THA:75:008/80/WP/12 THE DIET OF PRAWNS

by

Michael B. New
Senior Fisheries Biologist (Aquaculture)
Programme for the Expansion of Freshwater
Prawn Farming in Thailand

(FAO/UNDP/THA/75/008)

Bangpakong, Chacheongsao
Thailand
1980

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THE DIET OF PRAWNS^a

by

Michael B. New^b

LECTURE I: WHAT IS KNOWN ABOUT PRAWN NUTRITION?

1. INTRODUCTION

The knowledge of shrimp nutrition is less than that about fish nutrition. This first paper deals with the result of ‘controlled’ experiments; the second is on a more practical level and deals with real commercial, rather than research, conditions.

I first reviewed this topic four years ago (New, 1976). In the same year, Biddle presented a paper on the same topic, but restricted to the nutrition of freshwater prawns, at a meeting in the USA. His paper was largely based on mine but contained some additional publication and formed part of a book on shrimp farming (Biddle, 1976). Today's lecture has been based on these two reviews, updated by an examination of the literature published since 1976. A bibliography on this topic is in preparation (New, 1980).

The notes that follow are a record of my lecture notes and therefore only summarise what was actually said.

Brief comments were made on the confusion in the terminology (i.e., the word shrimp versus prawn); the differences between marine and freshwater prawns and between penaeids and carideans; the method of crustacean growth, etc.

Crustaceans absorb nutrients directly from the water as well as from ingested material; thus physiology and nutrition is likely to be complex. This lecture deals only with nutrition (strictly dietary experiments), not with physiology.

Most dietary work has been carried out by feedstuff manufacturers; the results are therefore proprietary.

The importance of diet in prawn culture must be borne in mind…food will be the single most expensive running cost in rearing prawns when the labour of feeding and of producing live feeds is taken into account. Feeding probably represents 50–60% of running costs.

^a This working paper is based on two lectures given to the 2nd Inland Aquaculture Course given at the National Inland Fisheries Institute, Bangkhen, Thailand (June-August 1979).

^b Senior Fisheries Biologist (Aquculture, (UNDP/FAO/THA/75/008).

Some of the statements which I make about prawn nutrition may be apparently conflicting…This only demonstrates the paucity of knowledge on the subject and the fact that crustacean nutritional research is very young.

2. POSTLARVAL NUTRITION

a) Palatability and Physical Dietary Structure

• Both palatability and structure of the diet affect ingestion.

• The manner in which prawns eat influences the need for dietary water stability.

• Wet diets appear to give superior growth rates and survival.

Carideans lack the gastric milling apparatus present in the anterior chamber of the proventriculus of penaeids. Dry feeds may congest the proventriculus and hamper enzymatic mixing with foods. Penaeids may tolerate dry foods better. Wet diets however pose manufacturing and storage problems.

The processing of dietary ingredients can damage their nutritional quality; there is some evidence that compounding does too. Care must therefore be taken in ingredient selection.

Many binders are used in experimental rations for postlarval prawns; there is no evidence that those used are inhibitory…some are in fact nutritious as well as being binders. Some examples of binders:

Agar
Gelatin
Carboxy methyl cellulose
Poly vinyl alcohol
Guar Gum
Sodium alginate
Manucol (Glycol poly ester alginate)
Collagen
L Starch
Sticky Rice
High Gluten Wheat
Amaizo Starch

Diets which are not water stable pollute environment more.

Ingestion rate is governed by the filling of the digestive gland…this is regulated by food size. Therefore the animals' milling ability is important.

More work on food form and size is needed; prawns will accept most foods presented; this does not mean that this criterion can be used to judge the efficacy of a feed.

Wet diets are less practical; however, water stability may be less important if the labour to feed several times a day or automatic feeders are available.

For any diet, the final arbiter of its characteristics will be its efficiencv…by this I do not mean food-conversion rate; protein conversion rate, etc.,…I mean the cost of the food needed to produce each kilogram of marketable prawns.

The chemosensory properties of diets are also important. Diets should release attractant substances. Live feeds and fresh animal products do this. Prawns are attracted even to other prawns which are dying. This phenomenon is quite separate to the topic of pheromones.

Prawns are stimulated into feeding activity by substances such as betaine (trimethylammonium hydroxide), morin (a fragrant aromatic compound), egg-white proteins and individual non-essential amino acids such as glutamic acid, glycine and taurine. One American company has even patented (for what that's worth¡¡) a method of inducing hunting and feeding reactions in prawns by the use of a mixture of mono-sodium glutamate with sodium or potassium aspartate.

b) Lipids and Sterols

Lipids

With carbohydrates, they form sources of energy…if feed energy is too low animals will utilise other nutrients, e.g. protein, to satisfy demand for energy, which is very costly; if feed energy is too high, reduced food intake will result and therefore not enough protein will be ingested to give good growth.

