Detailed Explanation of Techniques for Raising Fish Fry in Reservoir Cages Without Feeding
In reservoirs with fertile water quality, using cages to raise silver carp and bighead carp fry without feeding is not only practical and feasible, but also has advantages such as not occupying fry ponds, not spending on fish feed, and low cost. Without feeding, raising one silver carp or bighead carp fry of about 13cm in a cage in a reservoir costs less than 0.10 yuan, while raising one fry of the same size in a pond costs more than 0.10 yuan; converted per mu, the yield is about 150,000 fry/mu or more, which is 30-40 times that of internal ponds. Therefore, raising fry in reservoir cages without feeding can not only produce large-sized fry, but also has low cost and significant benefits.
The success and feasibility of raising fry in reservoir cages without feeding has its ecological and physiological basis. From an ecological perspective, since a reservoir is a relatively large body of water, the flow of water keeps various ecological factors and feed in the cage constant, and they will not change too much due to the high density of fry. From a physiological perspective, because the highest density is concentrated in a very small area, activity is reduced, nutritional accumulation is increased, and this is beneficial to fish growth. The main cultivation techniques are introduced as follows:
1. Selection of cages. The commonly used material for cages is polyethylene netting. Netting with an area of 30 sq m to 50 sq m and a depth of 2 m to 3 m is suitable; too large is inconvenient to manage. The mesh size depends on the size of the fry stocked, and it is best to replace it multiple times as the fish grow. For example, 4cm summer fry require cages with a mesh size of 1.1cm, and when the fish grow to about 13cm, cages with a mesh size of 3cm can be used.
2. Selection of location. Usually, cages should be placed in calm bay areas sheltered from the north and facing south, with a water depth of about 4 m, because these places have a high density of plankton and abundant suitable feed for fish. If possible, it is best to have a certain water flow, but when the water flow speed reaches 2 m/s it is not suitable, because at this time the fish will swim against the current, consuming energy, which is not conducive to fish growth.
3. Density of cages. Even in waters with relatively fertile water quality, the area of cages should not exceed 1% of the water area, and the spacing between cages should not be less than 30 m, because without feeding or fertilizing, plankton in the natural ecological environment is limited after all.
4. Density and combination. The stocking density and species combination of fry are related to the composition and quantity of plankton in the reservoir and the water exchange volume inside and outside the cages. Generally speaking, it is suitable to stock 200-300 summer fry per cubic meter of water. In terms of species combination, when the water quality is very fertile and small plankton are abundant, mainly raise silver carp; when large plankton account for a large proportion, mainly raise bighead carp.
5. Daily management. The main work of daily management is checking the cages. After a cage is placed in water for a slightly longer time, there will be many attachments, mainly composed of filamentous algae and some smooth gelatinous substances. These attachments will clog the mesh of the cage and prevent the exchange of water inside and outside the cage. Therefore, they must be removed frequently. The main removal methods include mechanical washing (once every half month), sun exposure, biological removal (polyculture of some tilapia and blunt snout bream that eat these attachments), chemical removal (treating the netting with copper salts), and sinking the cage (sinking it 3-5 m underwater to stop photosynthesis of the attachments), etc.