How Does a Fish Ladder Work?

A fish ladder is a structure with a slope, baffled raceway or naturalistic slope with boulders that is constructed around an artificial  or natural barriers like dams, waterfalls or locks. 

Fish ladders also called as fishways or fish pass or fish steps provide hydrodynamic refuges for resting and it functions as uphill stairways for the fishes. 

Need for Fish Ladders

The need for fish ladders can be clearly understood by realizing the effect of hydroelectric projects on fish life cycle and fish migration. Fish migration is a part of fish life cycle that range from daily to annual basis. Fish migrate to feed or spawn. 

The migratory fish once hatched they take a journey downstream. At their end of their life, they travel upstream to span. This cycle is very important to balance the aquatic ecosystem and fish population. As dams restrict this movement of fish to the upstream, fish ladders were are introduced. 

Note: Catadromous fishes are species of fishes that live in rivers and spawn in sea. Anadromous fishes are the ones that live in stream and spawn in rivers by travelling upstream. 

Hydroelectric projects affect the fishes in the following ways:

1. Block the migration of the fishes upstream. ( Downstream migration through turbines or spillway is fatal)
2. Many fishes and specifies cannot adapt to artificial lakes
3. Dams changes the downstream river flow that affect many species
4. Sedimentation, low oxygen level deteriorates water quality and kill fishes
5. Mobility of species (Fresh water molluscs, crustaceans, and other benthic organisms) is limited 

The main purpose of fish ladders is to act as an alternative passage to provide easy access to the fishes over or around a barrier or obstruction. 

A Salmon Fish Ladder

Structure of Fish Ladders

The structure takes the form of a ladder, that consist of a series of pools arranged in ascending order as shown in figure below. The fishes swim through this against the stream of water.

Fishways enable the fishes to pass through the barriers or baffles by swimming and leaping up to the steps or ladder to the water on other side. The velocity of the water falling should be high enough to attract the fishes away from the barrier and towards the fish ladder. But the velocity of water should not be so high that it would wash out the fishes downstream or exhaust them.

Also Read: 

1. Buttress Dam- Working Principle, Features and Components
2. What is Diversion Dam?