FISH LADDER (to use approx. 200 liters per minute
Fish migration is very important. Even the smallest of fish go up and down streams following their prey, spawning and evading predators.
To allow small fish (7 to 15 cm) to pass the dam, a v notch can be used. This entails having a notch in the top of the dam. It should be a minimum of 9 cm deep and 18 cm wide. Such a notch will allow 200 liters per minute through when the dam is full. The water from this notch should go into a box (untreated wood) 30 to 40 cm wide, 50 cm long and 40 cm deep. A v-notch 10 cm deep and 20 cm wide is cut in the end of this box. 2 boxes are required for every 60 cm fall. The water from the last box should fall into a pool or another box! at least 30 cm deep.
These sizes and depths are needed to allow the fish to rest and to take a "run" at each jump.
For a long run, you could make the boxes all together with two long planks at the sides, one or 2 at the bottom and the partitions at an angle so that they are vertical when it is put in place. Netting or wire must be placed over it to protect the fish from birds.
Perhaps your boxes can be made from large plastic containers or half barrels. The important thing is to maintain the minimum dimensions. The edges of the v-notches should be rounded to prevent injuries to the fish as they do their jumps.
The fish ladder can be inset into the stream bank for stability and leak proof operation. The exact details are up to you. Nobody knows the perfect solution! Freshwater shrimp and some types of aquatic insect larvae can sometimes climb over or between several layers of cloth or sack material draped over the dam. Perhaps they should be allowed to migrate too.
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