However, optimum calorie-protein ratios for prawns are not yet understood (although one group of scientists claim 40% protein and 3.3 Kcal/g is a good combination for Penaeus monodon).

I am going to talk about carbohydrates later; let us consider lipids first.

There seems to be some evidence that prawns cannot tolerate high levels of dietary fat. If this is so, then it is in marked contrast to fish. The fact that lipase activity in prawns is limited has been demonstrated (palmitic acid is incorporated into tissue three times as rapidly as tripalmitin).

One worker showed that 10% or more of a ⅓ beef tallow, ⅓ corn oil and ⅓ menhaden oil mixture depressed growth and survival. Another found that to increase the dietary level of cod liver oil over 5% produced no benefits; the same applied to corn oil. Confusingly, my own personal experience is that Macrobrachium can tolerate high oil diets, at least when they originate from accelerated freeze dried egg. I used a diet containing 20% oil. Generally, 5–7% levels are suggested; certainly not more than 10%.

The type of lipid is also important. Tissue lipids mimic dietary lipids in quality, although w3 fatty acids are retained in the tissue and w6 fatty acids are metabolised for energy. The predominant fatty acids in prawns are palmitic acid and w3 polyunsaturated fatty acids. Oils with high w6 saturated fatty acids and low levels of w3 fatty acids seem inhibitory. Fats which are suitable include:

Linseed oil
Linolenic acid (1%)
Clam oil
Fish oils
Shrimp head oil

Fats that give poor results if used alone are:

Soybean oil
Corn oil
Coconut oil

Shrimp fed diets with shrimp head oil also have more carotenoid pigment; an important consumer characteristic. The use of shrimp meals as a source of w3 fatty acids is cautionary because of the high variability of these meals.

Both linoleic (w6) and linolenic (w3) acids are essential fatty acids for prawns but the nutritive value of linolenic is higher.

Sterols

Prawns, like other crustaceans, cannot convert (synthesise) acetate into sterols.

Dietary levels of 0.5% cholesterol seem advantageous; 1.0% levels are no better; 5% cholesterol depresses growth.

0.5% of stigmasterol, ergosterol or B-sitosterol give similar survival to 0.5% cholesterol but poorer growth rates. Shrimp can convert these to cholesterol and, in contrast to the fat, absorb other sterols well.

Sterols are important because they are:

• Elements of cellular structure

• Precursors of steroid hormones, brain hormones, ecdysones and vitamin D.

• Shrimp can absorb dietary ecdysones.

It isn't likely that there will be a deficiency of cholesterol in any mixed diet, especially if it contains fish meal; dietary excess must however be watched in formulation.

There is commercial interest in the possibility of using dietary sterols, such as ecdysone, etc., for suppressing gonadal development, reducing agression, synchronising molting, increasing growth rate, etc.

Substances such as mussels or clams are rich in steroids; I doubt if any compounded diet has yet produced such good results as mussel as a food for prawns.

c) Proteins and Amino Acids

Protein is of dominant nutritional importance and also is responsible for the greatest cost component in any diet. It has therefore received more attention than any other nutrient to date. In spite of this, no clear cut guides to optimum protein levels in prawn diets have yet emerged.

Crustacean proteases are mainly tryptic; no pepsin-like enzymes are present; however, a full enzymatic complement is unnecessary if predigested food is available. Shrimp digest detritus and faeces, for example: 85% of the micro-organisms in shrimp gut produce chitinase. These bugs multiply within the gut also to provide an extra source of nutrition; most importantly they give the ability to the prawns to digest chitin…a protein/carbohydrate complex.

Nutritional experiments using casein as the only source of protein, though used in some mineral and vitamin assessments, do not succeed well. Diets based on chemically defined ingredients, e.g., containing mixtures of pure aminoacids or peptides, do not produce good growth. Early attempts to use chemically defined diets for prawn nutritional studies therefore quickly gave way to dietary experiments using complex and variable feedstuff ingredients. For this reason, reports of optimum protein level for prawn diets reported in the literature are difficult, if not impossible, to interpret because of the possible effect of concomitant changes in other dietary components.

There is thus a very wide range of optimum protein levels claimed by different workers. Depending on the ingredients used, optimum protein levels between 15% and more than 60% have been recorded. Comparison is also difficult because experimentation has been with different species.

