A fish ladder, also known as a fishway, is a specialized structure designed to help aquatic animals, primarily fish, bypass obstacles in rivers. These structures are typically built around human-made barriers like dams and weirs, or sometimes natural impediments such as waterfalls. They provide a navigable path, allowing fish to continue their upstream or downstream migrations.
Purpose of Fish Ladders
Fish ladders address the challenges migratory fish encounter due to human alterations of river systems. Many fish species, including salmon, shad, and sturgeon, undertake extensive migrations as part of their life cycle, moving between freshwater and saltwater environments or within freshwater systems. Dams, culverts, and other barriers obstruct these vital journeys, preventing fish from reaching their traditional spawning grounds, feeding areas, or safe habitats. This disruption can severely impact fish populations.
By providing an alternative route, fish ladders help maintain these migrations, which is fundamental for the survival and reproduction of many aquatic species. The primary purpose of these structures is to ensure the continuity of fish passage, thus supporting the ecological health and biodiversity of river ecosystems.
Operational Principles
Fish ladders operate by creating a series of manageable steps that allow fish to gradually ascend or descend past an obstruction. The principle involves managing water flow to create conditions that attract fish and enable their movement without exhausting them. Water flows through the ladder, typically from the higher water level above the barrier to the lower level below it. The design ensures that the water velocity within the ladder is strong enough to attract fish, but not so strong that it becomes impassable or washes them downstream.
Within the ladder, resting pools are often incorporated at intervals. These pools provide areas where fish can pause, recover from their exertion, and gather energy before continuing their journey upstream. This design mimics natural river conditions where fish might find calmer spots to rest. The consistent flow and presence of resting areas encourage fish to navigate the structure. Auxiliary water systems are sometimes used to enhance attraction flow at the ladder’s entrance, further guiding fish towards the passage.
Variations in Design
Fish ladders come in various designs, each tailored to different river conditions, fish species, and the characteristics of the barrier they circumvent.
One of the most common types is the pool-and-weir ladder. This design features a series of small pools arranged like steps, separated by low walls or weirs, over which water flows. Fish ascend by leaping over these cascades and resting in the pools between each rise. This design is effective for moderate elevation changes and allows fish to conserve energy during their ascent.
Another design is the Denil fishway, which uses a series of closely spaced, symmetrical baffles within a channel. These baffles redirect the water flow, creating a uniform, low-velocity current that fish can swim against continuously, without needing to leap. Denil fishways are often used where a more consistent flow is required or for fish species that are not strong jumpers. They can also be designed with or without resting pools.
Vertical slot ladders represent another common type, characterized by a narrow opening or slot in each baffle or wall that extends from the bottom to the top of the water column. This continuous slot allows fish to swim through each section without leaping, providing a more constant passage. The design offers flexibility in water level fluctuations and can accommodate a wider range of fish sizes and swimming abilities. These varied designs facilitate fish migration across diverse riverine landscapes.