9() FISH HATCHERY MANAGEMENT 



and water temperature. Biological influences include type of plant and an- 

 imal life present and the food habits of the fish crop. Chemical elements al- 

 ready present in the water supply, composition of the bottom mud, pH, 

 calcium, magnesium, and chemical interactions have significant effects on 

 fertilizer response. 



Not all ponds should be fertilized; fertilization may be impractical if a 

 pond is too large or too small. Turbid or muddy ponds with light penetra- 

 tion less than six inches should not be fertilized, nor those having a high 

 water exchange rate. Ponds having low water temperatures may not give a 

 good return for the amount of fertilizer applied. If the species of fish being 

 reared is not appreciably benefited by the type of food produced, fertiliza- 

 tion should not be considered. In cold regions where winterkill is common 

 in shallow productive ponds, fertilization may be undesirable. 



Ponds should be thoroughly inspected before they are fertilized. Includ- 

 ed in the inspection may be a secchi disc reading to determine the water 

 turbidity; close examination for the presence of filamentous algae, rooted 

 aquatic vegetation, and undesirable planktonic forms; oxygen determina- 

 tions on any pond where low oxygen concentrations are suspected, and ob- 

 servation of nesting locations in spawning ponds. 



Fertilizer to be applied should be weighed or measured on platform or 

 hanging scales, or with precalibrated buckets. It is necessary to calibrate a 

 bucket for each type of fertilizer used, because fertilizers vary considerably 

 in density. Small amounts of fertilizer may be dispensed with a metal 

 scoop, large amounts with a shovel or a mechanical spreader. 



Distribution of the fertilizer in the pond will vary with wind direction, 

 size of the pond, whether organic or inorganic materials are used, and the 

 particular reason for fertilizing. On a windy day (which should be avoided 

 when possible), fertilizers should be distributed along the windward side of 

 the pond. In general, organic fertilizers (especially heavy forms such as 

 manure) should be given a more uniform distribution than the more solu- 

 ble inorganic ones. However, when insufficient phosphorus is thought to be 

 responsible for plankton die-off, an inorganic phosphate fertilizer should be 

 evenly distributed over most of the pond. Ordinarily, inorganic fertilizer 

 need not be spread over any greater distance than about half the length of 

 the pond on one side. If a pond is being filled or if the water level is being 

 raised, it may be advantageous to apply fertilizer near the inlet pipe. 



Avoid wading through the pond while spreading fertilizers, if possible. 

 Wading stirs up the bottom mud and some of the fertilizer nutrients, par- 

 ticularly phosphates, may be adsorbed on the mud and temporarily re- 

 moved from circulation. A wader may destroy fish nests, eggs, and fry. Fer- 

 tilizer should not be spread in areas where nesting activity is underway or 

 into schools of fry. Larger fingerlings can swim quickly away from areas of 

 fertilizer concentration. 



