222 FISH HATCHERY MANAGEMENT 



Body fat of a hatchery trout fed production feeds is harder (more satura- 

 ted) than that of a wild trout, but after stocking the body fat gradually 

 changes to a softer (unsaturated) type. This can be attributed to both the 

 change in environment and feed. 



Linolenic fatty acids (omega-3 type) are essential for trout and salmon, 

 and should be incorporated at a level of at least 1% of the feed for max- 

 imum growth response. This may be supplied by the addition of 3-5% fish 

 oil or 10% soybean oil. 



The level of dietary lipid required for salmon or trout depends on such 

 factors as the age of the fish, protein level in the feed, and the nature of 

 the supplemental lipid. The influences of age of the fish and protein level 

 of the diet are interrelated; young trout require higher levels of both fat 

 and protein than older trout. For best performance, the recommended per- 

 centage of fat and protein for different ages of trout and salmon should be 

 as follows: 



% protein % fat 



Starter feeds (fry) 50 15 



Grower feeds (fingerlings) 40 12 



Production feeds (older fish) 35 9 



Hatchery personnel can check the protein and fat content of trout feeds ei- 

 ther on the feed tag for brand feeds or in the feed formulation data for 

 open-formula feeds to determine if these recommended nutrient levels are 

 being supplied by the feed they are using. 



High levels of dietary fat and, to a lesser degree, excess protein or car- 

 bohydrates can cause fatty infiltration of the liver. Fatty infiltrated livers 

 are swollen, pale yellow in color, and have a greasy texture. The level of 

 fat in affected livers may be increased to several times greater than normal. 

 This condition usually is accompanied by fatty infiltration of the kidney 

 and can lead to edema and death by reducing the elimination of wastes 

 through the urinary system. 



Fatty infiltrated livers should not be confused with fatty degeneration of 

 the liver or viral liver degeneration. Fatty degeneration of the liver is 

 caused by toxins from rancid feeds, chemical contaminates, certain algae, 

 or natural toxins. This condition is typified by acute cellular degenerative 

 changes in the liver and kidney. The liver is swollen, pale yellow in color 

 with oil droplets in the tissue, but does not feel greasy (Figure 7l). Rancid 

 fats in feeds stored for long periods (more than six months) or under warm, 

 humid conditions are the primary cause of this disorder in hatchery- reared 

 trout. Rainbow trout are most severely affected and brook trout to a lesser 

 degree, but brown trout are rarely affected by rancid oils in the feed. Viral 

 liver degeneration differs from the others by the presence of small hemor- 

 rhagic spots in the liver and swelling of the kidney. Anemia is characteris- 

 tic of advanced stages of all three liver disorders. 



