60 



Immediately upon any suspicion of trouble and always in 

 the even,t of increasing losses, the fish should be carefully 

 examined for gross lesions and all possible extraordinary factors 

 such asbad food, silt, and sudden fluctuations in water tempera- 

 ture, etc., should be checked. If nothing can be found, the fish 

 should be examined for parasites and microscopic lesions. If 

 still no demonstrable source of trouble can be found, a few 

 obviously affected fish should be preserved in a ten percent 

 solution of formalin and forwarded, with full particulars, to 

 the nearest pathological laboratory. Following this, all fish 

 should be salted. In the early stages of disease when relatively 

 few fish are as yet affected, these should be removed to an 

 isolated "hospital trough" to prevent spread of the disease to 

 the healthy fish. Needless to say, strict quarantine must be 

 employed to keep the disease from spreading through the rest of 

 the hatchery stock. 



In table 9 are presented the formulas for several 

 solutions used in the treatment of ailing fish. 



Table 9. — Formulas for solutions 



Control of predation 



INSECTS 



Minnow ponds may become overpopulated with aquatic 

 insects that prey on fish fry. The beetle larva called 

 "water tiger" and the adult insect known as the "back- 

 swimmer" [l^otonecta) are the most destructive. As both 

 these forms come to the surface of the water for air, 

 they can be controlled by covering the ponds with a 

 film of oil. Kerosene, fish oil, No. 2 fuel oil, and 

 cod-liver oil can be used for this purpose. The 