From an economic point of view, perhaps the most encouraging result was that of one worker, working with Macrobrachium, who reported that best results were obtained with a soy-tuna-shrimp meal diet with only 15% protein. The diet proved better with older prawns (more than 120 days old); young animals appear to require protein levels in excess of 35%, using this range of ingredients.

A vast range of high animal protein ingredients have been used in prawn rations…virtually all the conventional feedstuff ingredients plus squid meal, prawn meal, poultry faeces (be careful of toxicity), single cell proteins, etc. Macrobrachium, which seems more vegetable inclined than Penaeus, thrives well on rations containing coconut meal, soybean meal and acacia meal.

The consensus of opinion seems to be that protein levels in the 27–35% region are the best, but commercial rations for shrimp used in the USA have 20–25% levels.

It has not yet been shown experimentally whether lipid spares dietary protein for tissue production rather than energy utilisation. Work with isocaloric diets (increasing lipid levels while reducing protein to produce diets with the same level of energy) has not been done yet. Optimum protein-calorie ratios are unknown.

The quantitative amino acid requirements of prawns are comparatively unknown yet…therefore much of the protein fed in rations is sure to be unbalanced in its amino-acid composition. This masks the results of protein experiments and is probably the cause of reports that extremely high levels are required.

Preliminary results indicate dietary requirements for:

1.4% lysine
1.3% arginine
3.0% menthionine
0.6% tryptophane

Qualitatively, the amino acids which are essential in the diet of prawns seem to be similar to those for other animals although there are some disagreement on this. Apparent non-essentiality of some amino acids is suspected to be caused by the ability of the gut bacteria to synthesise them. The following amino acids have been shown to be essential for Palamon serratus, Penaeus setiferus and Macrobrachium ohione:

Arginine
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophane
Tyrosine
Valine

The value of free amino acids (i.e., not bound in protein) as attractants in prawn diets has been mentioned before. Free amino-acids in the body of prawns play an essential role in osmo-regulation.

Prawn meal has, in many rations, been experimentally shown to be a particularly useful high protein ingredient which, up to now, has been under-utilised in animal feedstuffs. One research worker, working with P. indicus, an omniverous species, suggested that a 60:40 ratio of fish meal: prawn meal gave the best results in his series of experiments.

d) Carbohydrates

Many carbohydrases exist in crustacea including L and B amylase, maltase, sacdharase, chitinase and cellulase.

Carbohydrates, with lipids, form a source of dietary energy and are also important in the storage of energy through glycogen, in chitin synthesis, and in steroid and fatty acid synthesis.

Wheat starch, dextrin, and oyster glycogen are assimilated better than potato starch. Not all species assimilate specific carbohydrates with the same efficiency.

Cellulose is also partly digested…thus prawns can utilise marine algae.

Crustaceans are able to utilise complex polysaccharides (starches) much more efficiently than some simple sugars such as glucose. This is in marked contrast to fish. It is postulated that glucose is rapidly but inefficiently utilised while polysaccharides are absorbed more slowly but more efficiently. Some recent work indicates again that glycogen is a good source of carbohydrate for prawns; so also is sucrose, a simple sugar…this is a confusing picture therefore.

Starches are also used in diets as binders, as has been mentioned previously.

Extra-cellular chitinases enable the digestion of chitin from dietary sources or from cast exuviae.

Dietary carbohydrate has a sparing effect on the carbon chains from amino acids, and therefore on dietary protein, for chitin synthesis. Chitin synthesis is required for the exoskeleton and for the periotrophic faecal membrane in penaeid prawns.

Some workers have concluded that glucosamine is non-essential in prawn diets but others have found that replacement of glucosamine with glucose checked growth rate. Glucosamine is an intermediary between glucose and chitin, which is a protein-carbohydrate complex.

Little is known about fibre requirements in prawn diets but one worker managed to restore decreases in growth rate caused by excessive levels of protein by increasing dietary fibre (vegetable) content in the diet. However, the gastro-intestinal tract in prawns is short. Therefore the passage of food is rapid and time for digestion limited. There is the possibility that fibre may increase motility and reduce the efficiency of the diet.

Although optimum protein-calorie ratios are not yet quantified adequately, there is some evidence that, at a 25% protein level, 1:1 and 2:1 ratios of carbohydrate:fat result in protein catabolism, whereas 3:1 and 4:1 ratios reduce this effect and seem sufficient to satisfy energy requirements.

e) Minerals

Few studies have reported on this subject but many diets have used mineral premixes as a kind of insurance policy.

