Fish Hatchery Management - fisheries & aquaculture

FISH HEAI,TH MANAGEMEN 301flared opercula, riding high in water, and distribution of individualsequidistant from each other are typical signs of fish infected with bacterialgill disease. Gills show proliferation of the epithelium that may result inclubbing and fusing of lamellae or even filaments. Microscopic examinationof affected gill tissue reveals long, thin bacteria arranged in patchesover the epithelium. Necrotic gill tissue may be visibly grayish-white andmany of the filaments may be completely eroded. Often, only the gills onone side are affected.A combination of large numbers of bacteria and gill epithelial proliferationdifferentiates bacterial gill disease from other gill problems. Etiology ofthe disease has not been proven conclusively because induction of thedisease with flexibacteria isolated from diseased fish has not been consistentlyachieved. Other common soil and water bacteria, such as Aeromo'nas sp., also may cause bacterial gill disease'Crowding, mud and silt in the water supply, and dusty starter diets areimportant stress factors that contribute to outbreaks of the disease. Watertemperatures above 56'F are favorable for the bacteria. Yearling and olderfish are less susceptible than fry, but outbreaks can be acute in all ages offish.Water supplies should be kept free of fish, silt, and mud. The accumulationof fish metabolic products due to crowding apparently is the most importantfactor contributing to bacterial gill disease problems, and should beavoided.The most reliable and often-used treatments for bacterial gill disease areRoccal, Hyamine1622 (98.8y0 active), and Hyamine 3500 (50% active).These treatments are not registered by the Food and Drug Administration.The effectiveness and toxicity of these compounds depends on water hardnessand temperature, so caution must be used to prevent losses due toover-treatment and to insure that the treatment is effective. The recommendedtreatment level is I to 2 parts per million of active ingredient inwater for I hour. Prophylactic treatments should be repeated every 7-I4days. If bacterial gill disease is diagnosed, treatment should be repeateddaily for 3 to 4 days.Bacterial gill disease seldom is a problem among warmwater fish, particularlythose being reared in earthen ponds. It occasionally becomes a problemwhen young channel catfish, largemouth bass, bluegills, or redearsunfish are held in crowded conditions in tanks or troughs for extendedperiods. This can be corrected by treating with 1-2 parts per millionRoccal for t hour daily for 3 or 4 days or with 15-25 parts per millionTerramycin for 24 hours. After the problem is under control, the fish populationshould be thinned or the water flow increased. Unless the managementpractice that precipitated the outbreak is corrected, bacterial gilldisease will reappear.