sport fishermen. This subject is dis- 

 cussed in a following section. 



Upon absorption of the yolk sac 

 (see fig. 4), fry usually are fed in 

 the troughs for a week or more be- 

 fore being placed in the rearing 

 ponds. To secure maximum pro- 

 duction, the ponds are stocked to a 

 safe carrying capacity and some of 

 the growing fish are removed at in- 

 tervals and stocked into native wa- 

 ters. By this practice the quanti- 

 ties of salmon being reared may be 

 near the safe carrying capacity at 

 all times. If adequate rearing space 

 is available, or if all of the fish must 

 be reared to a specific size or for a 

 definite period before releasing, the 

 initial stocking of the ponds is lim- 

 ited to the numbers of salmon young 

 that can safely be held in the ponds 

 until released. 



It is the practice at all Fish and 

 Wildlife Service salmon hatcheries 

 to use pounds as the measure for 

 calculating the carrying capacities 

 of ponds, troughs, or fish-distribu- 

 tion tanks, and the quantities of 

 food to be presented to the fish. At- 

 tempting to determine carrying ca- 

 pacities and quanities of food to be 

 fed when only numbers of fish are 

 known is inconvenient and inaccu- 

 rate. 



The objective of Fish and Wild- 

 life Service hatcheries is to produce 

 annually 1 pound of salmon for each 

 cubic foot of water available in 

 troughs and ponds for rearing. Oc- 

 casionally, at some hatcheries, this 

 goal is exceeded, but the average is 

 still below the objective. When at- 

 tempting to achieve an annual pro- 

 duction of 1 pound per cubic foot 

 of water, it is necessary to carry 



near the maximum poundage in all 

 rearing facilities throughout the 

 year. 



One important factor that may 

 limit the efficient utilization of rear- 

 ing equipment is the incidence of 

 disease. Some hatcheries may ex- 

 perience persistent disease among 

 the stocks of fish held, usually at- 

 tributable to the water supply. 

 Although the usual diseases at the 

 modern hatchery need not cause ex- 

 cessive mortalities if prophylactic 

 measures are routine procedure, the 

 methods of applying treatments 

 often require that the rearing facili- 

 ties be stocked with fish at levels 

 considerably below capacities. 

 Such routine control measures also 

 require labor. Diseases and their 

 control wdll be discussed in a follow- 

 ing section. 



Obviously the foods presented to 

 young salmon in ponds are of ex- 

 treme importance, not only for ade- 

 quate growth but also to maintain 

 the fish in a healthy condition. 

 This subject is discussed separately. 



Cleanliness in the troughs and 

 ponds in which salmon or trout are 

 being reared is desirable. Al- 

 though some hatcheries appear to 

 be able to produce fish successfully 

 and efficiently with but a minimum 

 of labor expended on the cleaning 

 of ponds, experience has shown the 

 advantages of cleanliness. Only 

 the concrete ponds in general use 

 for the rearing of salmon can be 

 cleaned satisfactorily. At some 

 liutcheries the growth of algae on 

 pond walls and floors is removed by 

 weekly brushing. Another proce- 

 dure is to apply special paint or a 

 solution of copper sulfate and salt 



31 