Results of early experiments involved different mineral supplement levels and placed emphasis on the Ca:P ratios in the premix without reference to the content of these two minerals in other dietary ingredients, especially in materials like fish and prawn meal.

To date, workers report that dietary CA:P ratios of above 2.4:1 seem to depress growth. However, one worker with marine shrimp claims that the Ca the prawns need can be totally absorbed from the (sea) water they live in and that dietary Ca is non-essential. He also says that Mg and Fe are non-essential but 2% P; 1% K and 0.2% of a trace element premix proved useful. The level of P in seawater is low and it is therefore claimed to be essential in the diet because prawn flesh is high in it.

f) Vitamins

Many diets for prawns have also used vitamin mixes designed for other animals as an insurance policy but, in many cases, enormously high levels of vitamins, particularly of Vitamin D, choline and Vitamin C have been given and the potential for acute hypervitaminosis or antagonistic reactions must have been high.

Study of the vitamin requirements of prawns has, as yet, received little attention, as is the case with mineral requirements.

Qualitatively, most of the B group of vitamins seem essential in the diet of crustacea, as are vitamins C and E. Vitamin D may come partly from dietary sources but it can be synthesised from ergosterol. It has been shown to be non-essential in insect diets. Vitamin K may actually be antagonistic to some species of crustacea.

Vitamin A is probably inessential but its precursors may be essential. Often the origin is B-Carotene, which is converted into astaxanthin as well and is indirectly responsible for crustacean pigmentation. Prawn meal can be used as a source of carotenoids but these can be destroyed by excessive heat during the production of prawn meal and also by exposure to light and atmospheric oxygen. The quality of prawn meal utilised is therefore important. Spirulina, a blue-green alga, is a better precursor of astaxanthin than alfalfa or corn gluten. Artemia is, of course, a good source of canthaxanthin.

Vitamin C seems to be essential in prawn diets and appears to accelerate growth at low levels but depress growth at high levels. Stress (through high density culture of disease or bad water quality) can cause vitamin C depletion in tissues; tissue levels can be maintained by dietary inclusion of Vitamin C. One worker has claimed that 0.5–1.0% dietary vitamin C is the quantitative requirement but another, testing a range of levels between 0% and 1% suggested that 0.3% was optimal and that higher levels depressed growth.

The only other vitamin which seems to have been examined in prawn diets is inositol and a 0.4% level has been postulated as optimal.

g) Other Substances

So far, I have only seen one report of an antibiotic used in prawn diets. This was in an experiment designed as part of a disease investigation but there were some interesting side effects. In a short term experiment, using dietary levels of 100–1000 mg/kg of dry food of oxytetracycline, feed intake dropped to between ⅓ and ¼ of that of the controls. Food conversion efficiency however increased and, while larger shrimp (av 0.46g) grew less well, small shrimp (av.0.14g) actually grew faster than the control.

h) Feeding Rate and Frequency

Finally, in this section on the nutrition of postlarval prawns, a few words about feeding methods.

Dietary experiments have been carried out mostly by feeding ‘to demand’ but feeding based on estimates of biomass would be more efficient.

Ingestion rates are inversely related to animal size. Feeding rates of 10% of biomass (sometimes as high as 100%) are employed in the first two postlarval months but these decline to 3–5%.

Little is known about the effects of varying environmental conditions on food consumption and the efficiency of food utilisation.

In nature, prawns eat frequently and rapidly. They also have short digestive tracts and little time for nutrient absorption. Feeding three or four times per day, rather than once, may prove better and also solve some of the dietary stability problems. Decreased ingestion rates have been observed after more than 6 hours exposure by prawns to the same food. Another important observation is that, even in an apparently well bound diet, more than 90% of its thiamine content leaches from the diet within 18 hours. This sort of effect is probably experienced by other water soluble nutrients.

3. LARVAL NUTRITION

Although penaeid and caridean larvae have different food requirements (this topic is dealt with more fully in my second talk), the emphasis in large scale larval production has in both cases been on the use of mixtures of live feeds with other natural materials. I am not going to say very much about the topic of experimental ‘artificial’ diets for larvae, because not much is known yet. I will be talking more about practical diets in my second talk.

There has been some preliminary work on the development of compounded diets for prawn larvae and it is to be expected that this type of research will grow, with the high cost of live foods (the cost of live food production, as well as the cost of Artemia cysts, for example, must be borne in mind) acting as an important stimulant to success.

