Figure 15. — Cleaning a raceway p 



to tlie dry pond, and then tiush the 

 pond tlioroughly before the intro- 

 duction of fish. This inliil)it8 or 

 prevents growth of algae during the 

 rearing season. 



Algae are not harmful to salmon 

 in ponds and usually (as in the 

 wild) harbor small animal organ- 

 isms upon which the fish can feed, 

 but the large numbers of fish held 

 in a rearing pond minimize the con- 

 tribution of this food source. Also, 

 the algae growth is a lodging place 

 for fish excrement, unconsumed 

 food, and disease organisms. 



With water adequate in tempera- 

 ture, quality, and quantity, with dis- 

 ease control, proper diets, and exper- 

 ienced care, the maximum produc- 

 tion of healthy salmon may be 

 expected. 



Diseases 



Throughout their lives all fishes 

 are subject to attack by a great 



tingerlii 



variety of disease organisms. In 

 tlie wild, diseases rarely become epi- 

 demic, because of tlie Avide disper- 

 sion of the fishes, which greatl}' 

 decreases the probability of direct 

 or indirect transmission of disease 

 organisms. 



In the hatchery pond or trough, 

 the fish usually are exposed to the 

 same diseases as are hsh in the wild, 

 })articularly if— common in the 

 l)ropagation of salmon — the hatch- 

 ery water supply is taken from a 

 stream. Disease organisms among 

 hatcher}^ fish have an ideal oppor- 

 tunity to spread because crowded 

 conditions permit easy transmission 

 l)etween lish. To combat disease 

 epidemics or even relatively minor 

 losses of hsh, the fish-culturist elim- 

 inates the possibility of disease 

 breeding or being retained in the 

 algae and excrement in ])onds and 

 troughs. He has as a tool an ade- 

 (jiiate flow of water tliat washes 



32 



