Fish Hatchery Management - fisheries & aquaculture

FISH HATCHf,RY MANAGEMEN']Body fat of a hatchery trout fed production feeds is harder (more saturated)than that of a wild trout, but after stocking the body fat graduallychanges to a softer (unsaturated) typ". This can be attributed to both thechange 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 l% of the feed for maximumgrowth response. This may be supplied by the addition of 3-570 fishoil or 10%r soybean oil.The level of dietary lipid required for salmon or trout depends on suchfactors as the age of the fish, protein level in the feed, and the nature ofthe supplemental lipid. The influences of age of the fish and protein levelof the diet are interrelated; young trout require higher levels of both fatand protein than older trout. For best performance, the recommended percentageof fat and protein for different ages of trout and salmon should beas follows:Starter feeds (fry)Grower feeds (fingerlings)Production feeds (older fish)% protein % fat50 1540 t2359Hatchery personnel can check the protein and fat content of trout feeds eitheron the feed tag for brand feeds or in the feed formulationdata foropen-formula feeds to determine if these recommended nutrient levels arebeing supplied by the feed they are using.High levels of dietary fat and, to a lesser degree, excess protein or carbohydratescan cause fatty infiltration of the liver. Fatty infiltrated liversare swollen, pale yellow in color, and have a greasy texture. The level offat in affected livers may be increased to several times greater than normal.This condition usually is accompanied by fatty infiltration of the kidneyand can lead to edema and death by reducing the elimination of wastesthrough the urinary system.Fatty infiltrated livers should not be confused with fatty degeneration ofthe liver or viral liver degeneration. Fatty degeneration of the liver iscaused by toxins from rancid feeds, chemical contaminates, certain algae,or natural toxins. This condition is typified by acute cellular degenerativechanges in the liver and kidney. The liver is swollen, pale yellow in colorwith oil droplets in the tissue, but does not feel greasy (Figure 7l). Rancidfats in feeds stored for long periods (more than six months) or under warm,humid conditions are the primary cause of this disorder in hatchery-rearedtrout. Rainbow trout are most severely affected and brook trout to a lesserdegree, but brown trout are rarely affected by rancid oils in the feed. Viralliver degeneration differs from the others by the presence of small hemorrhagicspots in the liver and swelling of the kidney. Anemia is characteristicof advanced staees of all three liver disorders.