The physical problems in making artifical larval food include:

1. Nutrients must not leach out

2. The food must not pollute the rearing water

3. The food must be available physically close to the larvae

4. It must be of a size which larvae can tackle; the zoea stages of marine prawn larvae can only eat algae at approximately 3–10 u but the mysis stages can eat Artemia nauplii, at ten times the size; Macrobrachium larvae on the other hand can tackle Artemia naupalii almost immediately after hatching: they can also use larger particles of food but if the particles are too large, the weight of several larvae drags larvae and food to the bottom of the tank.

5. It must attract the larvae

6. It must be digestible

If a larval diet for Macrobrachium is to consist of particles of a size range between 250 u and 1500 u for example and if each particle is to represent a balanced mixture of all the dietary ingredients, the latter have to be ground extremely fine indeed.

Neutral dietary buoyancy in water is difficult to achieve; binding the diet without making it inedible is also difficult.

Some larval diets have however been formulated:

1. Microcapsules with cross linked nylon- protein walls: these can be produced in a range of sizes from 5 u to 250 u with the property of neutral buoyancy. Macrobrachium larvae, Artemia and crab zoea have been shown to ingest them.

2. Flaked and gel (alginate) and freeze dried feeds have been tried. Gel and freeze dried diets were neutrally buoyant.

So far, results of larval production experiments using such diets have not been good unless live Artemia are also fed or an extract of Artemia is included in the diet.

LECTURE II: WHAT FEEDS ARE USED IN COMMERCIAL PRAWN PRODUCTION?

1. INTRODUCTION

In my first talk I spoke about research work designed to increase our basic knowledge of the nutritional requirements of prawns. In the case of postlarval diets, such knowledge is fundamentally important to those who wish to culture prawns in environmentally controlled systems where natural feeds may be unavailable. In the case of larvae, ‘artificial feeds’ are critically needed to remove or lessen dependence on scarce and costly resources of suitable live feed. In this second talk I am going to speak about those feeds which are used in hatcheries and ponds where prawns are reared.

In the previous talk, I spoke first about postlarval diets because more progress has been made with them than with the development of ‘artificial’ larval diets. In this talk I am going to reverse the order, since primary attention has been paid to hatchery diets rather than to diets for use in ponds for postlarval rearing, where natural foods are also available.

2. FEEDS FOR LARVAL PRAWNS

In their natural environment it is likely that prawn larvae consume a vast range of materials which are present in the water table with them; anything probably which presents itself to them and is of the right particle size. This includes live phytoplankton and zooplankton of many types and non-live particles.

In my first talk I generalised between caridean and penaeid dietary requirements as much as I could. In this section of this talk I must separate the two groups since they have rather different requirements in terms of both food and feeding techniques. The reason for these differences can be understood through an examination the difference in the life history of Penaeus spp. and Macrobrachium spp. In making these comparisons I must make it clear that there are differences between different species in each group as well so, when I talk about the length of each larval stage, for example, I am not talking about any particular species but trying to point out major differences between the two groups of prawns. Basic differences are summarised in the following table:

Thus it is necessary to culture algae in marine shrimp hatcheries but not in freshwater prawn hatcheries (although this comment is qualified later). The larval life of penaeids is shorter and Artemia are only required for about 5 days; Macrobrachium require Artemia for the whole of the larval life of 18 days +. Conversely, penaeids have a requirement for live foods in their early postlarval life, whereas Macrobrachium does not.

Thus hatcheries designed for marine prawn production use quite different techniques to those in freshwater prawn hatcheries; this does not only apply to feeding regime. Perhaps these differences are so great as to make hatcheries attempting to rear both types of prawn inadvisable. I will therefore deal with the feeding of marine and freshwater prawns separately. It must be stressed that every different hatchery uses a different feeding regime and frequent changes are made, based on local experience and on food availability. I cannot hope to cover all these variations and therefore I am just talking about examples for illustrative purposes.

a) Penaeid Larvae

Newly hatched marine prawn larvae live off their egg yolk and the nauplii do not feed. However, chemical fertilisation of their rearing tanks is practised from the first day so that phytoplankton are available for consumption as soon as the animals become zoea (in some hatcheries, phytopplankton are cultured in separate tanks).

After 2–3 days the nauplii become zoea and begin to feed on phytoplankton. Diatoms are best. Species such as Skeletonema, Thalassiosira, Melosira and Nitzschia have been used, as well as Tetraselmis. When grown with the larvae, phytoplankton concentrations of 5,000–20,000 cells/ml are used. 2–3g/m³ of KNO₃ and 0.2–0.3g/m³ of NaH₂ PO₄ will achieve this level. If diatoms fail, as sometimes unpredictably happens in phytoplankton culture, formula feed, yeast and soybean cake, crumbled to less than 100 u, has been successfully substituted.

