Data and Input Variables

Prices of eighteen different feed meals are used as a vector of independent variables. From the pool of oilseed products and animal byproducts, the following variables were chosen since they are all feed ration ingredients: soybean meal (high protein), soybean meal (low protein), soybean hulls, whole cottonseed, cottonseed meal, cottonseed hulls, linseed meal, poultry byproduct meal, hydrolyzed feather meal, meat and bone meal, canola meal, sunflower meal, corn gluten meal, prime tallow, yellow grease, bleachable fancy tallow, vegetable-animal blend, most importantly, menhaden fishmeal.

Weekly prices from January 2005 to the fourth quarter of 2012 were used in the analysis. Price observations at Fort Worth, Texas, for fourteen commodities were obtained from the Miller Publishing Company publication Feedstuff. Weekly prices for menhaden fishmeal and corn gluten meals and weekly prices for canola meal and sunflower mean were obtained from the same source but for Chicago, IL and Minneapolis, MI, respectively (due to data unavailability for Fort Worth). The nutritional compositions of each feed meal represent the independent variable in the hedonic regression. Thus, the first step of the model was to carefully select independent

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variables for the regression which was based on the relative importance of each characteristic for the fish feed ration.

5.3.1. Proteins

The proteins are among the most important nutrients of all living cells and excluding water, represent the largest chemical group in the animal body, a whole fish carcass contains on average 75% water, 16% protein, 6% lipid, and 3% ash. Tacon (1987) describes proteins as “essential components of both the cell nucleus and cell protoplasm, and thus account for the bulk of the muscle tissues, internal organs, brain, nerves and skin.”

According to Tacon (1987), the function of proteins for fish nutrition may be summarized as follows:

 “To repair worn or wasted tissue (tissue repair and maintenance) and to rebuild new tissue (as new protein and growth).

 Dietary protein may be catabolized as a source of energy, or may serve as a substrate for the formation of tissue carbohydrates or lipids.

 Dietary protein is required within the animal body for the formation of hormones, enzymes and a wide variety of other biologically important substances such as antibodies and hemoglobin.”

In contrast to livestock diets, fish diets are very high in protein. The typical diet usually contains at least 50% of protein. Protein levels on grow-out diets often approach or exceed 40% crude protein, while maintenance diets may contain as little as 25-35%. Currently, fish meal is a primary source of protein in both fish and shrimp diets. The

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reason is that in addition to protein, fish meal contains essential amino and fatty acids that usually are not present in tissue from terrestrial plants or animals.

Thus, protein is a vital nutrient for the maintenance and growth of fish. In literature, usually protein is reported as crude protein (CP), which represents approximate total nitrogen multiplied by 6.25 in the diet. Since crude protein is the closest estimation of the percentage of protein in the sample, it is considered to be a candidate in the regression.

5.3.2. Energy

Fish and shrimp require food to supply the energy that they need for movement and all the other activities. In literature, energy is defined as the capacity to do work. For example, New (1987) explains that “energy is required to do mechanical work (muscle activity for movement), chemical work (the chemical processes which take place in the body), electrical work (nerve activity) and osmotic work (maintaining the body fluids in an equilibrium with each other and with the medium, whether fresh, brackish or seawater in which the animal lives).” Hence, energy is not a nutrient. It is rather an end-product of absorbed macroorganic nutrients when they are oxidized and metabolized. The National Research Council reports Total Digestible Nutrients (TDN) as the most frequently used measurement of energy content. TDN was considered to be candidate in the equation from the energy measurements.

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5.3.3. Lipids

Lipids (fats) are high-energy nutrients that can be used to partially substitute protein in aquaculture feeds. Usually, lipids supply approximately twice the energy as proteins and carbohydrates. Fish diets contain about 15% of lipids since they also supply essential fatty acids that cannot be synthesized by the organism (Gatlin 2010).

Ether Extract (EE) is equivalent to lipids and represents an estimate of total fat or oil content, which is a rich source of energy. This nutrient group consists of several different compounds. Neutral lipids (fats and oils), in the form of triglycerides, provide a concentrated source of energy for aquatic species. Thus, EE were chosen to be also one of the candidates in the equation.

5.3.4. Minerals

Minerals are inorganic elements and they are necessary in the feed diet for normal body functions. Minerals are divided into two groups: macro-minerals and micro-minerals. Common macro-minerals are sodium, chloride, potassium and phosphorous. Micro-minerals such as copper, iodine, and zinc are required in small amounts. Usually the key minerals in feeding diet are calcium and phosphorous. However, different from livestock and other animals, fish are able to absorb majority of the minerals from water, thus requiring lower amount in their diet. More about

considering minerals into the equation will be discussed below.

5.3.5. Amino Acids

The nutritional value of a particular protein, as described previously, is based on the types and amounts of its amino acids, because proteins are formed by linkages of

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individual amino acids. Having high protein content in a particular feed meal does not imply that it will have well balanced amino acids. Although over 200 amino acids occur in nature, only about 20 amino acids are common. Of these, 10 are essential

(indispensable) amino acids that cannot be synthesized by fish. The 10 essential amino acids that must be supplied by the diet are: methionine, arginine, threonine, tryptophan, histidine, isoleucine, lysine, leucine, valine and phenylalanine. Of these, lysine and methionine are often the first limiting amino acids that are common in plant type based meals. For example, fish feeds prepared with plant (soybean meal) protein typically are low in methionine and lysine; extra methionine is usually added to soybean-meal based diets to promote optimal growth and health (Craig 2009). Animal proteins are more complete, providing well balanced amino acids which are not available in plant based proteins.

Even though all essential amino acids are required for proper fish nutrition, including all of them in the model would not be practical. Two of the most crucial amino acids were picked according to their deficiency signs in fish and shrimp: methionine and lysine. Deficiency of those amino acids in feed meal causes dorsal/caudal fin erosions; increased mortality, scoliosis, iordosis, and decreased carcass lipid content. Additionally, methionine deficiency is one cause of lens cataracts (Poston et al. 1977). Between these two amino acids, methionine is the most desirable amino acid in fish diets, since they are available only in limited amounts from plant based proteins.

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upon the amino acid composition of the protein and the biological availability of the amino acids present.