64 



I'lSH HATCHERY MANAGEMENT 



oxygen in water, or low oxygen concentration in the source water. At 

 hatcheries with chronic low oxygen concentrations and comparatively high 

 water temperatures, production should be held down to levels that safely 

 utilize the available oxygen, or supplemental aeration will be required. A 

 depleted oxygen supply can occur at night in ponds that contain large 

 amounts of aquatic vegetation or phytoplankton, and fish kills may occur 

 after the evening feeding. Here again, aeration may be necessary to 

 increase the oxygen supply. 



The carrying capacity of a rearing unit is usually stated as pounds of fish 

 per cubic foot of water. Reference is also made to the pounds of fish per 

 gallon per minute water inflow. In warmwater fish culture the carrying 

 capacity as well as production is usually expressed in pounds per acre. 

 Although these criteria are commonly used to express carrying capacity, they are 

 often used without regard for each other. This can be misleading. The term Flow 

 Index refers to the relationship of fish weight and size to water inflow and 

 the term Density Index refers to the relationship of fish weight and size to 

 water volume. There are clear distinctions in the affects of these two ex- 

 pressions. The Flow Index deals specifically with the amount of oxygen 

 available for life support and growth. The Density Index indicates the spa- 

 cial relationship of one fish with another. Even though water flows may be 

 adequate to provide oxygen and flush wastes, too much crowding may 

 cause behavioral and physical problems among the fish. 



AVERAGE WEIGHT IN GRAMS 



Figure 24. Effect of fish size on maximum loading density of 

 salmon, expressed as pounds of fish per cubic foot of water. 

 (From Burrows and Combs 1968.) 