You can alter the predominant species of phytoplankton present but this is best achieved if the algae are grown separately from the larvae. The variable quality of seawater often causes problems in phytoplankton culture and artificial seawater is sometimes used in hatcheries. In marine shrimp larval tanks, brown water is taken as an indication that predominantly diatoms are present, which is good; green water heralds a failure. The converse is the case with freshwater prawn larval tanks.

After another 5 days, the zoea change into the mysis stage and are able to take Artemia. They are now about the same size as Macrobrachium larvae are when they are first hatched. Artemia are usually supplied to the mysis larvae at densities varying from 0.3–3 nauplii/ml.

After a further 4–5 days or so, the larvae metamorphose into postlarvae but continue to be presented with Artemia over the first 4–5 days, while they are changing from an omnivorous to a mainly carnivorous diet. At this stage they are consuming 50–90 Artemia nauplii each per day. After that they are fed a natural food such as processed molluscan flesh or a formula feed as well as Artemia, which is discontinued after two more days.

It is important to realise, as is the case with the feeding of Macrobrachium larvae, that it is the density of food particles per unit of water volume that is most important, not the number of particles per larva present in the tank. This is why, for efficient utilisation of food available, optimisation of larval rearing density is sought.

The above describes a typical larval feeding system for penaeids; one worker has however claimed successful rearing of two species of Penaeus fed entirely on a monoculture of the diatom Chaetoceros gracilis at concentrations of 30,000–100,000 cells per ml.

b) Caridean (Macrobrachium Rosenbergii) Larvae

Unlike penaeids, they have no essential requirement for algae.

They will eat living animal foods like Artemia, the rotifer Brachionus and prepared foods almost immediately after hatching.

By the time Macrobrachium larvae have reached the ninth stage they are, like postlarval penaeid prawns, consuming about 80 Artemia naupalii per day.

Once again, the density of food is important and, where Artemia nauplii are the sole source of food, a density of nauplii of between 5 and 10/ml is essential, whatever the density of larvae in the tank.

Larvae will grow and survive well and metamorphose on a diet of Artemia nauplii alone, but there is evidence that the presence of other foods after the first few days results in better production. For this reason and to economise in the use of Artemia, other foods are used to replace part of the Artemia, such as sieved fish flesh, cooked egg custard, frozen adult Artemia, etc. Although there is evidence that the nutritional value of Artemia nauplii can be improved by allowing them to graze on phytoplankton before feeding, adult Artemia are unsuitable as a food as many evade the larvae.

Other live foods have been used, such as Brachionus, fish eggs, Daphnia, oyster larvae, etc., but none of them and no ‘artificial’ food has yet been found which can successfully totally replace Artemia as a larval food for prawn larvae. The critical requirement of these and some fish larvae for Artemia makes the supply, demand and cost of brine shrimp an overwhelmingly important topic. Successful insulation of Artemia, leading to the production of “Thai-grown cysts in salt ponds, has been reported (Tunsutepanich, 1979).” This work given the possibility of cheaper, local supply of Artemia which is, after all, an ideal pre-packaged larval food.

Although Macrobrachium larvae do not require phytoplankton (although they will ingest algae, evidence shows that they do not derive any nutrients from the phytoplankton tested), there is evidence that the presence of phytoplankton in larval rearing water (‘green water’), contrary to penaeid larval culture, is beneficial. Green water is claimed to improve water quality and to reduce disease risks, but the real reasons for its success are unknown. Phytoplankton are capable of converting the excretary products toxic to larvae (un-ionised NH₃), which are produced by the larvae themselves and also by the Artemia nauplii present and by the degradation of other decomposing feeds, to less harmful nitrates. Many hatcheries however, including the government one close to us here, are currently successfully using a clear water system which does not involve green water, using only Artemia nauplii and fish flesh or mussel/egg custard as feeds. One of the big problems with the use of a green water, or polyculture of phytoplankton species is that its production is unpredictable.

As mentioned above, the use of other feeds enables the quantity of Artemia needed to be reduced. For example, in our hatchery in England, which by necessity was environmentally controlled, we originally used the nauplii from 50g of Artemia cysts per day per 600 litre tank. This was equivalent to approximately 8 nauplii/ml/day and an availability (not a consumption) of 200 nauplii/larvae/day. By using sieved mackerel flesh (a fish similar to skipjack tuna in its characteristics as a food), graded to less than 250 u and fed at a rate of 50g per 600 litres per day, we were able to reduce the quantity of Artemia cysts used to 5g per tank per day… the equivalent of nearly 1 nauplii per ml per day or the availability of 20 nauplii per larvae per day. In each case we were raising batches of 20,000–30,000 larvae per tank (equivalent to 33–50 larvae stocked per ml) and we were using some 5 litres of phytoplankton (Chlamydomonas) at an original density of 1,500,000 cells per ml. Thus the density of phytoplankton in the rearing tanks was approximately 12,500 cells/ml only: these algae were added to improve the quality of Artemia, not as a food for prawn larvae.

