Brine Shrim- Artemia

 Brine shrimp or Artemia

 

Artemia is Brachiopod, forms an important zooplankton that can be stored in the form of cyst. It can be transported from one place to another and can be again rejuvenated whenever required.

8.1. Brine shrimp classification

    Kingdom: Animalia
      Phylum: Arthropoda
        Sub-phylum: Crustacea
          Class: Branchiopoda
            Order: Anostraca
              Family: Artemiidae
                Genus: Artemia

 

8.2. Brine shrimp-Introduction
Brine shrimp is the English name of the genus Artemia of aquatic crustaceans. Artemia, the only genus in the family Artemiidae, have evolved little since the Triassic period. Artemia were recorded from Lake Urmia, Iran, while Schlösser was the first person to give drawings of Artemia in 1756. Artemia are found worldwide in inland saltwater lakes, but not in oceans. Artemia are zooplankton, like copepods and Daphnia, which are used as live food in the aquarium trade and for marine finfish and crustacean larval culture. The cost of good quality cysts fluctuates with supply and demand. Normally 200,000 to 300,000 nauplii might hatch from each gram of high quality cysts.

 

8.3. Brine shrimp-morphology and physiology

Morphology of Artemia cyst: There is a variation in the size, dry weight and energy content of the strains. Hatching quality, percentage hatching rate and efficiency varies although hatching quality mainly depends on collection site. Temperature and salinity significantly affects survival and growth. Total lipid content and amino acid composition also varies depending on the strain.

 

Cyst consists of three layers: The first layer is hard with lipoprotein, chitin and hematin. The hematin imparts dark brown colour to the layer. This layer provides protection against any kind of mechanical and UV radiation. This layer can be removed by oxidation (decapsulation) by hypochlorite. The Second layer is the outer cuticular membrane which is a multilayered membrane with special filter. This protects the embryo from molecule larger than CO² and acts as permeability barrier. The Third layer is the embryonic cuticle which is transparent, highly elastic. It develops into hatching membrane during hatching.

 

8.4.1. Embryo : Undifferentiated gastrula which is a metabolic at <10% H₂O in the absence of oxygen. Presence of oxygen or cosmic radiation results in formation of free radicals which destroy specific enzyme system in the ametabolic artemia cyst.

 

8.4.2. Physiology of the hatching process

The development of an Artemia cyst from incubation in the hatching medium till nauplius release is shown in the figure.

8.4.3. Development of an Artemia cyst: Development of an Artemia cyst from incubation in seawater until nauplius release, when incubated in seawater the biconcave cyst swells up and becomes spherical within 1 to 2 h. After 12 to 20 h hydration, the cyst shell (including the outer cuticular membrane) bursts (= breaking stage) and the embryo surrounded by the hatching membrane becomes visible. The embryo then leaves the shell completely and hangs underneath the empty shell (the hatching membrane may still be attached to the shell). Through the transparent hatching membrane one can follow the differentiation of the pre-nauplius into the instar I nauplius which starts to move its appendages. Shortly thereafter the hatching membrane breaks open (= hatching) and the free-swimming larva (head first) is born. Dry cysts are very hygroscopic and take up water at a fast rate i.e. within the first hours the volume of the hydrated embryo increases to a maximum of 140% water content. However, the active metabolism starts from 60% water content onwards, provided environmental conditions are favourable.

The aerobic metabolism in the cyst embryo assures the conversion of the carbohydrate reserve into glycogen (as an energy source) and glycerol.

 

8.4.4. Ideal conditions for hatching Artemia cysts

 The optimal conditions for hatching Artemia are: temperature above 25^oC with 28^oC being optimum; salinity of 5 ppt; heavy continuous aeration; constant illumination (example: two 40 watt fluorescent bulbs for a series of four 1-liter hatching cones); and pH of about 8. Stocking density is set by adding no more than 5 grams of cysts per liter of water. Good circulation is needed to keep the cysts in suspension. A container that is V-shaped or cone-shaped is best (2-liter bottles work well; glue a valve on the bottle cap and invert it). The best container is a separation column, found in any lab supply, although it is more expensive. Unhatched cysts, empty shells and hatched nauplii can be easily removed separately. The hatching percentage and density are usually a function of water quality, circulation, and the origin of the cysts.

