Book excerpt: Many fish species, like salmon and sturgeon, undertake extended migrations as part of their basic behavior, and other fish and invertebrates also undertake short-term or small-scale migrations at certain phases of their life cycles. Activities such as dam construction for water supply and power generation, channelization for navigation and flood control, land drainage and wetland reclamation for agricultural and urban use all have profound impact on the aquatic ecosystem and thus on natural fish populations. Fish passes are often the only way to make it possible for aquatic fauna to pass obstacles that block their up-river journey. Based on knowledge and experience from mainly Europe and North America, this book describes the various types of fish passes, with special emphasis to "close-to-nature" solutions.