Why fish need ladders
Upstream and downstream - some fish need to migrate over long distances. But weirs, dams, locks and other manmade obstacles block their way. Monorails, ladders and even elevators help fish get where they need to go.
Award-winning fish elevator
For the structure on the right, a German company last week received the German Innovation Award for Climate and Environment. It is a fish elevator, which transports fish from the lower to the upper water level - and vice versa - of a weir. This elevator enables fish to cross the man-made obstacle and continue their journey along the river.
In and out
The fish elevator, constructed by the German company Baumann Hydrotec, runs mainly on water power. Fish enter a container inside a water-filled chamber at the bottom of the weir. Eventually a latch closes. Water enters the chamber and lifts the container up. The latch opens, and the fish continue their journey.
A long way to go
But why do fish need an elevator at all? The reason is that they migrate up and down rivers. Salmon are famous for their long journey. They travel hundreds of kilometers from the sea up the rivers to reach spawning grounds, where they lay their eggs. Freshwater eels also travel a long way to reproduce. And a lot of other fish species migrate just to search for food.
End of the line
Migration ends as soon as something like this shows up: a dam. Without help, fish are unable to reach the upper level. And they might not survive being washed down with the rushing water from the upper to the lower level.
Travel by train
Conservationists might come up with a solution like gathering fish on one level of a dam and transporting them to the other level - by monorail. This is what actually happens at the Hwacheon Dam in South Korea. The dam interrupted an important fish migration route when it was built in 1944.
A great help
Fish ladders are more natural. They offer an alternative route from the upper to the lower level and vice versa, allowing fish to overcome the man-made obstacle themselves. Though manmade, fish ladders don't have to be elaborate, expensive constructions - they can be small and simple, like this one.
25,000 fish a day
When the weir, dam or lock is economically important and quite big, the fish ladder will look very impressive. This one in Geesthacht near Hamburg, Germany, on the river Elbe, is 550 meters (1,800 feet) long. It is the biggest fish ladder in Europe, consisting of 45 interconnected pools that the fish have to pass through.
Spiral staircase
Fish ladders can take the form of spiral staircases. Fish work their way up from one pool to the other. This construction, though, needs less space than the usual kind of fish ladder. An added benefit: the roundish pools prevent the fish from hurting themselves in the course of their journey.
Aiming high
One of these spiral staircases for fish is to be found at a hydropower station near Kiel in northern Germany. Looking like some futuristic industrialstructure, it is 200 meters (656 feet) long, consists of 36 pools and covers an incline of 3 percent.
Special treatment
Eels are weak swimmers, they just slither up- and downstream. They need special eel ladders: watery, ascending ramps with brush-like structures on the ground. Special rest areas allow them to have a break during their climb. Eels' ladders are often built alongside ordinary fish ladders.