 

 8.4.5. Brine shrimp-decapsulation and hatching of cyst

Process involved in decapsulation and hatching of cysts and its direct usage to produce nauplii, Artemia cysts are either hatched naturally by incubation in seawater for 24–48 hours or hatched after decapsulation. Decapsulation is the removal of the outer membrane of a cyst called the chorion by dissolution in hypochlorite, without affecting the viability of the embryo. The outer shell often causes problems when not removed since it can harbour bacteria and other organisms which may be harmful to the species feeding on Artemia. Also, non-hatched cysts and their shells cannot be digested and may cause blockage of the gut in fish and crustaceans.

 

8.5.1. Hydration: Hydrate the dry cysts in natural seawater. Use a transparent conical tank or funnel-shaped container (e.g., glass or plastic cylinder, thick plastic bags formed into the desired shape) and keep the cysts in continuous suspension by aerating from the bottom of the apparatus for one hour. Upon hydration, the dry cysts which are deflated like bean seeds become round-shaped. Full hydration is necessary to insure that the inner part of the indented dry cyst shell will be completely exposed when the decapsulation solution is added.

 

8.5.2. Reaction with decapsulation solution (Hypochlorite): Prepare the decapsulation solution using 1N NaOH, Sodium hypochlorite (NaOCI) and seawater. Allow the hydrated cysts to react with the decapsulation solution (hypochlorite) for 7–15 minutes. To prevent damage of embryo, keep the temperature below 40°C by adding ice cubes to the suspension or by using a water bath. A change in colour of the cysts from brown to white to orange usually indicates that the reaction is complete. Check under the microscope if possible.

 

8.5.3. Sieving and washing: Drain the suspension of decapsulated cysts into a fine-mesh sieve and rinse immediately with seawater 6–10 times or until the smell of the hypochlorite is removed. Decapsulated cysts may be fed directly to the cultured fish / crustacean or stored in saturated brine solution at low temperature for future use.

 

8.5.4. Incubation: Incubate the decapsulated cysts for 24–48 hours in natural seawater at a density not greater than 5 grams cysts per liter of incubation medium. For optimum hatching, keep the temperature at 30°C and the pH at 8–9. Provide sufficient light at least during the first two hours or preferably continuous illumination of about 1000 lux (attained with 40-watt flourescent light tube, 20 cm away from the hatching container). Maintain the dissolved oxygen at levels close to saturation, with cysts kept in suspension throughout the incubation period. If culture to the adult stage is not intended, it is preferable to feed the Artemia to the fish or shrimp larvae immediately upon hatching to take advantage of the yolk in the nauplii.

 

8.5.4.1. Harvesting processing and packaging of Artemia cyst

After 15-20 hours of incubation, most of the cysts will be hatched and there will be noticeable colour change in the culture from brown to orange. At this time soft aeration and the pinkish orange nauplii will be seen swimming. An empty or undissolved shell tends to float, while the unhatched cyst and debris sink. A siphon also can be used to remove first the debris and then the nauplii from the bottom. The nauplii should then be collected on a 100-120 micrometer screen, washed with clean water, and placed in a small volume of water. Washing removes contaminants and hatching metabolites. Wash harvested nauplii for feeding.

 

brackish water fishes for aquaculture

 Important brackish water fishes

 

             Coastal aquaculture is one of the high potential areas of increasing overall fish and shellfish production of India. With a long coastal line of 8,129 km India Indian water harbor rich brackish water fish biodiversity inclusive of fin fishes, crustaceans, mollusks and sea weeds. Physical resources include a number of brackish water lakes like Chilika, Vembanad and Pulicate lake; esturine system like Hooghly Matlah, Mahanadi, Godavari, Krishna and Kauvery- Coleroon esturine systems on the east coast. The Narmada – Tapti estuarine system on the west coast and an estimated potential area of 1.2 million ha is amenable for coastal aquaculture. The coastal are of the country also has 2.54 million ha of salt affected soil which are unfit or marginally fit for agriculture excluding 0.57 million ha under mangroves. Biological resources include diverse species of crustaceans, fishes, mollusks and aquatic plant. The important species of crustaceans are  P.monodon, P.indicus, P.merguiensis, P.pencillatus,  P.semisulcatus, P.japonicus, P.monoceros and M.dobsonii. Important fin fish species are Lates calcarifer, Mugil cephalus, Liza macrolepis, Chanos chanos, Epinephalus tauvina and E. malbaricus, Lutjanus spp.  Important crab species are Scylla tranquebarica and S.serrata. Important sea weed species are Gelidiella acerosa, Gracilaria edulis, Sargassum spp. and Turbineria species.

