The Farming Of The Giant Freshwater Prawn
Macrobrachium rosenbergii popularly known as ‘scampi’ has been expanding in India recent years. Scampi farming gained momentum after the set-back in shrimp farming due to disease outbreaks and other factors. The infrastructure available to produce shrimp seed and process the shrimp was helpful in providing support to scampi farming.
The existing culture system includes both monoculture and polyculture with Indian major carps in ponds. Grow out stocking densities range from 0.5-2.5 scampi per m2 in polyculture and 1-5 per m2 in monoculture. The culture period is 6-8 months starting at the beginning of southwest monsoon (June-July, 27-30°C ). The scampi are fed with farm-made or commercial feeds. This article summarizes the nutrition and feeding of freshwater prawns (grow out), with special reference to culture conditions in India.
There is a fairly good amount of information on the nutrient requirements of freshwater prawn. The prawns are capable of digesting a wide range of foods of both plant and animal origin. Characterization of the activities of the digestive enzymes in the alimentary tract indicates the presence of enzymes like trypsin, amino peptidases, proteases, amylases, chitinase, cellulase, esterases and lipases.
Proteins and Amino Acids:
Diets with about 35-40% protein and gross energy level of about 3.2 kcal/g diet and protein:energy ratio of about 125-130 mg protein/kcal are suitable for growth of M. rosenbergii in clear water systems that do not have any supply of natural foods. Broodstock reared in ponds having natural food (benthic micro- and macro fauna) require about 30% protein in the diet. Many commercial feeds for grow-out contain 24-32% crude protein. Protein/starch ratio of 1:1 is known to be effective for better feed efficiency and growth rate.
The prawn requires the same ten essential amino acids as other crustacean and fish species, but quantitative requirements have not been determined. The amino acid composition of the prawn muscle is used to provide guidance values in feed formulation.
The comparatively high specific activity of amylase found for M. rosenbergii supports the fact that the species efficiently utilizes carbohydrates as a source of energy. During fasting, energy metabolism in the prawn is dominated by carbohydrates, followed by lipids and proteins. Complex polysaccharides including starch and dextrin are more effectively utilized than simple sugars. Dietary glucosamine (an amino sugar and intermediary between glucose and chitin) facilitates molting followed by enhanced growth. Dietary protein is efficiently utilized at dietary lipid-carbohydrate ratio of 1:3-1:4. The prawns are also known to utilize as high as 30% dietary fiber.
Lipids and Fatty Acids:
In freshwater prawn that uses dietary carbohydrate efficiently as energy source, protein sparing by lipids is not considered to be crucial. The dietary lipid level in prawn diets can be as low as 5 % provided the lipid source contains sufficient levels of essential fatty acids. There is a dietary requirement for highly unsaturated fatty acids (HUFA) although in very small quantities. Both n-3 and n-6 HUFAs at dietary levels of 0.075% are known to increase weight gain and feed efficiency remarkably. In addition both 18:2n-6 and 18:3n-3 are also required.
M. Rosenbergii, like other crustaceans, is unable to synthesize cholesterol due to the absence of the enzyme 3 hydroxy 3 methylglutaryl CoA reductase. The dietary requirement for cholesterol is approximately 0.3-0.6% in diet.
Substitution with 0.6% ergosterol or stigmesterol is generally not so effective compared to 0.6% cholesterol. However, a mixture of phytosterols (sitosterol, campesterol and dihydrobrassi-casterol) has been found to be as effective as cholesterol. So, unlike in penaeid shrimp feeds, there is no need to add high levels of purified cholesterol in freshwater prawn feeds provided the ingredients contain sufficient levels of phytosterols.
Low level of dietary cholesterol in broodstock diet is known to adversely affect egg quality resulting in inferior quality of seed production The cholesterol content in the eggs and hepatopancreas, and total lipid content in the ovary and hepatopancreas of pond reared broodstock fed with a diet containing 30% crude protein and 5% lipid was significantly lower when compared to the eggs from wild broodstock collected from the lower reaches of the river Brahmini in Orissa, India. Higher levels of lipids and cholesterol are probably key factors in egg maturation and egg quality.
The freshwater prawn also has limited ability to biosynthesize phospholipid (PL) de novo. A basal level of 0.8% dietary PLs is required to meet the demand of the scampi broodstock. A dietary source of phosphatidylcholine (PC) in the form of soy-lecithin is essential for larval growth and survival. Supplementation of larval diets with 5% soy-lecithin along with 1 % cod liver oil and 1% groundnut oil improved growth rate by164%. In the absence of sufficient levels of bile salts during development, dietary PC may also enhance the assimilation of ingested fats by acting as temporary emulsifier.
Vitamin requirements of M. rosenbergii are probably similar to other crustaceans and fish species. The prawn requires 60-150mg vitamin C/kg diet. Levels of 60mg ascorbic acid and 300 mg -tocopherol per kg diet are considered sufficient for proper reproduction and offspring viability in prawn broodstock. However, feeding female prawn with higher levels of both these vitamins (each around 900 mg/kg) might improve larval quality including higher tolerance to ammonia stress. It has been reported that vitamin E at 200 mg/kg diet modulated some of the antioxidants defense system by decreasing lipid peroxidation in the hepatopancreas.
Information on quantitative mineral requirement of freshwater prawn is limited. Dietary supply of calcium seems to improve growth of freshwater prawn. Performance of the prawns were better when calcium was provided at 3% level in soft water (Calcium concentration at 5 ppm). Even when the calcium concentration was higher at 74 ppm, performance improved when calcium was provided at 1.8%. The optimum level of zinc is at 50-90mg/kg diet. Growth and feed conversion efficiency declined at higher dietary doses (> 90mg/kg) of zinc.
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