oi'gaiiisins from the pond as I'apidly 

 us ])()ssib]e. Furllu'r. and most im- 

 portant, lu' nst's propri' diets to Ivccp 

 his lish in healthy condition ami 

 thus less snsct'pt ihle to disease. lie 

 has. too. the i-ecommendations of 

 researchers who ha\ c identified dis- 

 ease ortjanisnis and methods of 

 control. If unable to cope with a 

 <lisease problem, tlie liatcherynnin 

 usually can call upon a ])atliol()f»;ist 

 for assistance. 



In some salmon and trout hatch- 

 eries the control of organisms that 

 nuiy destroy eggs or fish is a con- 

 tinuous routine, and at all stations 

 constant vigilance is required to rec- 

 ognize and control diseases at first 

 ;ippearance. 



The most serious losses among 

 eggs are occasioned by fungus 

 growths. Other diseases of eggs, 

 such as white spot and soft eo:g, are 

 less well known, although white spot 

 is believed to be caused by mechani- 

 cal injury to the egg. 



One group of plant organisms 

 and four groups of animals com- 

 ])rise the principal organisms caus- 

 ing the diseases of salmon and trout. 

 The bacteria, considered in the plant 

 group, ai'e the most important in 

 connection with fish diseases, al- 

 though the protozoans, trematodes, 

 cestodes, and crustaceans — all ani- 

 mals — are represented in the fish- 

 disease groups. Recently a virus 

 infection has caused excessive mor- 

 talities among red-salmon finger- 

 lings, and a similar infection may 

 have been responsible for losses 

 among other species in hatcheries 

 throughout the country. 



Parasitic infections may ap})ear 

 on the external surface of the fish. 



or they may be internal. Those on 

 the exterior surfaces usually can be 

 controlled or eliminated by chemi- 

 cal (reatment. Intei'nal j)arasites 

 ai-e much mort' diflicidt to recognize 

 and coiUi-oI. The incorpoi'ation of 

 api)ropriate chemical disinfectants 

 in the food is elfective with some in- 

 teiiial parasites. Davis (1947) 

 gives detailed descriptions of vari- 

 ous disease organisms and their 

 control, and includes an excellent 

 list of references. Since the publi- 

 cation of Davis' work, many other 

 researchers have provided addi- 

 tiomd information and references 

 including Rucker and associates 

 (1952) on gill disease, and on kid- 

 ney disease (1951). 



Too much stress cannot be i)laced 

 upon the necessity for adequate 

 diets for the production of disease- 

 resistant fish. Nutritional deficien- 

 cies open the door to infection. 

 When fish become weak from infec- 

 tion, they usually stop feeding and 

 thus become weaker and less able 

 to resist disease. 



Foods for Hatchery Salmon 



Until comparatively recent years 

 the requirements of salmon and 

 trout for the diet components neces- 

 sary to life — carbohydrates, pro- 

 teins, fats, minerals, and vitamins — 

 were practically unknown. Re- 

 search has not provided much spe- 

 cific information regarding the 

 needs of fish, but the work of re- 

 searchers on other animals, a lim- 

 ited amount of fisheries work, and 

 trial-and-error methods, have im- 

 proved tremendously the informa- 

 tion available to the fish culturist. 

 A review of recent publications will 



33 



