Nutritional components can be divided into 5 main categories: carbohydrates, proteins, fats, vitamins and minerals. Each provide fish with essential energy requirements and nutrients required for growth and everyday metabolism. Each constituent will be discussed below in more detail.
Carbohydrates are often added as starches to fish feed formulations as binding and bulking agents, but are in actual fact not a major energy source for most fish. From 1 gram of carbohydrate, fish can only extract about 1.6 kcal, as opposed to mammals which can extract approximately 4 kcal (Craig, 2009). This is said to have arisen from the evolution of fish in aqueous systems in which carbohydrate sources were scarce, so their digestive and metabolic systems became more equipped to utilise proteins and fats for their energy needs (Lovell, 1998). Some fish however, such as warm water herbivores and omnivores are able to metabolise carbohydrates reasonably well.
Proteins in the diet are essential for body maintenance and muscle growth (Kesena & Arimiche, 2010) and are the primary, most efficient source of energy in fish. Proteins in fish feed are usually labelled as fish meal or shrimp meal. This component is generally the most expensive and so, some manufacturers substitute this with alternatives such as and soybean meal. Since protein is the major source of energy, it is important in assessing adequate energy needs. If a diet is deficient in energy content relative to protein content, a proportionate amount of dietary protein will be used for energy rather than tissue building. This is because energy needs for body maintenance and voluntary activity must be satisfied before energy and remaining protein is available for growth.
Amino acids are the building blocks of all proteins. Fish require twenty amino acids for growth, ten of which must be included in the diet as they cannot be synthesised within the body. These are known as essential amino acids.
Protein requirements generally are higher for small fry (baby fish), however as they grow larger, their protein requirements will usually decrease. Requirements will also vary with water temperature, water quality, genetic composition and feeding rates (Craig, 2009).
– Fats (lipids)
Lipids in fish feed provide a concentrated energy source whilst providing various other nutritional functions (Lovell, 1998). Lipids supply approximately twice the amount of energy as proteins and carbohydrates and some foods may contain up to 15% of fish diets (Craig, 2009). They also serve as a vehicle for the transportation of fat soluble vitamins (A, D, E and K) and play a crucial role in the structure and formation of cell membranes (Lovell, 1998). While warm-blooded animals have a major requirement for omega-6 fatty acids, most species of fish require omega-3 fatty acids and must be included in the diet (Cho, 1999). An explanation for this difference in fatty acid requirement is that the omega 3 structure allows a greater degree of unsaturation which is necessary to maintain flexibility and permeability characteristics at low temperatures such as in water (Lovell, 1998).
However, if a diet contains excess energy the fish may become full before they consume the necessary amounts of protein, vitamins and other nutrients for subsequent growth (Kesena & Arimiche, 2010). Excess energy, relative to protein content can cause excess amounts of visceral and body fat (Soyaqua, 2008). As such, it is recommended that fat content in ornamental fish foods do not exceed 10%.
Vitamins are organic micro-nutrients required in trace amounts and are essential for normal fish growth, health and general maintenance of metabolism (Cho, 1999). Most are not synthesised in the body and so must be included in the diet. Vitamins are divided into two categories: water soluble and fat soluble. Water-soluble vitamins include: the B- group vitamins, choline, inositol, folic acid, pantothenic acid, biotin and ascorbic acid (vitamin C). The fat-soluble vitamins include: the A vitamins, retinols (responsible for vision), the D group vitamins, E vitamins, the tocopherols and K vitamins (Craig, 2009). The requirements for most vitamins will depend upon the intake of other nutrients, size of the fish, species, life stage, growth rate and external environmental stresses (Soyaqua, Fish Nutrition, Feeds and Feeding, 2008).
Minerals are inorganic micro-nutrients necessary in the diet for normal body functions (Craig, 2009). In fish systems, minerals play pivotal roles in osmoregulation, intermediary metabolism, and are important in the formation of the skeleton and scales (Cho, 1999).
Minerals can be divided into two subgroups; those that are required in large quantities are termed major minerals and those that are required in small amounts are terms trace minerals (Lovell, 1998). Major minerals include calcium, sodium, chlorine, magnesium, potassium and phosphorous, while trace minerals include copper, chromium, iodine, iron, zinc and selenium (Craig, 2009). In the food product labelling, major minerals are often listed.
Fish are able to absorb dissolved minerals from the surrounding water across the gill membrane or through the digestive tract and skin. In fact, most of the calcium requirements of fish are extracted from the water (Lovell, 1998). This is why it is so important to maintain the water’s general hardness within their optimal ranges.