328 FISH HATCHERY MANAGEMENT 



Figure 98. Characteristic signs of whirling disease in older fish that have sur- 

 vived the disease are a sunken cranium, misshapen opercles, and scoliosis of the 

 spine due to the destruction of cartilage (arrow). (Courtesy G. L. Hoffman, Fish 

 Farming Experimental Station, Stuttgart, Arkansas.) 



which may not appear for 4 months after infection. In heavy infections, 

 spores readily can be found in wet mounts or histological sections (Figure 

 99). They are ovoidal (front view) or lenticular (in profile), and have two 

 pyriform polar capsules containing filaments at the anterior end. 



Because of the seriousness of whirling disease, control and treatment 

 measures must be rigorous. Ideally, all earthen rearing units and water sup- 

 plies should be converted to concrete, followed by complete decontamination of 

 facilities and equipment with high concentrations of such chemicals as sodium 

 hyprochlorite or calcium oxide. Allow the treated area to stand 4 weeks, clean 

 thoroughly, and ref>eat decontamination. New eggs or fry must be obtained from a 

 known uncontaminated source and raised in spore-free ponds or raceways for the 

 first 8 months. 



PLEISTOPHORA 



Several species of Pleistophora infect hatchery fish. As the name of the class 

 Microsporidea indicates, these are exceedingly small protozoans. Pleisto- 

 phora spores are about the size of large bacteria, 3-6 micrometers long and 

 somewhat bean shaped. Severe infections have been reported in the gills of 

 rainbow trout and in the ovaries of golden shiners. In golden shiners, the 

 parasites infest up to about half of the ovary and significantly reduce the 

 fecundity of broodstock populations. 



The only known control for Pleistophora in rainbow trout is prevention. 

 Rainbow trout or their eggs should not be transferred from infected to 

 uninfected hatcheries. Broodstocks known to be infected should be phased 

 out and the rearing facilities decontamination. 