In general, in mixed food regimes, other foods are presented during the daytime, when utilisation can be judged by eye, at several different feeding times. The Artemia nauplii portion of the diet is normally presented in the late evening as the last feed as a means of ensuring that food supplies do not run out during the night. It is essential that larvae are able to feed continuously.

3. DIETS FOR POSTLARVAE

Under natural conditions, postlarval prawns consume a very wide range of foods.

Penaeids are considered to be either omnivorous scavengers or detritus feeders. The natural diet of such species as P. mondon, P. indicus and P. merguiensis consists of molluscs, small crustaceans, polychaetes, algae and detriuts. P. indicus can eat larger crutaceans. All can be cannibalistic. Metapenaeus spp. seem to be more vegetarian and detritus inclined. Thus, polyculture of penaeids, rather than monoculture, may be a more efficient way to utilise a pond.

Macrobrachium postlarvae seem to feed most actively at night and are omnivorous, eating aquatic insects, fish, molluscs, other crustaceans, algae, leaves and stems of aquatic plants, plant seeds, detritus, etc. There is a belief that Macrobrachium spp. are more vegetarian in their diet than penaeids, but they will not thrive on a diet of algae alone.

Both marine and freshwater prawns have access to chitin from insects, crustacea and bacteria. Thus prawn meal may not be so essential in diets designed to be fed in ponds as it seems to be in cases where the compounded diet is the sole feed. Obviously, in the pond culture of prawns, diet can be very varied as there is a great deal of natural food available. The composition of supplementary diets may not have to be as carefully considered as might at first be expected. However, in highly intensive systems, where natural food will be insufficient, the composition of supplementary feeds may become critical.

I am first going to deal briefly with the stimulation of natural pond food before going on to talk about supplementary feeds.

In the case of marine prawn culture, natural food production is normally encouraged by fertilisation. Generally fertilisation and/or feeding is necessary if production levels are to be raised above 400–500kg/ha/yr.

The practice of fertilisation brings with it some problems such as dense phytoplankton blooms which may cause (in addition to gross turbidity) supersaturation of DO₂ by day and DO₂ depletion at night. The subject of pond fertilisation is a whole separate topic on its own and I suspect that you will be receiving other lectures about this in relation to fish culture generally.

When talking about fertilisation regimes, there can be no true generalisation since the system of fertilisation suited best to each individual pond depends on the type of fertiliser locally available, on water quality, on soil conditions, on the feeding regime to be employed, etc. The following are given as examples of some fertilisation programmes which have been recommended for marine shrimp culture:

1. Sun-dry the ponds; apply 170kg/ha of cottonseed meal; add a further 56kg to the water every week.

2. For nursery ponds (the first two months), encourage the growth of ‘lab-lab’, which is a basis of blue-green algae in which diatoms and other plants and animals grow, by applying 350kg/ha of chicken manure to the dried pond; alternatively, 50–100kg/ha of 18:46:0 (N:P:K) or 100–150 kg/ha of 16:20:0 (N:P:K) can be used; these amounts are then added to the water again every two weeks.

3. In production ponds, to encourage the growth of ‘lumut’, basically a filamentous green algae, add an initial 180–200kg/ha of 16:20:0 (N:P:K) fertiliser to the water, followed by 90–100kg/ha of the same fertiliser every other week.

4. For phytoplankton growth, diatoms being preferable for penaeids, use of 20:1 or 30:1 ratios of N:P are suggested. Ratios of 1:1 are best for flagellate production.

Note: Ammonia fertilisers are mostly absorbed by the soil. They are therefore better for blue-green algae production. Nitrate fertilisers tend to remain dissolved in the water and are therefore better for phytoplankton production.

The amount of fertiliser to use for phytoplankton production is best assessed on a trial and error system. To start with, 1ppm of N and 0.1ppm of P is suggested; further applications should be based on results.

Ponds for cultivation of Macrobrachium are often not intentionally fertilised but some farmers do so mainly to encourage a phytoplankton bloom to discourage filamentous algae and provide cover. Water depth and flushing rates are also manipulated to encourage phytoplankton growth rather than filamentous algae; an interesting contrast to the use of ‘lumut’ in marine shrimp culture; harvesting techniques also, of course differ.

