Size and construction
The size of the pond varies with species, fish size and site conditions, from fractions of a hectare to several hectares. As the ponds become larger, control becomes more difficult. The same is the case with harvesting. To carry out the harvesting the pond must normally be emptied and/or drained several times, otherwise the fish density will be too high when lowering the water level and harvesting. A seine net may also be used for harvesting. Commonly, relatively small ponds are used for brood stock, fry and juvenile production, while larger ponds are used for on-growing.
Pond depth is usually between 0.5 and 2.4 m, depending on what the pond is used for. For on-growing fish it is normal to choose a depth sufficient to prevent any light reaching the bottom of the pond. In this way growth of vegetation at the bottom is prevented and harvesting is easier. Ponds for fry are normally shallower because the bottom vegetation may function as shelter. However, the depth must not be so great that temperature layers occur; deep water will also increase the pressure on the sides and bottom of the pond together with possibilities for increased seepage. This increases demands for compaction of the material used for the bottom and sides when constructing the pond.
It is important to have a slope towards the outlet on the bottom to make drainage possible and harvesting easier: this can be in the range 1/1000 to 1/100, with the largest slope in the smallest ponds. The slope of the walls inside the pond is between 2.5 : 1 and 4 : 1; 3 : 1 is quite common, but this varies with the composition and angle of repose of the material used for construction. Exterior walls are normally slightly steeper than 2 : 1, but of course this also depends on the angle of repose for the material. The recommended width of the pond crest varies with depth; for 3 m deep ponds a crest width of 1.8 m is recommended;11 for shallow ponds the crest width can be reduced depending on the material used for construction.
The length–width ratio of ponds is normally about 2 : 1, but of course is adapted to the site conditions. The shape of watershed ponds depends on the terrain. Harvesting with a seine net is easier if the pond is rectangular. If ponds are too wide, harvesting will be more difficult.