Protein Requirement of Fish: Properties and Biological Functions

The word protein comes from the Greek word ‘proteios’ which means first. It is an important biological molecule (biomolecule) that is made up of smaller units called amino acids. The amino acids are connected to each other by peptide chains and the chains are joined by sulfhydrol bonds, hydrogen bonds and Vander Wallace energy. The chemical composition of proteins varies greatly from the compounds of any biologically active group. Protein consists of 50% carbon, 16% nitrogen, 21.5% oxygen and 6.5% hydrogen. For proper body structure, Protein Requirement of Fish is very important.

Proteins vary in size or shape. Some polypeptides carry only 20-30 amino acids, others carry a few thousand amino acids. In every living cell, skin, hair, the bone marrow, muscles, tendons and ligaments contain proteins. Proteins combine with each other to form body structures and take part in defense. Enzymes, hormones, antibodies and globulins affect, control and protect the body’s chemical reactions. Important organic molecules such as hemoglobin, myoglobin and various lipoproteins carry oxygen and other elements in the body and provide energy to the body.

Protein is a complex molecule. It breaks down when the body needs it. That is why they are the slowest and longest source of energy from carbohydrates. During digestion, proteases break down proteins under the influence of enzymes to form amino acids. Some absorbed amino acids are converted into carbohydrates through glucogenesis. When hungry, the organism can produce and use glucose from its own body proteins. This condition is especially seen in muscles.

You might also read: Amino Acid Requirements of Fish

Classification of Protein

Proteins can be divided into different parts based on their structure and composition, e.g.

(A) Classification of Protein on the Basis of Structure

Protein can be divided into 3 parts on the basis of structure, viz.

1. Fibrous protein: This type of protein consists of long, parallel polypeptide chains. It is water soluble. Such proteins form fibers and sheets and also make cellular structural components, such as collagen, elastin, keratin, etc.

2. Globular protein: In this type of protein, the polypeptide chains fold firmly to form a spherical structure. They are water soluble. They perform physiologically important functions. Examples of such proteins are: serum gabulin, insulin, etc.

3. Intermediate protein: This type of protein consists of fiber-shaped polypeptide chains. They are water soluble. They help with physiological activities such as blood clotting.

(B) On the Basis of Structural Components

Proteins can be divided into two parts, viz

1. Simple protein: A protein that contains only amino acids is called simple protein. They are again of different types, viz

(i) Albumin: They are neutral in nature and soluble in water. Example: Egg albumin

(ii) Globulin: They are neutral in nature and insoluble in water. Example: Serum globulin

(iii) Histones: They are alkaline in properties and soluble in water. Example: Chromatin

(iv) Scleroproteins: They are neutral in properties and insoluble in water. Examples: keratin, collagen

2. Conjugated protein: This type of protein contains non-protein prosthetic groups and amino acids. Such proteins are again of different types, viz

(i) Phosphoprotein: The protein which contains phosphate along with amino acids is called phosphoprotein. For example: milk casein, egg yolk etc.

(ii) Glycoprotein: A protein that contains carbohydrates along with amino acids is called glycoprotein. For example: saliva mucin.

(iii) Nucleoprotein: The protein that binds to DNA is called nucleoprotein. E.g. chromatin.

(iv) Chromatoprotein: The protein which contains pigment is called chromoprotein. Such as: hemoglobin, cytochrome.

(v) Lipoprotein: A protein that contains lipids along with amino acids is called lipoprotein. Such proteins are present in the cell membrane.

(vi) Flavoprotein: The protein that contains flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) is called flavoprotein. They play an important role in electron transfer. Such as riboflavin.

(vii) Metaloprotein: A protein that contains metallic elements along with amino acids is called metalloprotein. Such as: hemoglobin, myoglobin.

 (C) Based on Biological Activity

Proteins are divided into the following different types:

(i) Structural protein: The protein that plays a role in building the structure of the body is called structural protein. Such proteins are found in tendons and cartilage, such as collagen. This type of protein is also found in hair, nails and skin, such as keratin.

(ii) Contractile protein: This type of protein plays a role in the contraction and expansion of muscle fibers, such as myosin and actin.

