Common problems and countermeasures during the black fry stage of Japanese eel

At the turn of spring and summer, Japanese eel farming in various regions is mostly in the black fry stage, and it is commonly reported that there are phenomena such as a decline in feeding rate and a tendency for gill rot and tail rot syndrome to occur, seriously affecting the healthy, sustainable, and steady development of Japanese eel farming. The main problems existing in the black fry stage of Japanese eel farming are now analyzed, and some corresponding countermeasures are proposed for reference.

1. Decline in feeding rate:

In Japanese eel black fry farming, the phenomenon of a decline in feeding rate or difficulty in increasing the feeding amount occurs. The main triggering factors are unstable water quality, liver and gallbladder problems, spore parasite infection, enteritis, gill congestion and inflammation, and environmental stress. Farmers can take corresponding measures according to different triggers:

Cause 1: Unstable water quality. Some farmers lack experience in Japanese eel farming and copy the farming methods for European eel. Their daily water quality management habitually involves large water exchanges and does not focus on cultivating algae in the pond water, resulting in pond water that is relatively clear and lean, with transparency above 30cm, and insufficient dissolved oxygen in the farming water body, which is not conducive to the feeding and growth of Japanese eel.

Countermeasures: Appropriately reduce the amount of water exchanged, use spirulina 3-5kg/mu + compound fertilizer 2-3kg/mu or water-regulating agent 0.6-1ppm or water-improving agent 1ppm, and splash over the whole pond to cultivate water quality; at the same time, orally administer VC 3-5g/kg feed + liver-protecting agent 5-8g/kg feed.

Cause 2: Liver and gallbladder problems. In diseased eels, bile secretion is disordered. Necropsy reveals gallbladder atrophy, congestion around the liver or relatively black bile, and gallbladder enlargement (this situation is usually caused by drug poisoning).

Countermeasures: Focus on detoxifying water quality. If the pond water is acidic, use Shuixian 2-3ppm and splash over the whole pond; if the water is alkaline, use Shuibao 1-2ppm and splash over the whole pond; at the same time, orally administer liver-protecting agent 5-8g/kg feed + swelling-reducing and bleeding-stopping powder 5-8g/kg feed, with 10 days as one treatment course.

Cause 3: Caused by massive parasitism of spore parasites (also called Myxidium) in the kidney.

Countermeasures: Orally administer Chongdi Zhibao Powder (Ling) 3g/kg feed + liver-protecting agent 5-8g/kg feed, with days as one treatment course.

Cause 4: Enteritis. Generally there are three symptoms: bacterial, white dysentery, or colitis.

Countermeasures: First orally administer Yukangbao (sarafloxacin hydrochloride) 5g/kg feed + Changyanling (Andrographis paniculata powder) 3g/kg feed, with 7-10 days as one treatment course; afterward use BD-602 (eel-specific probiotics) 3-5g/kg feed, with 10 days as one treatment course.

Cause 5: Congestion and inflammation of the eel gills. Diseased eels often show a preference for rushing (climbing) toward the foam of the aerator.

Countermeasures: External use of potassium permanganate 2-3ppm + Chongdi Zhibao Powder 3ppm + insecticide Ling 2#, splash over the whole pond; oral administration: liver-protecting agent 5-8g/kg feed + swelling-reducing and bleeding-stopping powder 5-8g/kg feed.

Cause 6: Triggered by external environmental stimulation. For example, when the water quality is too clear and lean, it is prone to sunlight stimulation on sunny days, causing spitting of bait and biting; or ammonia nitrogen and nitrite in the pond water are relatively high.

Countermeasures: Cultivate water quality, use water-improving agent 1-2ppm or ammonia-quieting agent 0.3-0.5ppm (can reduce NO2- and NH3+ in pond water), and splash over the whole pond; orally administer VC 3-5g/kg feed.

2. Gill rot and tail rot syndrome:

Cause: Improper operation during pond-circling and sorting of eels causes mechanical damage; or deterioration of pond water quality (especially likely to occur in aged ponds or cement-bottom ponds).

Countermeasures: Stop feeding for 1-2 meals, use potassium permanganate + oxytetracycline hydrochloride 8-10ppm or florfenicol (10%) 3-5ppm or Juwei iodine (10% PVI) 1ppm, and splash over the whole pond; after 1-2 days, use Shuixian 2-3ppm + chlorine dioxide 0.2-0.3ppm, for 2-3 consecutive times.

Oral administration: compound vitamin B tablets 10-20 tablets/kg feed + sarafloxacin hydrochloride 5g/kg feed, with 7-10 days as one treatment course.

If the tail rot symptoms are relatively severe, lower the water temperature as much as possible, ventilate the heat-preservation shed, and stop feeding for 2-3 meals.

External use: salt 3-5‰ + oxytetracycline hydrochloride 10-15ppm, splash over the whole pond, once every 16 hours, for 2 consecutive times; afterward carry out water quality adjustment: Shuixian 2-3ppm + chlorine dioxide 0.2-0.3ppm; oral administration as above.