Role of nutrient in fish
nutrition
In aquaculture, feed represents 40-50% of the total
operational costs. Although many aqua culturist attempts to enhance natural
food supplies in pond through fertilization, despite the demand and need for
prepared aquatic animal feeds is increasing continuously. The development of
semi-intensive and intensive farming methods necessitates a thorough
understanding and application of wide range of different disciplines and
related technologies including nutrition, reproduction, physiology, and
genetics and rearing systems. In particular intensive farming systems are
totally dependent upon the external provision of nutrient inputs in the form of
high quality nutritionally balanced complete diets.
Science of fish nutrition has advanced in recent years
with the development of new, nutritionally balanced commercial diets that
promote optimal fish growth and support better health. Feed formulations
according to the nutrient requirement of fish under culture, its life stage and
type of culture being followed and from good quality locally available
ingredients are preferred mostly. The most common feed used for carp culture in
India are rice bran and oil cake. However, a strong database on nutritional
aspects of cultivable fish species has been developed and several feed
formulations have been successfully undertaken to optimize growth in order to
gain best possible production. Nutritional and quantitative feed requirement is
also affected by species being cultured, life stage of particular species ,
culture methods used, feeding methods, processing losses or feed storage
losses, unique water quality conditions and utilization capacity of aquatic
animals. Therefore, a complete understanding of nutrient requirement, feed
ingredient available, their proximate composition, processing methods, feed
formulations, storage, application methods and their consequent effect on
aquatic environment becomes necessary for sustainable production.
Energy and nutrients
The main energy
yielding nutrients are protein, carbohydrate and lipids. The main function of
feed is to provide energy for body growth, reproduction and replacement of old tissues. Non energy
yielding nutrients include vitamins and minerals that to support the optimal
growth of fishes therefore, feed must be balanced and complete in energy
yielding and non energy nutrients. When fish are reared in high density
intensive culture systems or confined in cages and cannot forage freely on
natural feeds, they must be provided a complete nutritionally balanced diet. In
contrast, supplemental (incomplete, partial) diets are intended only to support
the natural food available (planktons, insects, algae, small fish) to fish in
ponds produced as a result of ponds own capacity to produce biomass. Therefore,
it becomes important to know about the energy yielding nutrient and their major
role in fish production.
Protein:
Proteins are a large complex
molecules made up of various amino acids joined by peptide bonds. They are essential components that perform a
central role in the structure and functioning of all living organisms. Proteins are the major organic material in
some animal tissues, making up about 65-75 % of the total on a dry weight
basis. Proteins are composed of carbon (50%), nitrogen (16%), oxygen (21.5%),
and hydrogen (6.5%). Animal must consume protein to furnish a continual supply
of amino acid. After protein is consumed, it is digested or hydrolyzed to
release free amino acid that are absorbed from the intestinal tract of the
animals and distributed by the blood to the various organs and tissues. These
amino acids are then used to synthesize new proteins. Since proteins are
continually being used by animals, either to built new tissues ( as during
growth and reproduction) or to repair worn tissues, a regular intake of protein
or amino acids is required.
20 amino acids are common in nature. Nutritionally the
various amino acids can be divided into two groups, dispensable (non-essential)
and indispensable (essential). Certain amino acids are considered indispensible
because the animal cannot synthesize them at all, or they are not synthesized
in sufficient quantity to support maximum growth. The dispensable amino acids
are those that can be readily synthesized in amount adequate to support maximum
growth. Most animals, including fish, require the same 10 indispensible amino
acids. These 10 essential amino acids 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. In
addition to differing in size and function, proteins differ in the relative
proportions of the amino acids they contain.
Because protein is the most expensive part of fish feed,
it is important to accurately determine the protein requirements for each
species. Fish feeds prepared with plant protein typically are low in
methionine; therefore, extra methionine must be added to soybean-meal based
diets in order to promote optimal growth and health. Protein requirements
usually are lower for herbivorous fish (plant origin food consuming) and
omnivorous fish (plant-animal origin food consuming) than they are for
carnivorous (animal origin food consuming) fish. High protein diets are also
required for fishes being cultured in intensive culture systems where growth
depends on balanced fish feed. Protein requirements generally are higher for
smaller fish. Protein requirements also vary with rearing environment, water
temperature and water quality, as well as the genetic composition and feeding
rates of the fish. Protein is used for fish growth if adequate levels of fats
and carbohydrates are present in the diet. If not, protein may be used for
energy and life support rather than growth. Fish are capable of using a high
protein diet, but as much as 65% of the protein may be lost to the environment.
Most nitrogen is excreted as ammonia (NH3) by the gills of fish, and
only 10% is lost as solid wastes. Accelerated eutrophication (nutrient
enrichment) of surface waters due to excess nitrogen from fish farm effluents
is a major water quality concern of fish farmers.
