Key Technical Points for Rearing Snakehead Fry in Net Cages

1. Stocking density: generally 600-700 3-cm summer fry per square meter, with a harvest size of about 6-8 cm. The summer fry stocked must be strictly graded and screened so that the size is uniform at stocking, and the stocking density should be adjusted appropriately according to size.

2. Feed and feeding: The diet of summer fry is still mainly zooplankton. In areas where water earthworms are produced, or where earthworms, maggots and other animal feeds are cultured, these can all serve as feed for summer fry. Use a polyethylene mesh sheet to set up a feeding platform inside the net cage, 15-20 cm below the water surface, for feeding and for collecting residual bait. To feed dead water fleas, use the hanging-bag method, with about 500 grams per bag, and each bag can feed 600-700 fish. As the fry grow, gradually increase the amount, by about 250 grams roughly every week. As a principle, use less per hanging bag and more bags, to increase the dispersion of the fry when feeding, encourage uniform feeding, improve feeding effectiveness, and reduce stunted and weak fry. If other animal feeds are fed, they can be chopped to an appropriate size and scattered on the feeding platform. From this point on, pre-feeding “signal” conditioning can be carried out to summon the fry, such as playing music, an electric bell, tapping and other signals, with feeding starting when the signal is given. Afterward, when the fry hear the signal they will gather near the feeding platform waiting to be fed; this makes it easy to form a feeding frenzy, so feeding is good, feed utilization is high, the feed is not easily lost within the net cage, the fry grow fast, the feeding amount is easy to control, and it is also easy to observe the health status of the fry, among other advantages. When the fry reach more than 4 cm in length, minced fish meat can be used to replace the above several types. If compound feed is used, conditioning also starts from this stage. The conditioning method is: before alternating zooplankton (or water earthworms, etc.) with minced fish meat, first mix the two feeds in different proportions and feed them, using the same feeding method as before. Afterward, gradually increase the proportion of minced fish meat until it is entirely minced fish meat. Conditioning takes about 3-4 days and they will adapt quickly. 3. Regular net brushing: brush the nets every morning to prevent dirt and parasitic filamentous algae from clogging the mesh, which would affect water exchange inside and outside the cage and cause deterioration of water quality and oxygen deficiency inside the cage. Remove residual bait and clean the feeding platform every day to prevent accumulation and putrefaction of residual bait and dirt. Replace the net cage once every week, and thoroughly clean and disinfect the replaced net cage, drying it in the sun before reuse. While changing cages, carry out screening and separate rearing so that the fry in the cage are uniform in size; after 3-4 weeks of culture this way, the fry can reach 6-10 cm, and can be stocked as small-sized fry for further rearing of large-sized fry or for grow-out farming. 4. Pond patrol: patrol twice a day, morning and evening, to observe the activity, feeding and water quality of the fry, keep records, and propose measures to solve problems. At the same time, pay attention to whether the net cage facilities have been altered by wind, water currents, manual operations, etc., such as broken tying ropes or the bottom floating up; these should all be repaired promptly, otherwise serious mortality may result. Finally, do a good job of shift handover.