 

            A brief description of important brackish and marine water fin fishes are as follows

 

11.1. Elops mechnata:- Body elongate head conical. Maxilla extends behind the eyes and lower jaw is projecting. Dorsal profile is concave. Body is silvery in colour and fins are yellowish with grayish tinge coloration. It is a carnivorous fish feeding mainly on crustacean. It ascends estuaries and rivers attain maximum size of 700mm.

 

11.2. Megalops cyprinoides:- Body is oblong and slightly compressed laterally. Ventral profile is more convex than dorsal. Eye with narrow adipose lids. Vomer, palatines, pterygoides and sphenoid are with villiform teeth. Dorsal fin originates between mid way of snout and caudal base. Caudal fin is deeply forked. Top of head is dark olive, back bluish- green abdomen is silvery with bluish markings. This is euryhaline fish that tolerate 0-40ppt salinity. This fish migrate to estuaries and river, feeds on fish, crustaceans and other animals. Maturity is attained at 250 mm of length and breed twice a year in coastal waters.

11.3. Etroplus suratensis :-Body is oblong and compressed. Cleft of mouth is small. Maxilla extends more than mid way to lower orbit. Lower jaw is slightly longer than upper jaw. Dorsal fin is single with the spinous portion greater in extant than the soft portion. Lateral line is present in upper part of body. Teeth are lobate and are arranged in single row on each jaw. Caudal fin is slightly emarginated. Colour of fish is light green with eight oblique bands arranged vertically. The dorsal, caudal, ventral and anal fins are of dark leaden colour but the pectoral fin is yellowish with a black base. The fish has strong spine on the dorsal and anal fins to defend it from its predators. It attains maturity at 2^nd year of its age when the fish become more than 100 mm in size and can breed in impoundments like pond. During spawning female cleans algae and other growths from a small area on submerged objects and lays eggs which are fertilized by males. Mother guard eggs which remains attached singly to submerged flat surface. Fecundity is 1200-2000 eggs and growth is 120mm attained in first year with 110 g weight. Maximum size recorded is 450mm. It is also a euryhaline fish tolerating 0-40 ppt of salinity. It matures within a year of its life and breed in captive water almost throughout the year. The eggs are attached in some substratum like weed, twigs, bamboo poles stones and husk. The eggs are fertilized by male and further guarded by female. Eggs hatch in seven days. The seed has vertical bands and a spot on caudal peduncle. The fry feeds on zooplankton and insects. The juveniles and adult feed mainly on filamentous algae and food matters of plant origin including Spirogyra.

 

11.4.  Lates calcarifer:- It is also known as “Giant Pearch” and popularly known as “Bhekti”. It is distributed mainly in central and eastern Indian Ocean region. Its existence is common in Australia, Burma, India, Indonesia, Malaysia, Papua, Phillippines, Singapore and Thailand. In India it is available on east and west coast but is more available in West Bengal region.

Body is oblong, moderately compressed; head is depressed and upper profile concave. Mouth is slightly oblique. Canine teeth are absent. Vomer and palate are equipped with villiform teeth. Two dorsal fins are united at the base of dorsal fins. Pectoral is shorter than ventral and rounded, caudal fin is also rounded with a fan shape. The spinuous and soft part of the dorsal fin are separated by a deep notch. The lateral line extends on the tail. In juveniles the colour is olive brown above with silver belly while in adults it is greenish or bluish above and silvery below. It is carnivorous in habit and feed on fish crustaceans, snails and worms. It also shows cannibalistic habit when there is scarcity of food. It matures at second year of its age when it is more than 400 mm in size. It breed only once in a year in open seas. Eggs are heavy that sink to bottom. It is also euryhaline fish species tolerating 0-40 ppt of salinity. It ascends frequently brackish water and tidal rivers. It can grow up to 200 cm and can attain 100 kg weights. Most common sizes are 25-100cm.  Fishes are reported to be caught by seine, gill nets along the coastal areas, lagoons and estuaries. In culture ponds it is known to attain 1.5 to 3.0 kg first year and 5.0 kg in second year.

