Fish Hatchery Management - fisheries & aquaculture

214 FISH HATCHERY MANAGT]MENTof protein in the feed resulting onlv in reduced growth, whereas the lack ofany one of several vitamins produces well described deficiency signs. Nutrientssuch as protein and vitamins should be present in feeds at levels tomeet minimum requirements, but not in an excess which might be wastedor cause other health problems.The nutrients to be discussed in this chapter include (1) protein, (2) ca.-bohydrates, (3) fats, (4) vitamins, and (5) minerals.Protein RequirementsThe primary objective of fish husbandry is to produce fish flesh that isover 50% protein on a dry weight basis. Fish digest the protein in most naturaland commercial feeds into amino acids, which are then absorbed intothe blood and carried to the cells.Amino acids are used first to meet the requirements for formation of thefunctional body proteins (hormones, enzymes, and products of respiration).They are used next for tissue repair and growth. Those in excess of thebody requirements are metabolized for energy or converted to fat.Fish can synthesize some amino acids but usually not in sufficient quantityto satisfy their total requirements. The amino acids synthesized areformed from materials released during digestion and destruction of proteinsin the feed. Certain amino acids must be supplied in the feed due to theinability of fish to synthesize them. Fish require the same ten essential a-minoacids as higher animals: arginine; histidine; isoleucine; leucine;lysine; methionine; phenylalaninel threonine; tryptophan; valine. Fish fedfeeds lacking dietary essential amino acids soon become inactive and loseboth appetite and weight. When the missing essential amino acids arereplaced in the diet, recovery of appetite and growth soon occurs.In fish feeds, fats and carbohydrates are the primary sources of energy,but some protein is also utilized for energy. Fish are relatively efficient inusing protein for energy, deriving 3.9 of the 4.65 gross kilocalories pergram from protein, for an 84'/o efficiency. Fish are able to use more proteinin their diet than is required for maximum growth because of their efficiencyin eliminating nitrogenous wastes through the gill tissues directlyinto the water. Nutritionists must balance the protein and energy componentsof the feed with the requirements of the fish. Protein is the mostexpensive nutrient and only the optimal amount should be included formaximum growth and economy; less expensive digestible fats and carbohydratescan supply energy and spare the protein for growth.Several factors determine the requirement for protein in fish feeds.These include temperature, fish size, species, feeding rate, and energy contentof the diet. Older fish have a lower protein requirement for maximum