Now we come to the point where I should say something about supplemental food for postlarval prawns and I could easily end my lecture simply by saying “you name it, they feed it”¡¡. Probably much of the supplemental food added acts as a fertiliser and increases the biological productivity of the pond as a whole rather than acting as a true prawn feed. Certainly it seems that, particularly in Macrobrachium culture, much of the pelleted food added is eaten by small fish, which themselves form a source of food for the prawns. Larger fish, of course, not only become competitors for total pond nutrients but may be predatory.

Supplemental foods fall mainly into three categories:

1. Raw materials and waste products

2. Compound feedstuffs designed for other animals but used for prawns

3. Compounded prawn feeds

I propose only to give some examples of each, without making judgements about which is ‘best’. This judgement must depend entirely on the interrelating factors of food availability, food cost and productions rates of marketable prawns achieved in each specific location; generalisation is therefore unwise.

a) Examples of raw materials and waste products:

Rice bran
Broken rice
Trash fish
Fish offal
Dead poultry
Slaughterhouse waste (poultry, beef, hog, etc.)
Fish processing waste
Prawn processing wastes
Snails
Mussels
Clam
Squid
By-products of vegetable oil manufacture
All types of plant waste

b) Examples of compounded diets for other animals:

Chicken feed (18–26% protein)
Hog feed (probably 14–20% protein)

c) Examples of compounded prawn diets:

A few diets are on the market but their formulae are kept secret. Although major ingredients may be listed, the quantities of each are not given. Ralston Purina sell 20%, 25% and, I believe, 30% diets for shrimp (marine), which have also been used for freshwater prawns.

I believe some proprietary prawn compounds are on sale in Japan, the Philippines and Taiwan. I am hoping that, in our discussion time in a few minutes, you will be able to tell me what foods are used in prawn production in your own countries.

The following is a list of the ingredients of a typical formula for the Purina 25% marine shrimp ration. In examining it one must remember not only that the quantities are not stated but that it is normal policy for feedstuff manufacturers to alter the ingredient composition of any of their compounds, without notice, according to current availability and ingredient cost.
The list is as follows:

The following mixture is used by some Hawaiian freshwater prawn farmers:

90% Broiler Starter Pellets
10% Prawn meal

The following is a shrimp ration used for pond culture experiments for Penaeus monodon in Tahiti:

Those of you who are good at mental arithmetic and are still awake at this late stage in the afternoon will notice that this list of ingredients only adds up to 92%. Another diet quoted on the same page as this one was recorded added up to 101%… a beautiful example of the problems in reviewing and trying to interpret the literature on prawn nutrition. I could give many others.

I could provide other formulae for prawn diets; there are many in the literature but most have been used for tank or laboratory experiments rather than for commercial prawn production. In most prawn farms feeding is an art, not a science, and those materials which are closest to hand and cheapest are utilised. While this may result in profit now, intensive culture will require a much fuller understanding of the dietary requirements of prawns in pond culture than we have available now.

It isn't really possible to say much about feeding rates for supplementary prawn rations because these always seem to be governed by eyeball judgements on ‘how much food is uneaten from yesterdays feeding?’. This technique is likely to lead to alternate periods of near starvation and satiation, the latter combined with dubious water quality. Tables of feeding rates, based on biomass weights and prawn sizes, such as are used in fish feeding, are required for prawns. Once compounded prawn diets become readily and commercially available it is to be expected that their manufacturers will research this subject. One feeding rate in ponds which was recorded in the literature was for Macrobrachium farming in Hawaii, where a production of 3 000kg/ha was achieved through regular cull harvesting techniques. Here supplemental feed was presented at rates between 28 and 45kg/ha/day. One more example of the problems of studying this topic: the writer of the report giving this information went on to say that “ ½ gallon of chow/acre/day is a good rate for a newly stocked pond.” Such a statement is almost incomprehensible! How much does a gallon of chow weigh?

4. REFERENCES

Biddle, G.N., 1976. The nutrition of freshwater prawns. In: J. A. Hanson and H. L. Goodwin (Editors), shrimp and Prawn Farming in the Western Hemisphere, Dowden, Hutchinson and Ross, Inc., Pennsylvania, USA., pp. 272–291

New, M. B., 1976. A review of dietary studies with and prawns. Aquaculture, 9 : 101–144.

New, M. B., 1980. A bibliography of shrimp and prawn nutrition Aquaculture, (in press).

Tursutapanich, A., 1979. Cyst production of Artemia salina in salt ponds in Thailand. FAO Working Paper THA: 75:008/WP/79/9.