 

11.5. Elutheronema tetradactylum:-Mouth is large reaching behind the eyes. Teeth are villiform. Preoperculum is serrated. First dorsal fin originates between origin of pectoral and ventral. Origin of second dorsal fin is opposite to that of anal fin. Anal fin margin is deeply concave. Pectoral fin is with four free rays extending to pelvic. Dorsal side colour is silvery green and abdomen is yellowish white. It is less hardy fish predatory in feeding habit mainly feeds on fish, prawn, mysids, amphipods, isopods, stomatopods. Male fish matures when it reaches to 225 mm and female fish reaches to 285 mm. It is tolerant to salinity changes. Enters and survive in back waters and in estuaries. It enters rives and backwater for spawining. In a span of year it attains annual growth of 190-300mm san can attain maximum length of 200cm.

11.6. Mugil cephalus- Mugil cephalus has a robust body and fatty tissue covering most of the eye. Body is oblong and compressed. Head is broader than its height. Lateral lime scales are 38-42. A band of teeth are present on both jaws. There are two dorsal fins. Pectoral fin originates above middle of body depth. Caudal fin is forked. It is blue green on back and silvery on sides and whitish ventrally. There is 6-7 distinct brown bands dawn the flanks and a dark purple blotch at the base of pectoral fin. Although this fish grows to a maximum of 90 cm the common size range from 35-45 cm. It is filter feeder fish mainly feeding on organisms that occupy lower tropic level in food chain. The main food consists of algae, diatoms, crustaceans, decaying organic matter, detritus, and occasionally on zooplankton. It matures at 250-240 mm of length. Male mature in first year and female mature in second year. Breed in offshore. It is euryhaline fish which can tolerate a salinity of 0-75 ppt. They grow fast some species attain weight of 750gm and length of 45 cm by end of first year.  Egyptians and Romans are the pioneers in Mullet culture. It is an important food fish distributed in tropical and subtropical regions of the world including countries Italy, Israel, Egyptian the west and Korea and Japan in the east.  Among 14 valid species of mullet most important species are Mugil cephalus, Liza microlepis, L.subviridis, and L. tade, M. chelo, M. capito, M. saliens, M. oligolepis, M. cephalus, M.dussumieri, M. troschelli and M. corsula, M.tade and M.parsia and M.oligolepis.

 

11.7. Milk fish (Chanos chanos):- Milk fish is fast growing, eurythermal, euryhaline fish mostly feeding on algal growth at bottom of culture pond. It is hardy fish species that survive in even wide fluctuation of dissolve oxygen and it is also less sensitive to disease. Body is elongated, spindle shaped and moderately compressed. Dorsal portion of head is flat, upper jaw over hanging lower jaw and lower jaw also has a tubercle on its tip. Mouth is small, without teeth anterior and transverse. Pectoral fin is pointed with scaly appendages at base. Ventral fins have a long basal scale inserted under middle of dorsal. Anal fin is small with two rows of scale at base. Upper lobe of deeply forked caudal fin is slightly larger than lower lobe. Dorsal side of body is olive green in colour and abdomen in silvery and whites. Dorsal, anal and tail fins have dark margins. Small size fishes enters the back water, estuaries, lagoons and rivers which serve as nursery grounds and spent early part of its life till about one year. For attaining maturity they return back to the sea and spawn annually or biannually in 5^th or 6^th year of its age releasing about 3-7 millions of eggs each time. The seed of this fish is abundant in calm, clear coastal water near estuaries or lagoons where microscopic algal food is available in plenty. It stays here for about one year when and grows to about 50cm weighing to 500-800gm. Generally they feed on lab-lab. The algal mat consisting of a complex animal-plant combination material. The young larvae feed on algae belonging to bacillariophycea, myxophycea and chlorphyceae. Fry and fingerlings feed upon diatoms, algae, lamellibranches, fish eggs etc. It is primarily a phytoplankton feeder. This fish can be induced bred, fecundity ranges between 2.0to 6.0 million. Larvae migrate to coastal water, estuaries and swamps. It is also a euryhaline fish that can tolerate a salinity of 0-40 ppt. It grows to 200-400 mm and gain weight of 800gm in a year. It is especially cultured in South East Asian countries like Indonesia, Philippines and Taiwan for centuries. The ponds facility in which it is cultured is termed as “Tombak”. 

 

Fish nutrition basics

 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 C_x(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 1^st 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