If adequate protein is not provided in the diet, there is
rapid reduction or cessation of growth or loss of weight because the animal
withdraws protein from some tissues to maintain the functions of more vital
ones. On the other hand, if too much protein is supplied, proportionally less
will be used to make new proteins and rest more be metabolized to produce
energy.
Protein requirement
of commonly cultivable fishes
|
Fish name
|
Life stage
|
Required protein
in diet (%)
|
|
Catla catla
|
Fry
|
40-45
|
|
|
Fingerlings
|
35-40
|
|
Labeo rohita
|
Fry
|
40-45
|
|
|
Fingerlings
|
35-40
|
|
Cirrhinus mrigala
|
Fry
|
40-45
|
|
|
Fingerlings
|
40-45
|
|
Cyprinus carpio
|
Fry and finger lings
|
40-45
|
|
Ctenophsryngodon
idella
|
Fry and fingerlings
|
35-40
|
|
Hypophthalmichthys
molitrix
|
Fry and fingerlings
|
35-40
|
|
M.rosenbergii
|
Post larvae and juveniles
|
35-40
|
Carbohydrates:
Carbohydrates are one of the major class of natural organic compounds
with the general formula Cx(H2O)y. They are
considered least expensive form of dietary energy also act as pellet binder.
Mainly they include sugars, starch and cellulose form. The simplest
carbohydrates are sugars (such as ribose and glucose). These are called
monosaccharide and are the basic units from which all other carbohydrates are
built. When two of these simple sugars bonded together they form compounds
called disaccharides, these include sucrose and maltose compound.
Polysaccharides are formed by joining together ten or more monosaccharide’s.
Carbohydrates are essential component of the diet of fish and may be used as a
source of energy or modified by being combined with fats or portions. The most
important source of carbohydrate in
fish feed is wheat, rice bran, oil cakes, grasses and maize flours. Enzymes
like amylase have been detected in several fishes for carbohydrate digestion.
The carbohydrates are absorbed as simple sugars. All the enzymes involved in
major pathways like glycolysis, tricarboxylic acid cycle, pentose phosphate
shunt, gluconeogenesis and glycogen synthesis have been demonstrated.
Carbohydrate
utilization in fish:
Carps, tilapia, milk
fish and prawns efficiently utilize carbohydrate as source of energy. However,
the ability of fish to utilize dietary carbohydrate varies considerably with complexity of carbohydrate. Most of carnivorous fishes have poor
ability to digest carbohydrates.
Dietary starches are useful in the extrusion manufacture of floating feeds.
Cooking starch during the extrusion process makes it more biologically
available to fish. In fish, carbohydrates are stored as glycogen that can be
mobilized to satisfy energy demands. They are a major energy source for
mammals, but are not used efficiently by fish. For example, mammals can extract
about 4 kcal of energy from 1.0 gram of carbohydrate, whereas fish can only
extract about 1.6 kcal from the same amount of carbohydrate. Channel cat fish have been reported to
utilize polysaccharides such as starch or dextrin more readily than
disaccharide or simple sugars. Studies have also indicated that common carp,
channel catfishes, red sea bream utilize higher levels of dietary carbohydrates
than yellowtail and salmonids. The formulated feed for carnivorous fishes must
contain carbohydrate level less than 20% because they produce very low amount
amylase. Therefore, they are not able to utilize food containing carbohydrates.
In contrast, omnivorous and herbivorous species (such as Indian major carp,
tilapia, channel catfish and others) are able to utilize more than 45%
carbohydrate in the form of cooked starch or mixture of cereal bran’s and oil
cakes. Glucose, maltose and sucrose are however are utilize by fish at varying
degrees. Carbohydrate serves as the least expensive source of dietary energy
and help in improving the pellet quality. Therefore, some form of digestible
carbohydrate should be included in fish diet. Carbohydrate may also serve as
precursor for the various metabolic intermediates necessary for growth that is
dispensable amino acids and nucleic acids. Thus in the absence of adequate
dietary carbohydrates or lipids fish have only protein available to meet their
energy needs. When other sources of energy are available, some protein may be
utilized for growth instead of energy. This relationship between protein and
carbohydrate has been referred as protein-sparing action of carbohydrates.
Lipids : Lipids are organic molecules me up of carbon, hydrogen and oxygen. Fatty acids have
a general structure consisting of a chain of carbon atoms with their associated
hydrogen atoms ending with a carboxylic acid group. They are the rich source of
energy and are insoluble in water but soluble in solvents like acetone, benzene
etc. Chemically fats are triglycerides. Along with important source of energy
essential fatty acids and phospholipids lipids provide a vehicle for absorption
of fat soluble sterols and vitamins. They also play a major role in the
structure of cell and cellular membrane and serve as the precursor of several
hormones synthesis. They are highly digestible in fish and are reported to
spare proteins. Feeding excess lipids may produce fatty fish and it will have
deleterious effect on flavor, consistency and storage life of finished
products.
