Dietary Components
The main components of fish diets, man-made or natural are:
Protein
Carnivorous fish, such as salmonids, require diets rich in protein
(about 40 - 50% content) compared to
omnivorous or herbivorous fish such as carps (typically 25 - 35% content) for optimal growth.
Proteins are composed of a number of building blocks known as amino acids. Different proteins are formed from around 20 known amino acids, the calorific value (energy worth) depending on the amino acids present, their arrangement(s) and availability following absorption. Fish appear to be incapable of synthesizing 10 amino acids totally or in sufficient quantities to meet their needs - these are referred to as essential amino acids and must be present in their original, pure form in any diet. The non-essential amino acids can be either synthesized from their basic raw materials (carbon, oxygen, hydrogen and nitrogen) or interconverted from other amino acids. These essential and non-essential amino acids are listed in below.
| Essential Amino Acids | Non-essential Amino Acids |
| Arginine | Adenosine |
| Histidine | Alanine |
| Isoleucine | Aspartic Acid |
| Leucine | Cystine |
| Lysine | Glutamic Acid |
| Methionine | Glycine |
| Phenylaline | Proline |
| Threonine | Serine |
| Trytophan | Tyrosine |
| Valine | ---- |
Provided the correct ratio and amount of essential amino acids are provided, protein synthesis can occur and new tissue can be laid down. As amino acids cannot be stored, they must be present in the correct amounts. If one amino acid is limiting or the protein component of the diet is in excess, proteins may be used as an energy source for maintenance and movement. This process, known as deamination, is energy- expensive and also leads to increased nitrogen waste production which can have environmental consequences.
Protein sources, mainly as fish meals, are the most expensive components of diets and are also poor as an energy source. It therefore pays to utilise or spare all dietary protein for growth and supply maintenance and movement energy from a cheaper and more readily accessible source. This is the accepted approach with lipids, and to a lesser extent carbohydrates, meeting daily energy requirements and demands. Levels of protein in formulated pelleted diets have changed little over the last 20 or so years since they were introduced, remaining at around levels of 40 - 47%.
