NUTRITION AND FEEDING 255 



the trough and maintain a position in the water column. A considerable 

 amount of work has been conducted to determine when various salmonid 

 fry first take food. Brown trout begin feeding food approximately 31 days 

 after hatching in 52°F water, while food was first found in the stomachs of 

 rainbow trout fry 21 days after hatching in 50°F water. 



The upper alimentary tract of rainbow trout fry remains closed by a tis- 

 sue plug until several days before swim- up. Thus, feeding of rainbow trout 

 fry before swim- up is useless. Some fish culturists have observed higher 

 mortality in brook trout fed early than in those deprived of food for up to 

 5 days after swim-up. 



Yolk absorption is a useful visual guide to determine the initial feeding 

 of most species of fish. Most studies reported in the literature (Table 3l) 

 indicate that early feeding of fry during swim-up does not provide them 

 with any advantage over fry that are fed later, after the yolk sac has been 

 absorbed. Many culturists start feeding when 50% of the fry are swimming 

 up because if fry are denied food much beyond yolk-sac absorption, some 

 will refuse to feed. No doubt, starvation from a lack of food will lead to a 

 weakened fry that cannot feed even when food is abundant. 



It is apparent that the initial feeding time for warmwater fishes is much 

 more critical than for coldwater species because metabolic rates are much 

 higher at warmer water temperatures. This will lead to more rapid yolk ab- 

 sorption and a need for fish to be introduced to feed at an earlier date. 



Feeding Frequency 



The frequency at which fish should be fed is governed by the size of the 

 fish and how rapidly they consume the feed. When fish are started on feed, 

 it is desirable to give small amounts of small- sized particles at frequent in- 

 tervals. 



Several factors influence how quickly fish consume feed. The type of 

 feed, the way it is introduced, and the type of trough or pond in which it 

 is fed all will affect the rate of consumption. Feeds that are heavier than 

 water must be fed with more care than those that float. Once a sinking 

 feed reaches the bottom many fish will ignore it. To avoid their prolonged 

 exposure to water, sinking feeds should be fed slowly and at greater fre- 

 quency. 



Trout and salmon generally are fed small amounts at hourly intervals 

 throughout an 8- hour day when they first start to feed. Some fish culturists 

 feed fry at half- hourly intervals and gradually reduce the number of feed- 

 ings as the fish increase in length. The general practice has been to feed 

 trout three times a day until they are 5 inches long (20/pound). Larger 

 trout are fed twice daily and broodfish are usually fed once each day. 



