Fish Hatchery Management - fisheries & aquaculture

HATCHERY OPERATIONS 93a soft-water pond. Fertilizers are often spread on the pond bottom prior tofilling, and nuisance vegetation may also be sprayed at this time.WILD.FISH CONTROLWild fish must be kept from ponds when they are filled, as they competewith cultured species for feed, complicate sorting during harvest, may introducediseases, or confound hybridization studies. Proper construction ofthe water system and filtration of inlet water can prevent the entrance ofwild fish.A sock filter is made by sewing two pieces of 3-foot-wide material into al2-foot-long cylinder, one end of which is tied closed and the other endclamped to the inlet pipe (Figure 37). lt can handle water flows up to1,000 gallons per minute. This filter should be used only on near-surfacedischarges, to prevent excessive strain on the screening.A box filter consists of screen fastened to the bottom of a wooden boxeight feet long, three feet wide, and two feet deep (Figrr.e 38), and is suitablefor water flows up to 1,000 gallons per minute. The screen bottom issupported by a wooden grid with I x 2 foot openings, which prevents excessivestress and stretching. The filter may be mounted in a fixed positionor equipped with floats. If the inlet water line is not too high above thepond water level, a floating filter is preferred. This allows the screen toremain submerged, whatever the water level, which reduces damage causedby falling water.If the water supply contains too much mud or debris and cannot be effectivelyfiltered, ponds can be filled and then treated with chemicals tokill wild fish. Rotenone is relatively inexpensive and is registered and labeledfor this purpose. It should be applied to give a concentration of 0.5to 2.0 parts per million throughout the pond. Rotenone does not alwayscontrol some fishes, such as bullheads and mosquitofish, and it requires upto two weeks to lose its toxicity in warm water and even longer in cold water.However, 2 to 2.5 parts per million potassium permanganate (f Utror)can be added to detoxify rotenone.AntimycinA is a selective poison that eliminates scaled fishes in thepresence of catfish. It does not kill bullheads, however, which are undesirablein channel catfish ponds. The chemical varies in activity in relation towater chemistry and temperature; the instructions on the label must beclosely followed. Expert advice should be sought in special cases.Chlorine in the form HTH, used at concentrations of 5 parts per millionfor as little as one hour, will kill most wild species of fish that might enterthe pond. Chlorine deteriorates rapidly and usually loses its toxicity afterone day at this concentration. Chlorine can be neutralized if need be withsodium thiosulfate. Chlorine is a nonspecific poison, and will kill most ofthe organisms in the pond, not only fish.