84 FISH HATCHERY MANAGEMENT 



several buyers, who find them more attractive if the fish are of uniform size 

 within each lot. 



A number of commercial graders are available. Mixed sizes of fish may 

 require grading through more than one size of grader. Floating grading 

 boxes with panels of metal bars on the sides and bottom are commonly 

 used in fish hatcheries. Spacing between the bars determines the size of 

 fish that are retained; fish small enough to pass between the bars escape. 

 The quantity of fish in the grader at any one time should not exceed five 

 pounds per cubic foot of grader capacity. Small fish can be driven from the 

 grader by splashing the water inside the grader with a rocking motion. 



Recommended grader sizes for such warmwater fish as minnows and 

 channel catfish are as follows: 



Minnows Channel catfish 



Spacing Length Spacing Length 



between offish between offish 



bars held bars held 



(inches) (inches) (inches) (inches) 



A lT;-inch grader will retain j-1-pound channel catfish. Catfish pass 

 most readily through the bottom of a grader and minnows through the 

 sides. 



Fish Handling and Harvesting 



Handling of fish should be kept to a minimum to avoid injury and stress 

 that can lead to disease or death. Losses from handling can be substantial, 

 but they do not always occur immediately and can go unnoticed after the 

 fish have been stocked in natural waters. 



An adequate supply of oxygen must be provided in the raceway or pond 

 during harvest, and during transit in containers. Silt and waste material 

 such as feed and feces in the water should be avoided or kept to a 

 minimum. Overloading nets or containers will abrade the skin of the fish. 

 Extremes in water temperature should be avoided in the hauling containers 

 and between rearing units. Sudden changes in water temperature of 6°F or 