(iii) Transport protein: This type of protein transports essential substances all over the body. For example, hemoglobin transports oxygen and lipoprotein lipids to the body.

(iv) Storage protein: This type of protein stores nutrients. For example, casein stores protein and ferritin in milk and iron in the spleen and liver.

(v) Hormone type protein: This type of protein regulates metabolism and nervous system in the body. For example, insulin regulates glucose levels in the body and growth hormone regulates body growth.

(vi) Enzyme type protein: This type of protein accelerates the biochemical reaction in the cell. For example, the enzyme sucrase hydrolyzes sucrose and trypsin hydrolyzes proteins.

(vii) Protection protein: This type of protein detects and destroys foreign particles in the body. For example, immunoglobulins respond to the immune response.

(D) Based on Nutrition

 Protein is divided into the following two types: 

(i) Complete protein: The protein that can provide all kinds of essential amino acids is called complete protein.

(ii) Incomplete protein: Protein that cannot provide all kinds of essential amino acids is called incomplete protein.

Structural Levels of protein

The structural levels of proteins are of 4 types namely primary, secondary, tertiary and quaternary.

Primary structure of protein
Primary structure of protein

Primary: The linear sequence of amino acids in a polypeptide chain is the primary structure. Ribonuclease and protein myoglobin can only function in their primary structure.

Secondary: When a polypeptide chain is coiled to form a spiral or helix, it is called a secondary structure. Such as skin keratin.

Secondary structure of protein
Secondary structure of protein

Tertiary: A helixed polypeptide molecule folds itself to form a complex structure, but a specific structure, such as a spherical or rod-shaped structure, is called a tertiary structure. Such as blood globulins.

Quaternary: Some proteins have two or more polypeptide chains and each of them has primary, secondary and tertiary structure then it is called quaternary structure. Such as insulin and hemoglobin.

Tertiary and quaternary structure of protein
Tertiary and quaternary structure of protein

General characteristics of Protein

  • Protein is an organic substance that consists of nitrogen, oxygen, carbon and hydrogen.
  • It is an important biological molecule or biomolecule that acts as a key component of the cell’s cytoplasm.
  • It is the structural element of the body collar.
  • It makes amino acids.
  • Protein provides heat and energy to the body and plays an important role in maintenance.
  • Proteins are referred to as bricks which make bones, muscles, hair and other parts of the body.
  • It is an enzyme-like active ingredient that participates in metabolic action.
  • It makes antibodies, hemoglobin, etc. in the body.
  • Its molecular weight is 5-300 kilo Dalton.

You might also read: Lipid Requirements of Fish

Properties of Protein

  • Protein is a colorless and tasteless substance.
  • They are homogenous and crystalline.
  • Protein has variable size and shape.
  • Crystalloid proteins are commonly found in globules and plants. Fibrous proteins, on the other hand, are like threads and they usually occur in animals.
  • Due to its large size, it exhibits numerous colloidal properties.
  • Its diffusion rate is very slow.
  • It can act as an acid or alkali.
  • Most proteins are dissolved in water, alcohol, light alkali, or salts of various concentrations.
  • It is heat tolerant.
  • Proteins can be changeable or unchangeable.
  • The normal properties of proteins can be destroyed by heat, salt concentration, freezing, ultrasonic stress or stirring.
  • Proteins can form characteristic bonds with other proteins.

You might also read: Carbohydrate Requirements of Fish

Biological functions of Protein

  • Protein plays an important role in the growth of reproductive and small fish.
  • It helps in repairing tissues.
  • It builds immunity of the body.
  • It produces essential enzymes and hormones.
  • It conserves energy in the muscles.

Sources of Protein

  • Shrimp meal
  • Fish meal
  • Meat meal
  • Alfalfa meal
  • Poultry droppings
  • Dry skim milk
  • Dried Peas
  • Coconut shell
  • Cotton seeds
  • Sunflower seeds
  • Rape seed
  • Sesame
  • Nuts
  • Soybeans
  • Bone meal
  • Liver
  • Blood meal
  •  potato leaves
  • Epil epil etc.

