78 FISH HATCHERY MANAGEMENT 



culture is applied. Fry are usually stocked in earthen ponds that have been 

 prepared to provide an abundance of zooplankton. Fry are stocked at den- 

 sities of 50,000 to 70,000 per acre to produce 2- inch fingerlings in 30 to 40 

 days. Because of the aggressive feeding behavior of these species, especially 

 northern pike, care must be taken not to let the zooplankton decline or 

 cannibalism will occur and survival will be low. At a size of 2 to 3 inches 

 these fish change from a diet of zooplankton and insect larvae to one 

 predominantly of fish. At this stage, the fingerlings usually are harvested 

 and distributed. If fish larger than 2 to 3 inches are desired, the fingerlings 

 can be restocked into ponds supplied with a forage fish. Stocking rates do 

 not normally exceed 20,000 per acre, and generally average about 10,000 

 to 15,000. As long as forage fish are present in the pond, northern pike and 

 walleyes can be reared to any size desired. As the fish become larger, they 

 consume more and larger forage fish. Northern pike and walleyes are 

 stocked at lower densities if they are to be raised to larger sizes. Stocking 

 densities of 10,000 to 20,000 fingerlings per acre are used to rear 4- to 6- 

 inch fingerlings; 5,000 to 10,000 per acre for 6- to 8-inch fish; and usually 

 less than 4,000 per acre for fish 8 inches or larger. 



This method of calculating carrying capacities of ponds or raceways ig- 

 nores the effects of accumulative metabolic wastes. Where water is reused 

 through a series of raceways, the Flow Index would remain fairly constant, 

 but metabolic products would accumulate. 



Inventory Methods 



The efficient operation of a fish hatchery depends on an accurately main- 

 tained inventory for proper management. Whether weight data are applied 

 directly to the management of fish in the rearing units or used in an ad- 

 ministrative capacity, they are the criteria upon which most hatchery prac- 

 tices are based. 



Hatchery procedures that are based upon fish weight include feed calcu- 

 lations, determination of number per pound and fish length, loadings of 

 distribution trucks for stocking, calculations of carrying capacities in rear- 

 ing units, and drug applications for disease control. 



Administrative functions based upon weight of fish include preparation 

 of annual reports, budgeting, estimating production capability of rearing fa- 

 cilities, recording monthly production records, feed contracting, and plan- 

 ning for distribution (stocking). 



Some managers inventory every two or three months to keep their pro- 

 duction records accurate; others use past record data to project growth for 

 several months and obtain a reasonable degree of accuracy. An inventory is 

 essential after production fish have been thinned and graded, and one 



