A) PROTEIN AND PROTEIN QUALITY IN FISH FEEDS

By Carl D. Webster, Ph.D., Aquaculture Research Center, Kentucky State University

Nutrition involves the processes by which an animal is provided with nutrients needed for maintenance, growth, health, coloration, and reproduction. A nutrient is defined as an element or compound of dietary origin which is necessary to support the life and well-being of an animal. Nutrition concerns all aspects (assimilation, digestion, absorption, and utilization) of a food. There are several broad classes of nutrients in a food. These are proteins, lipids (fats), vitamins, minerals, carbohydrates, and energy. Since protein is generally the most important constituent of a fish feed, we will begin our discussion of fish nutrition with a look at protein and protein quality.

Protein is not only the most important constituent of a fish diet, it is generally the most expensive (compare the prices of a pound of steak to a pound of rice). Proteins are made up of amino acids linked together by chemical bonds. There are 20 amino acids which are common to most proteins, and 10 of these are essential for normal growth and health of the fish. These 10 essential amino acids are called, oddly enough, essential amino acids. They are: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. There are three types of proteins. Fibrous proteins are highly indigestible and include collagen (found in connective tissue and bone), elastin (found in blood vessels), and keratin (found in hair, hooves, scales, and feathers). A second type of protein is the contractile proteins which are found in the muscle. These proteins are highly digestible and are found in the flesh of all animals. The third group is represented by globular proteins which are found in hormones, enzymes, and blood.

Proteins serve many functions in fish. They are components of bones, skin, organs, and muscle. Generally, about 70% of the total weight of the fish is made up of protein. Like humans, fish cannot make the essential amino acids in the body. Therefore, fish need to have a dietary source of proteins as the source for essential amino acids. As the protein is broken down in the digestive tract, the amino acids that were once linked together are separated into individual amino acids, called free amino acids. These free amino acids are carried through the blood and travel to the various organs and tissues where they are rejoined together to make new proteins. If a fish gets too little protein in the diet, growth of the fish is reduced and, in severe cases, weight is lost. Feeding fish too much protein in the diet is not wise because protein is expensive and the cost of the feed will be higher than needed, and the excess protein will be used as an energy source. Lipid (fat) is a much better, and cheaper, source of energy for fish.

Fish require a certain percentage of protein and a certain level of essential amino acids for proper growth. These requirements depend upon the size of fish, water temperature, culture conditions, daily feeding rate, stocking rate, and species of fish. The last factor is different than in most animals. One breed of cattle requires the same protein level as another breed. Likewise, all breeds of dogs require the same protein level in their diet. This is true for goats, cats, hogs, etc. However, each species of fish have their own protein requirements. Thus, koi have a different protein requirement (actually essential amino acid requirements) than do largemouth bass, or carp, or tuna. This makes feed formulation for fish quite interesting.

Protein quality, or the nutritional value of proteins, is based upon the amino acid composition of the feed ingredients, specifically the essential amino acid content, as well as the availability of the amino acids. The percentage of protein that can be digested by a fish determines protein quality. For instance, poultry feather meal is an ingredient with a high percentage of protein (80%) and is not very expensive. Thus, one might think that a fish diet could use a high percentage of this ingredient and save the producer some money. However, this is not the case. Poultry feather meal is very poorly digested by fish and thus has a very poor protein quality value. However, if the feather meal is hydrolyzed (broken down) during processing, the protein quality is improved, but so is the cost of the ingredient.

Generally, marine fish meal represents the highest quality protein source used in fish diets. Marine fish meals can contain between 60-75% protein, of which between 80-95% is digestible. Poultry by-product meal is an animal-source protein ingredient that has attracted much attention from nutritionists and is used in numerous feed formulations as a partial or total replacement for marine fish meal. Soybean meal, cottonseed meal, corn gluten meal, soy protein products, and distiller’s and brewer’s by-products are commonly-used plant protein sources. For koi and goldfish diets, additional ingredients may be added to the diet to enhance color and health of the fish, such as algae meals, yeast, protective and chelated forms of vitamins and minerals, and ingredients that enhance color of the fish.

Thus, when one evaluates a feed, it is not enough to know the percentage of crude protein. One must know if the ingredients supplying the protein have a high protein quality that is digestible to the fish. Only by meeting these two criteria can a fish feed be considered adequate to meet the protein requirements of the fish.

B) FEEDING BASICS- TEMPERATURE RELATED FEEDING GUIDE

By Carolyn Weise, as per reported koi hobbyist generally accepted practices.

Under 50ºF No Feeding recommended.

50-65ºF Cold Weather Wheat Germ recommended- starting at 1-2x/week at

50ºF, gradually increasing to daily, then by 60ºF twice daily.

66-85ºF Summer Staple, Growth Food, Color Enhancing Food- feed small Portions 3-6 times daily.

Over 85ºF Cut back on feeding regimen. Feed 1-2x/daily as fish cannot Metabolize at high temperatures or low temperatures. Cold Weather formula can again be used for easier absorption.

Over 90ºF No feeding recommended. Any uneaten food will add to organic load and oxygen depletion in the water. Fish will begin to die.