HATCHERY OPERATIONS 93 



a soft-water pond. Fertilizers are often spread on the pond bottom prior to 

 filling, and nuisance vegetation may also be sprayed at this time. 



WILD-FISH CONTROL 



Wild fish must be kept from ponds when they are filled, as they compete 

 with cultured species for feed, complicate sorting during harvest, may in- 

 troduce diseases, or confound hybridization studies. Proper construction of 

 the water system and filtration of inlet water can prevent the entrance of 

 wild fish. 



A sock filter is made by sewing two pieces of 3- foot- wide material into a 

 12-foot-long cylinder, one end of which is tied closed and the other end 

 clamped to the inlet pipe (Figure 37). It can handle water flows up to 

 1,000 gallons per minute. This filter should be used only on near-surface 

 discharges, to prevent excessive strain on the screening. 



A box filter consists of screen fastened to the bottom of a wooden box 

 eight feet long, three feet wide, and two feet deep (Figure 38), and is suit- 

 able for water flows up to 1,000 gallons per minute. The screen bottom is 

 supported by a wooden grid with 1 x 2 foot openings, which prevents ex- 

 cessive stress and stretching. The filter may be mounted in a fixed position 

 or equipped with floats. If the inlet water line is not too high above the 

 pond water level, a floating filter is preferred. This allows the screen to 

 remain submerged, whatever the water level, which reduces damage caused 

 by falling water. 



If the water supply contains too much mud or debris and cannot be ef- 

 fectively filtered, ponds can be filled and then treated with chemicals to 

 kill wild fish. Rotenone is relatively inexpensive and is registered and la- 

 beled for this purpose. It should be applied to give a concentration of 0.5 

 to 2.0 parts per million throughout the pond. Rotenone does not always 

 control some fishes, such as bullheads and mosquitofish, and it requires up 

 to two weeks to lose its toxicity in warm water and even longer in cold wa- 

 ter. However, 2 to 2.5 parts per million potassium permanganate (KMn04) 

 can be added to detoxify rotenone. 



Antimycin A is a selective poison that eliminates scaled fishes in the 

 presence of catfish. It does not kill bullheads, however, which are undesir- 

 able in channel catfish ponds. The chemical varies in activity in relation to 

 water chemistry and temperature; the instructions on the label must be 

 closely followed. Expert advice should be sought in special cases. 



Chlorine in the form HTH, used at concentrations of 5 parts per million 

 for as little as one hour, will kill most wild species of fish that might enter 

 the pond. Chlorine deteriorates rapidly and usually loses its toxicity after 

 one day at this concentration. Chlorine can be neutralized if need be with 

 sodium thiosulfate. Chlorine is a nonspecific poison, and will kill most of 

 the organisms in the pond, not only fish. 



