HATCHERY OPERATIONS 113 



WATER LOSS 



Water loss by seepage is a problem at many hatcheries. A permanent solu- 

 tion is to add a layer of good quality clay about a foot thick, wetted, rolled, 

 and compacted into an impervious lining, (in small ponds, the same effect 

 can be achieved with polyethylene sheets protected with three to four 

 inches of soil.) Bentonite can be used effectively to correct extreme water 

 loss when applied as follows: 



(1) Disk the bottom soil to a depth of six inches, lapping cuts by 50%. 



(2) Harrow the soil with a spike- tooth harrow, overlapping by 50%. 



(3) Divide treated area into 10-foot by 10-foot squares. 



(4) Uniformly spread 50 pounds of bentonite over each square (20,000 

 pounds per acre). 



(5) Disk soil to a depth of three inches. 



(6) Compact soil thoroughly with a sheepsfoot roller. 



This procedure has reduced seepage over 90% in some cases. 



Evaporation is a problem in farm and hatchery ponds of the southwest. 

 Work in Australia indicates that a substantial reduction in evaporation 

 (25%) can be reduced by a film of cetyl alcohol (hexadecanol), applied at a 

 rate of about eight pounds per acre per year. The treatment is only effec- 

 tive in ponds of two acres or less. 



PROBLEM ORGANISMS 



Most plants, animals, and bacteria in a pond community are important in 

 fish culture because of their roles as fish food and in photosynthesis, 

 decomposition, and chemical cycling. However, some organisms are un- 

 desirable, and sometimes have to be controlled. 



Some crustaceans — members of the Eubranchiopoda group such as the 

 clam shrimp {Cyzicus sp.), the tadpole shrimp {Apus sp.) and the fairy 

 shrimp [Streptocephalus sp.)— compete with the fish fry for food, cause ex- 

 cessive turbidity that interfers with phytosynthesis, clog outlet screens, and 

 interfere with fish sorting at harvest. They usually offer no value as fish 

 food, because of their hard external shell and because of their fast growth 

 to sizes too large to eat. 



These shrimp need alternating periods of flooding and desiccation to 

 perpetuate their life cycles, and they can be controlled naturally if ponds 

 are not dried out between fish harvests. However, they usually are con- 

 trolled with chemicals. Formalin, malathion, rotenone, methyl parathion, 

 and others have been used with varying degrees of success; many of these 

 are very toxic to fish. The best chemicals today are dylox and masoten, 

 which contain the active ingredient trichlorfon and which have been re- 

 gistered for use as a pesticide with nonfood fish. Treatments of 0.25 part 

 per million dylox will kill all crustaceans in 24 hours, without harming 



