Prevention and Control Methods and Technical Key Points for Complications in Fingerling Fish

An unusual combination of complications - rotten gills, enteritis and hemorrhage - once occurred in fingerling fish in Sanhe Town, Hongze County, Huai'an City, Jiangsu Province, causing large-scale deaths of fingerlings. The specific details of the disease and its prevention and control measures are summarized below for everyone to discuss together.

Onset time and symptoms: In early August, scattered dead fish appeared, and the diseased fish had Trichodina on their gill filaments, with no other symptoms. Around mid-August, mass mortality occurred, and all were grass carp fingerlings. The diseased fish turned dark in body color and showed a behavior of lying at the edge, floating weakly by the pond bank and stopping feeding. When the diseased fish were scooped up and examined and the gill covers lifted, after deworming measures had been taken earlier there were no longer any parasites, yet the gill filaments were found to be rotten and stuck together, and there were bleeding spots at the base of the pelvic fins. After dissection, the intestinal tract was found to be congested and empty of food, and the fish died quickly, with no other obvious symptoms. All the dead fish showed three symptoms, namely rotten gills, hemorrhage and enteritis. When this syndrome appeared, it was also the peak period of fingerling deaths. This peak death period lasted about 20 days, with daily deaths per mu reaching 15-20 kg, and in severe cases as much as over 30 kg/mu. Throughout the course of the disease, the amount of food eaten gradually decreased, until in the end some ponds stopped feeding altogether.

The author believes there are 2 reasons causing the occurrence of this disease. (1) There were problems with the methods of disease prevention and treatment and with the dosage of medication. (2) Water quality and feed feeding were the chief culprits causing the outbreak of the fish disease.

First, the disease prevention and treatment measures were inadequate. As the saying goes, prevention is more important than treatment. Through investigation, the author found that the 4 neighboring households whose ponds did not develop the disease had carried out very thorough deworming in the early control of Dactylogyrus and Trichodina, whereas the ponds that developed the disease all began with an outbreak of trichodiniasis, which then triggered rotten gills and subsequently enteritis and hemorrhage. Therefore, deworming in the early stage is the key.

Second, because of the large amount of pelleted feed fed, when the fishermen saw the fish growing rapidly after eating the feed, they were delighted, and in pursuit of high yield they paid no attention to the feed conversion ratio. Protein intake exceeded the standard, and feed conversion ratios were all above 5-6. Not only that, some farmers fed feed with a protein content as high as 32%. The grass carp fingerlings were so stuffed that every one of them had a big belly and a round waist, losing their resistance and unable to withstand any invasion from the outside. At the same time, because of continuous overcast and rainy weather, insufficient light, and the large amount of pelleted feed fed, a large amount of organic matter was deposited, worsening the water quality. The water color of most fish ponds was thick dark green, and some had already given off a foul odor. Although a large amount of water was exchanged at this time, it could no longer play much of a role.

Therefore, the author summarizes the main measures for preventing the occurrence of complications during the fingerling breeding period as follows: (1) Deworm regularly, and do it thoroughly; (2) Keep the water quality good; (3) Feed pelleted feed reasonably. Treatment measures: disinfect and sterilize in a timely manner, and splash Yuyong Jundujing over the whole pond. At the same time, oral administration of "Grass Carp Four Diseases" can be observed to bring about an obvious improvement, and the number of dead fish is also greatly reduced. On this basis, deepening the pond water and lowering the water temperature can further achieve a control effect.