INTRODUCTION

There are two major reasons why non-living produced feeds for rearing larvae of aquatic animals do not yet have an advantage over live food organism. These are:

1. rapid deterioration of water quality due to disintegration of micropellets, which are usually fed in excess in order to achieve satisfactory growth and survival;
2. high mortality rates, due to malnutrition and/or incomplete digestion of diet components.

Cultivation of larval stages of various aquaculture species is still highly dependent on live food which is for herbivorous larvae, like molluscs and crustaceans, a fairly understood task. Many more difficulties have to be faced when live food animals are required, as is mainly the case in fish rearing, but holds true for latter stages of crustacean larvae as well. The reason why live food is so essential for larval growth has not yet been clearly defined. Enzymes present in phyto and zooplankton but not synthesized by the physiological system of a larvae are probably important. Also of importance are several essential biochemical compounds such as poly-unsaturated fatty acids, most of which have been defined as to species requirements. Primary producers of these fatty acids such as algae and bacteria form the base of the trophic pyramid, and as such constitute the largest link in the aquatic food chain. The large-scale, intensive production of microalgae and rotifers suffers from two major problems: it is expensive and often unreliable. Contributing to the problem is the fact that designs used for experimental and pilot scale units, which are the bulk of the published research, are usually inappropriate for larger system because of logistical problems, prohibitive cost of materials, or diminishing surface area to volume relationships which affect scale up performance. Scale up problems can arise in the bulk handling of materials such as animals, water and feeds which in a restricted laboratory situation are easily transported and held in small containers. Carrying out necessary life support functions can also become complicated, since daily work routines for large numbers of animals quickly becomes prohibitive. Routine maintenance and cleaning of culture units, while trivial in the laboratory becomes a major problem with increased scale. As hatchery managers try to stem the rising costs of production, the economic cultivation of live feeds or some alternative becomes ever more important. The sections that follow will attempt to illuminate various options and potentials for larval penaeid shrimp feeds. The summary section will then rate the most cost effective choices for management consideration.

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ALGAE

There has been widespread interest in the mass production of microalgae since the 1940's. Microalgae have been cultured as a source of oils, polysaccharides, fine chemicals and oxygen. They have been exploited for soil conditioning, eutrophication control, waste-water treatment, and consumption by humans, livestock and aquatic organisms. At one time, mass culture of algae was seen as a solution to the world protein shortage and numerous other global problems. In most cases, however, large scale microalgal culture has fallen short of economic expectations due, in large part, to the expense and difficulties associated with separation of the cells from the culture medium and processing. A clear exception to this is the cultivation of microalgae to feed aquatic organisms, i.e., its use as a feed as opposed to food. In general, the most important problems encountered in the large-scale production of microalgae as feed for bivalves, crustaceans and rotifers may be classified as either 1) economic in nature, or 2) related to the dependable output and consistent quality of large volume production. Algal production costs for aquatic animals may be determined indirectly by estimating the percentage of total hatchery costs used to grow algae. Very large volumes of select algae are required for the nursery culture of bivalve molluscs, while smaller amounts are used in the larviculture of commercially valuable molluscs and penaeid shrimp. From the data shown in Fig. 1, it is possible to see that penaeid shrimp hatcheries use more of the total hatchery costs to produce algae than do bivalve hatcheries.

Fig 1. Percent total hatchery costs used for algae production, ration of algae culture volume to target species culture volume.

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Another estimate of resources devoted to algal production may be derived from the ratio of algae culture volume to target species volume. The highest algae : target species ratios are for penaeid shrimp, which indicates that these facilities probably devote a great deal more resources to producing algae than to growing shrimp larvae or rotifers. Why are the economics of shrimp hatchery algae production higher than those of bivalve hatcheries? The answer lies in the efficiency of the mass algae culture systems used at these facilities. Most bivalve hatcheries are located in temperate areas where outside temperatures never exceed high end limits for the species being cultured. Their problem is one of too little light in the winter months to maintain algal division rates at acceptable levels. Generally, they have to add light to outside tanks during the winter months. On the other hand, penaeid shrimp hatcheries are located in tropical and sub-tropical areas where temperatures are in excess of optimal algae species criteria for at least half of the average production season. Light is also in excess in these tropical areas for outside cultures. Rather than design facilities around these environmental parameters as have in the bivalve hatcheries, shrimp culturists adapt phytoplankton species to their system irregardless as to whether they are of optimal nutritional value to the shrimp larvae. As a first step towards designing better production facilities, shrimp hatchery biologists and engineers need to take a critical look at existing systems, their yields, operating costs, reliability, etc., and consider ways of improving performance through better design. Mass algal production systems should be designed around the most nutritionally sound algae species available for a particular culture organism. What most penaeid hatcheries do is simply rely on culturing whatever grows best in their system. In most cases these "weeds" are not the best food for penaeid larvae. Culture age/growth phase, light intensity, temperature, nutrient limitation and source and cell density can all affect the chemical composition of the algae. The problem of consistent nutritional quality is also especially pronounced in outdoor cultures. When during up-scaling, an algal culture is transferred outdoors, the cells often suffer from photic shock. This is due to not being adapted to light of such a high intensity, and they require a period of time to adapt. After this "lag phase' if the algae survives, the higher temperatures and higher incident light levels in outside cultures generally cause accelerated growth rates which are not as easily managed as indoor cultures. The pH of the culture is not maintained within desired ranges and algae is more often than not fed at a less than optimal nutritional level or past exponential growth rates. Algal cells in exponential phase may have a different biochemical composition to those in stationary phase. Changes in basic culture parameters can also change the fatty acid profile of the algae. It is well known that lipids are the most energy rich of the nutrient classes, providing approximately 9 Cal/g compared to 4-5 Cal/g for carbohydrates. The principal components of most lipids are fatty acids. Many marine animals appear to have a limited ability to synthesize the poly-unsaturated fatty acids (PUFA) 20:5(n-3) or EPA and 22:6(n-3) or DHA from precursor fatty acids in the linolenic acid family. The growth and survival of penaeid shrimp has been shown to increase when foods rich in EPA and DHA are included in the diet. The fatty acid profiles of 10 algae species are shown in Fig. 2. These algaes were grown at 20 degrees C. with 79-80 uE/m2/s2 on 12:12 light:dark cycle with Guillard´s F2 medium. Three of the species shown were sampled 6 months later under similar conditions to test whether similar compositional data would be obtained. Noteworthy here are the 20:5(n-3) and 22:6(n-3) levels in the typical larval penaeid shrimp algaes C.gracilis, and T. suecica related to the bivalve larval feeds C. calcitrans, T. isochrysis, and P. lutheri. In a series of experiments conducted at the Hawaii Institute of Marine Biology, Penaeus vannamei and Penaeus monodon larvae were fed various phytoplankton species diets and to evaluate their performance against each other. Larvae fed Thalassiosira weisflogii had an 82% survival versus 76% survival for larvae fed Chaetocerous gracilis. More important was the fact the larvae fed Chaetocerous gracilis had a mean dry weight of 78.6 ug per postlarvae while those fed Thalassiosira weisflogii attained a mean dry weight of 132.7 ug per post larvae!!!. In separate experiments, mean dry weights of larvae fed T. weisflogii, exclusively, compared favorably to weights of animals fed phytoplankton and Artemia.

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Fig 2. Percentage composition of fatty acids in diatoms and prymnesiophytes.