Major kind of lipids include prostaglandins (regulate
metabolic reactions), steroid (Cholesterol, bile acids and many hormones),
waxes, fatty acids and fats. Simple
lipids include fatty acids and tri-acylglycerols. Fatty acids can be: a)
saturated fatty acids (SFA, no double bonds), b) polyunsaturated fatty acids
(PUFA, >2 double bonds), or c) highly unsaturated fatty acids (HUFA; > 4
double bonds).
Fatty
acids are denoted by formula Cx: y (n-z)
Where
x= number of carbon atoms
Y= number of double bonds in chain
Z= carbon at which 1st
double bond appears from non carboxyl end.
Oil and fats which are made up of combinations of fatty
acids and glycerol molecules are known as neutral fats or triglycerols. They
are the form which store metabolic energy mainly because they are less oxidized
than carbohydrates or proteins and hence yield more energy on oxidation. Some
of the fatty acids are required in the diet of most of the animals because
animals are unable to synthesize these fatty acids themselves so; they are
called essential fatty acids. These fatty acids are essential for normal
growth, moulting and maturation in aquatic animals. Examples are linoleic
(18:2n-6), and linolenic acid (18:3n-3), arachidonic acid, eicosapentaenoic
acid (EPA: 20:5n-3) and docosahexaenoic acid (DHA:22:6n-3). Some of the fatty
acids which can be synthesized in animal body and therefore, not necessary to
be included in diet are known as non- essential fatty acids. In general aquatic
animals raised in freshwater, brackish water and sea water require fatty acids
of the omega 3 and 6 (n-3 and n-6) families. Marine fish oils are naturally
high (>30%) in omega 3 HUFA, and are excellent sources of lipids for the
manufacture of fish diets. Freshwater fish do not require the long chain HUFA,
but often require an 18 carbon n-3 fatty acid, linolenic acid in quantities
ranging from 0.5 to 1.5% of dry diet. This fatty acid cannot be produced by
freshwater fish and must be supplied in the diet. Many freshwater fish can take
this fatty acid through enzyme systems elongate (add carbon atoms) to the
hydrocarbon chain, and then further desaturate (add double bonds) to this
longer hydrocarbon chain. Through these enzyme systems, freshwater fish can
manufacture the longer chain n-3 HUFA, EPA and DHA, which are necessary for
other metabolic functions and as cellular membrane components. Marine fish
typically do not possess this elongation and desaturation enzyme systems, and
require long chain n-3 HUFA in their diets. Other fish species, such as
tilapia, require fatty acids of the n-6 family, while still others, such as
carp or eels, require a combination of n-3 and n-6 fatty acids. One gram of fat
on oxidation gives about 9.0 kcal (37 kilojoules) of energy. Fatty acid sources
includes: Ghee, butter, fish oil, meat, egg, milk, cheese. Plant sources
includes vegetable oil from the seeds of coconut, mustard, sunflower,
safflower, nuts, soybean etc. A recent trend in fish feeds is to use higher
levels of lipids in the diet. Although increasing dietary lipids can help
reduce the high costs of diets by partially sparing protein in the feed,
problems such as excessive fat deposition in the liver can decrease the health
and market quality of fish.
Essential Fatty Acid (EFA) deficiency sign:
The amount of EFA required by warm water fishes is small
in relation to total dry diet weight. Long term feeding of EFA deficient diet
may results in severe erosion of fins in trout. Signs more especially related
to lipid function or dysfunction included altered permeability of membranes as
exhibited by increased rate of swelling of isolated liver mitochondria in
isotonic sucrose solution, fatty degeneration of livers, increased respiration
rate of liver homogenates, decreased hemoglobin levels, and decreased red blood
cell volume. The principal sign of EFA deficiency reported in studies with warm
water fishes have reduced growth rate, reduced feed efficiency and in some
cases increased mortality. Decreased feed utilization efficiency and growth
have been also observed in fish like common carp and in crustaceans.The excess
PUFA without stabilization with antioxidant increase susceptibility of diets to
oxidative rancidity and the production of toxic by-products.
Requirement of lipid of some fresh water fish:
|
Common
name
|
Lipid
level (g/kg) feed)
|
|
Indian carps
|
50-80
|
|
Chinese carps
|
50-80
|
|
Common carps
|
80-100
|
|
Tilapia
|
|
|
(a) Up to 0.5 g
|
100
|
|
(b) Up to 35 g
|
80
|
|
(c) More than 35 g
|
60
|
|
Rainbow trout
|
120
|
|
Cat fish
|
80-120
|
|
Eel
|
100
|
