FISH HEALTH MANAGEMENT 301 



flared opercula, riding high in water, and distribution of individuals 

 equidistant from each other are typical signs of fish infected with bacterial 

 gill disease. Gills show proliferation of the epithelium that may result in 

 clubbing and fusing of lamellae or even filaments. Microscopic examina- 

 tion of affected gill tissue reveals long, thin bacteria arranged in patches 

 over the epithelium. Necrotic gill tissue may be visibly grayish-white and 

 many of the filaments may be completely eroded. Often, only the gills on 

 one side are affected. 



A combination of large numbers of bacteria and gill epithelial prolifera- 

 tion differentiates bacterial gill disease from other gill problems. Etiology of 

 the disease has not been proven conclusively because induction of the 

 disease with flexibacteria isolated from diseased fish has not been con- 

 sistently achieved. 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 are 

 important stress factors that contribute to outbreaks of the disease. Water 

 temperatures above 56°F are favorable for the bacteria. Yearling and older 

 fish are less susceptible than fry, but outbreaks can be acute in all ages of 

 fish. 



Water supplies should be kept free of fish, silt, and mud. The accumula- 

 tion of fish metabolic products due to crowding apparently is the most im- 

 portant factor contributing to bacterial gill disease problems, and should be 

 avoided. 



The most reliable and often- used treatments for bacterial gill disease are 

 Roccal, Hyamine 1622 (98.8% 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 hard- 

 ness and temperature, so caution must be used to prevent losses due to 

 over- treatment and to insure that the treatment is effective. The recom- 

 mended treatment level is 1 to 2 parts per million of active ingredient in 

 water for 1 hour. Prophylactic treatments should be repeated every 7—14 

 days. If bacterial gill disease is diagnosed, treatment should be repeated 

 daily for 3 to 4 days. 



Bacterial gill disease seldom is a problem among warmwater fish, partic- 

 ularly those being reared in earthen ponds. It occasionally becomes a prob- 

 lem when young channel catfish, largemouth bass, bluegills, or redear 

 sunfish are held in crowded conditions in tanks or troughs for extended 

 periods. This can be corrected by treating with 1-2 parts per million 

 Roccal for 1 hour daily for 3 or 4 days or with 15-25 parts per million 

 Terramycin for 24 hours. After the problem is under control, the fish popu- 

 lation should be thinned or the water flow increased. Unless the manage- 

 ment practice that precipitated the outbreak is corrected, bacterial gill 

 disease will reappear. 



