A fish ladder is a structure which allows fish to move past an obstacle such as a dam. Without fish ladders, fish would be stuck downstream of the obstacle, and this could potentially have a negative impact on the breeding cycles and lifestyle of the fish. Many dams are equipped with fish ladders, often to satisfy requirements from government agencies, and some dams have turned their fish ladders into wildlife viewing areas, allowing visitors to watch the fish as they move up the ladder.
The term “ladder” might seem a bit confusing for people who are trying to figure out how fish could climb up a conventional ladder. Fish ladders are actually made by creating a series of stepped pools. The fish can jump from pool to pool, eventually coming out on the other side of the dam or obstacle. There are a number of different designs and configurations for the pools. For especially large dams, people may use a fish elevator instead. Just like a people elevator, a fish elevator fills with passengers and then rises to the top of the dam before opening to allow the fish out on the other side.
Depending on where one is, fish ladders may be referred to as fishways, fish passes, or fish steps. They are usually designed in such a way that the water becomes noisy, attracting the attention of fish who may be struggling at the foot of a dam. The water is kept calm enough for the fish to be able to swim in the pools, as the designers of fish ladders do not want the fish to get so tired that they cannot navigate the fish ladder successfully.
In some cases, the fish ladder may be mounted in a glass chamber on the side of the dam, allowing observers to watch the fish as they move up the ladder. This can be useful for biologists who want to study the movement of fish populations and look at how dams are impacting fish in a particular region. Visitors to the dam may also enjoy watching the fish as they leap from pool to pool, or hop down the fish ladder, depending on the time of year and the species.
Local wildlife also may become interested in fish ladders, since the ladder confines the fish in a narrow inlet, making it easy to swoop in and grab a snack. Bears, eagles, raccoons, and other fish-eaters tend to congregate around fish ladders, regarding them as a movable feast of fish species.
People have recognized the impact of dams on fish since at least the 1600s, when fish ladders first began to be constructed. Modern versions differ little from the fish ladders of the 18th and 19th century, illustrating the soundness of the basic fish ladder design.