In the United States, more than 2 million dams and other barriers block fish from migrating upstream to spawn as a result, many fish populations have declined. For example, Atlantic salmon used to be found in every river north of the Hudson River. Some manmade devices have helped like the fish ladder. What is a Fish Ladder?
Fish ladders found along fish migration routes are structures that are located in waterways, usually rivers, alongside manmade or natural obstructions like Dams, waterfalls, and locks that impede fish migration patterns, fish ladders are also known as fish steps, fish passes, or fishway.
Designs vary depending on the obstruction, river flow, and species of fish affected, but the general principle is the same for all fish ladders: the ladder contains a series of ascending pools that are reached by swimming against a stream of water. Fish leap through the cascade of rushing water, rest in a pool, and then repeat the process until they are out of the ladder.
What is a Fish Ladder
The survival of many fish species depends on migrations up and down rivers. Among anadromous fish such as salmon, shad, and sturgeon, downstream migration is a feature of early life stages, while upstream migration is a feature of adult life.
Among Catadromous (migrating down rivers to the sea to spawn) like the North American eel, the opposite is true. River obstructions such as dams, culverts, and waterfalls have the potential to slow or stop fish migration. Indeed, these impediments to fish migration are often implicated in the decline of certain fish stocks.
A fish ladder, also known as a fishway, gives a detour route for migrating fish past a particular obstruction on the river. Designs vary according to the obstruction, river flow, and species of fish affected, but the general principle is the same for all fish ladders:
the ladder contains a series of ascending pools that are reached by swimming against a stream of water. Fish leap through the cascade of rushing water, rest in a pool, and then repeat the exercise until they are out of the ladder.
Fish ladders have a mixed record of effectiveness. They vary in effectiveness for different types of species, with one study showing that only three percent of American Shad make it through all the fish ladders on the way to their spawning destination.
They are fish ladders, dams, weirs, etc. A fish ladder is constructed to allow fish to pass over a dam by providing water velocity which a fish can negotiate while pushing upstream.
But when the flowing water passes over the steps of the ladder, the action is like a cascade aerator. Even small dams like weirs that are a meter tall can stop fish from swimming upstream because they are higher than most fish can jump.
Types of River Water Fish Ladders
Different types of Fish ladders are as follows :
- Pool and Weir fish ladder
- Pool and Orifice fish ladder
- Vertical slot fish ladder
- Baffle fishway
- Rock ramp
- Fish elevator
- Siphon fishway
Pool Fish Ladder on Rivers
1. Pool and weir fishway or fish ladder is a widely known fish pass structure. It consists of a series of small overflow weirs and pools of regular length. The pools are constructed in the form of steps and these pools are divided by overflow weirs.
2. Pool and orifice fish ladders are almost similar to the pool and weir fish ladders and the only difference, in this case, is the overflowing weir is provided with a submerged orifice within its body. Hence, in this case, the fish can move upstream by just passing through each orifice rather than jumping over the weir crest.
3. Vertical slot fishway another variation of pool and weir fishway. In this case, the weirs are replaced by walls with vertical slots so that the fish can pass through these slots from pool to pool and upstream quite easily. In some cases, multiple vertical slots are also provided.
4. Baffle fishway is another type of fishway that is in the form of a rectangular channel with a series of equally spaced baffles perpendicular to the direction of flow. Generally, in the case of baffle fishways, water flows continuously without resting unlike in pool-type fish ladders however, pools can also be provided in between baffle walls if needed.
5. Denil Fishway is a classic baffle fishway in which baffles are provided on the sides and floor of rectangular passes. While in the case of the Larinier fishway and Chevron fishway baffles are provided only on the floor of the fish pass. Alaskan fish passes are recommended for steeper slopes.
6. Fish elevators also called fish lifts are another type of fishways in which fish are lifted by a water-filled chamber from downstream to upstream. Fish lifts are well-suitable for tall obstructions such as arch dams and high weirs.
7. A fish siphon or siphon fishway is a closed fish pass which is provided between two watercourses. The fish enters into the siphon tube which is partially filled with water and the flow rate in this tube is controlled by the siphon effect.
Solutions, like fish ladders or fish capture and transport, can address barriers to upstream and downstream passage of migrating salmon, while still providing flood control or water supply. Other solutions include removing or replacing culverts and removing obsolete dams.
Compared to ladder-type fish passes, the fish elevator facilitates a huge amount of fish migration at a given time. Some large-sized species may not travel upstream in ladder-type passes due to the small size of openings, and poor swimming capabilities. For such types of species, Fish elevators work best.
There are more than 80,000 dams in the U.S. and nearly all of them have some kind of fish pass. They range from multi-step ladders like the Seattle one to elevators that suck the fish upstream to nature-like diversion canals
To help fish surmount the looming wall of a dam and reach upstream waters, dams are fitted with stairlike structures called ladders (fish leap up a series of pools) and elevator-like contraptions called lifts (fish are channeled into a hopper that gets raised)
Conclusion:
A fish ladder, also known as a fishway, gives a detour route for migrating fish past a particular obstruction on the river. Designs vary according to the obstruction, river flow, and species of fish affected, but the general principle is the same for all fish ladders; the ladder contains a series of ascending pools that are reached by swimming against a stream of water. Fish leap through the cascade of rushing water, rest in a pool, and then repeat the exercise until they are out of the ladder.