Protein Requirements of Fish

Total operating cost of fisheries sector is 60% and the total production cost is 50%.  About 75% of this is spent on protein, which is the most expensive ingredient in a fish diet. This is why it is important to have enough knowledge on the protein needs of fish to prepare a low-cost balanced diet. Arbitrary wastage of protein and failure to determine the exact amount of protein required in fish adversely affects production. Fish feed requires more protein than cattle and poultry feed.

Commercial foods prepared for tilapia or catfish contain about 32-40% protein, compared to 44-50% for trout or salmon. Fish require less energy to conduct biological activity than other warm-blooded animals. Many species, such as salmon and trout, derive their energy from fats and proteins rather than carbohydrates. Fishmeal is used as a protein in most commercial fish foods which is more expensive than high quality vegetable protein such as soy protein.

As protein is the most expensive part of the fish diet, it is very important to accurately determine the protein requirement of each species of culturable fish. In the case of herbivorous fish, the demand for protein is always low. High density fish farming requires more protein than low density fish.

The demand for protein is higher in small fish. As the fish get bigger, the demand for protein decreases. Protein demand varies depending on the environment of the cultivation medium, water temperature, water quality and genetic material of the fish, and the rate of food intake.

Research on dietary fish fries, fingerling, and earling fish has shown that the demand for protein for early fries is highest. As the size of the fish increases, the demand decreases. Fish fries must be provided with almost half of the digestible material with balanced protein as food for maximum growth. At ideal temperatures (250 C), the demand for salmon and trout fry at 6-7 weeks decreases by 40%, and in the case of earling salmon, the demand increases by 35% (Graph 1 and 2).

The total protein requirement of baby catfish is lower than that of salmonids. In the beginning, 50% of the ration of digestible material should be protein for the food-eating fish fries, and this demand for protein decreases with increasing size. Some sources of salmon’s diet provide an idea of the protein needs of baby salmon with changes in water temperature. The maximum growth of chinook salmon requires about 40% egg protein at 7 0 C, while the same fish requires 50% protein at 15 0C.

Salmon, trout and catfish consume more protein than they need due to the removal of nitrogen-containing waste gel tissue directly into the aquatic environment as a soluble ammonia compound. This process of nitrogen removal is more effective than birds and mammals. Birds and mammals use energy to produce urea, uric acid or other nitrogenous wastes. These wastes are excreted urine through the kidneys.

Protein requirement of fish (Chinook salmon)  at temperature of 47 0 F.

Graph-1: Protein requirement of Chinook salmon at temperature of 47 0 F  . Top Curve – The initial starting weight of each fish is 1.5 g and lower Curve – The starting average weight of each fish is 5.0 g.

Protein requirement of fish (Chinook salmon)  at temperature of 58 0 F.

Graph 2: Protein requirement of Chinook salmon at temperature of 58 0 F. Top curve: The initial starting weight of each fish is 2.8 gm and lower curve:The initial starting weight of each fish is 5.8 gm. (Source: DeLong, D.C., J.E. Halver and E.T. Mertz, 1958, J.Nutr., 65:589-99)

In fact, fish need to be fed a high-grade protein, high-energy, high-fat, sufficient essential fatty acids, vitamins and minerals. The response curve of protein demand is obtained from the expected level / response curve through the Almquist plot. These differences in protein demand are due to differences in farming techniques and food ingredients.

Some fish, such as grass carp (Ctenophryngodon idella) and omnivorous fish called Brycon species, require relatively high levels of protein-rich food for maximum growth. Brycon species survive by eating foods rich in low-protein unwanted fruits and plant elements, and in this case, food intake from the natural food chain plays an essential role.

Uryhaline fish such as rainbow trout (Salmo gairdneri), coho salmon (Oncorhynchus kisutch) and 20 ppt salt-tolerant fish have the same protein demand as freshwater fish. There is no readily available data on demand for protein in marine fish (35 ppt).

Table: The demand for protein at different stages in the life cycle of some cultivable fish is shown

Fish Species

Protein Requirements of Fish ( % Dry weight)





Labeo rohita




Catla catla




Cyprinus carpio




Clarias batrachus




Clarias gariepinus




Pangasius hypophthalmus




Oreochromis mossumbicus




Macrobrachium rosenbergii




Penaeus monodon