Fish Hatchery Management - fisheries & aquaculture

2lu! ISH HAl CHERY MANA(IEMliN lthe digestibility and metabolism of carbohydrates by fish. All of the necessaryenzymes for digestion and utilization of carbohydrates have beenfound in fish, yet the role of dietary carbohydrates and the contribution ofglucose to the total energy requirement of fishes remain unclear.There is little carbohydrate (usually less than 1.0'li of the wet weight) inthe fish body. After being absorbed, carbohydrates are either burned forenergy, stored temporarily as glycogen, or formed into fat. Production ofenergy is the only use of carbohydrates in the fish system. No carbohydraterequirements have been established for fish because carbohydrates do notsupply any essential nutrients that cannot be obtained from other nutrientsin the feed.The energy requirement of a fish may be satisfied by fat or protein, aswell as by carbohydrate. If sufficient energy nutrients are not available inthe feed the body will burn protein for energy at the expense of growthand tissue repair. The use of carbohydrate for energy to save protein forother purposes is known as the "protein-sparing effect" of carbohydrate.Carbohydrate energy in excess of the immediate energy need is convertedinto fat and deposited in various tissues as reserve energy for use duringperiods of less abundant feed. Quantities in excess of needed levels lead toan elevated deposition of glycogen in the liver, and eventually will causedeath in salmonids.Fat-infiltrated livers and kidneys in salmonids are a result of fat depositionwithin the organ, resulting in reduced efficiency and organ destruction.This condition results primarily from excess levels of carbohydrates inthe feed.Carbohydrates also may serve as Precursors for the various metabolic intermediates,such as nonessential amino acids, necessary for growth. Thus,in the absence of adequate dietary carbohydrates or fats, fish may makeinefficient use of dietary protein to meet their energy and other metabolicneeds. In addition to serving as an inexpensive source of energy, starchesimprove the pelleting quality of fish feeds.Dietary fiber is not utilized by fish. Levels over 10')t in salmonid feedsand over 20'li, in catfish feeds reduce nutrient intake and impair the digestibilityof practical feeds.CARBOHYDRATES IN SALMONID FEEDSCarbohydrates are an inexpensive food source, and there is a temptation tofeed them at high levels. However, trout are incapable of handling highdietary levels of carbohydrates. The evidence for this is the accumulationof liver glycogen after relatively low levels of digestible carbohydrate arefed. Trout apparently cannot excrete excessive dietary carbohydrate. Inhigher animals, excessive carbohydrate is excreted in the urine' Such excretiondoes not occur in trout even though the blood sugar is greatly in-