otherwise unsurmountable or hardly overrun obstacles. The efficiency of a fish pass depends on the site, associated uses and fish species potentially using the path created. Several factors affect the choice of the type of fish pass to be installed, such as the biology and behavior of target species, water velocity, flow patterns and water level variations, topographical constrains or the slope to be overcome. Pool-type fish passes are the most frequent in small to medium height obstacles, while fish locks and fish lifts are more common in larger dams.