Nutrition and Feeding for Sustainable
Aquaculture Development in the
Third Millennium

M.R. Hasan1

Department of Aquaculture, Bangladesh Agricultural University,
Mymensingh 2202, Bangladesh

Hasan, M.R. 2001. Nutrition and feeding for sustainable aquaculture development in the third millennium. In R.P. Subasinghe, P. Bueno, M.J. Phillips, C. Hough, S.E. McGladdery & J.R. Arthur, eds. Aquaculture in the Third Millennium. Technical Proceedings of the Conference on Aquaculture in the Third Millennium, Bangkok, Thailand, 20-25 February 2000. pp. 193-219. NACA, Bangkok and FAO, Rome.

ABSTRACT: Over the last decade, the world has witnessed spectacular growth in the aquaculture industries of many developing countries. It is unequivocally agreed that global aquaculture production will continue to increase, and much of this will occur in the developing countries of Asia and Africa, through the expansion of semi-intensive, small-scale pond aquaculture. Nutrition and feeding play a central and essential role in the sustained development of aquaculture and, therefore, fertilizers and feed resources continue to dominate aquaculture needs. This paper reviews a number of specific issues in the fields of aquatic animal nutrition and feeding which are critical for sustainable aquaculture production in both industrialized and developing countries, e.g.: nutrient requirements of fish and their supply under practical farming conditions, availability and supply of feed resources and their implication on development of aquafeeds, forecasting of demand and supply of marine resources, and maintenance of environmental quality and sustainability of aquaculture systems. While discussing the nutrient requirement of fish under farming conditions, the possibility of accessing existing databases on nutrient requirements is examined, along with their application for establishing general nutritional principles. Particular emphasis is placed on understanding the contribution of naturally available food in semi-intensive aquaculture and its role on the development of on-farm feed management strategy. Other issues such as nutritional effects on immunocompetence and disease resistance of fish, understanding of broodstock and larval nutrition, role of nutrition on fish quality, and development of regional nutritional databases for aquaculture development are also discussed. Recommendations for improvement of nutrition and feeding protocols in support of sustainable aquaculture development in the third millennium are also made.

KEY WORDS: Aquaculture, Feeding, Aquafeeds, Nutrition.

 

 
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Introduction

Over the last decade, spectacular growth has taken place in aquaculture. Most production in developing countries is realized from pond-based or open-water extensive, improved extensive and semi-intensive practices using polyculture farming technologies. In contrast, the bulk of high-value freshwater and marine carnivorous finfish in developed countries is produced by intensive farming systems using high-cost nutrient inputs in the form of “nutritionally-complete formulated diets”.

It is unequivocally agreed that global aquaculture production will continue to increase, and much of the increased production in developing countries of Asia and Africa is likely to be achieved through the expansion of semi-intensive, small-scale pond aquaculture. Nutrition and feeding will play an essential role in the sustained development of this aquaculture. Therefore, it is imperative that fertilizers and feed resources continue to be produced and refined. Sustained development of aquaculture, however, must take into account and ensure that the needs of competing users are met, and that environmental integrity is protected. Therefore, sustainable aquaculture management should address allocation of inputs based on local circumstances, and balance maximizing profitability with social and environmental costs.

This paper will review a number of specific issues in the fields of aquatic animal nutrition and feeding that are critical for sustainable aquaculture production in both industrialized and developing countries. Some of the major issues are:

  • availability and cost of feed resources and development of aquafeeds;
  • increasing competition for resources with other users (e.g. agriculture and livestock industries);
  • forecasting of local and global market supply and demand; and
  • maintenance of environmental quality and sustainability of aquaculture systems.

Aquaculture development is also confronted with the choice between using established culture of herbivorous/omnivorous species under extensive or semi-intensive systems or developing more intensive systems to meet increasing production demands. Similarly, issues and conflicts, such as the demand for food verses availability of marine resources, productivity verses environmental quality, and choice of species verses biodiversity, warrant critical examination.

 

Nutrient requirements and supply under practical farming conditions

Growth, health and reproduction of fish and other aquatic animals are primarily dependent upon an adequate supply of nutrient, both in terms of quantity and quality, irrespective of the culture system in which they are grown. Supply of inputs (feeds, fertilizers etc.) has to be ensured so that the nutrients and energy requirements of the species under cultivation are met and the production goals of the system are achieved.

Complete data on nutrient requirements are only available for a limited number of species. Although dietary protein and lipid requirements and carbohydrate utilization are relatively well investigated for several fish and shrimp species, data on the requirements of micronutrients such as amino acids, fatty acids and minerals are only available for the most commonly cultivated carnivorous and selected omnivorous fish species. Available data on nutrient requirements for various fish species are presented: protein (Tables 1 and 2), amino acids (Table 3), essential fatty acids (Table 7), minerals (Table 8) and vitamins (Table 10).

Table 1 shows the protein and energy levels resulting in maximum growth for a few species of juvenile fish. The data show that the protein requirements for different fish species range from 28 to 56 percent of dry diets. Apparently, marine and freshwater carnivorous species require 40-55 percent dietary protein, while most freshwater omnivorous and herbivorous species require 30-40 percent of their dry diet to be made up of protein. Like finfish, most crustaceans studied to date have rather high protein requirements, ranging from 30 to 60 percent of the dry diet (Table 2).

Lipids are primarily included in formulated diet to maximize their protein sparing. There is convincing evidence that the degree of unsaturation does not appreciably affect digestibility or utilization of fats and oils as energy sources for coldwater or warmwater fish (Steffens, 1989). Carnivores like trout have natural diets rich in triglycerides and can adapt to high fat diets (upper limits have yet to be defined). Dietary lipid levels as high as 35 percent has been reported in some salmonid feeds (New, 1996). The maximum levels for other freshwater fish appear to be lower. In general, 10-20 percent of lipid in most freshwater fish diets gives optimal growth rates without producing an excessively fatty carcass (Cowey and Sargent, 1979).

 

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Carbohydrates are the least expensive form of dietary energy and are frequently used for protein sparing in formulated diets. Fish and shrimp vary in their ability to digest carbohydrate effectively (New, 1987). The utilization of dietary carbohydrate has also been found to vary with the complexity or chemical structure of the carbohydrate source used. Channel catfish (Ictalurus punctatus) and Kuruma shrimp (Penaeus japonicus) appear to utilize complex carbohydrates more readily than simple sugars (NRC, 1983; New, 1987) The ability of carnivorous fish species to hydrolyse or digest complex carbohydrate is limited due to the weak amylolytic activity in their digestive tract, thus for species such as trout, starch digestion decreases as the proportion of dietary starch is increased. For salmonids, carbohydrate digestibility also diminishes with increasing molecular weight (Steffens, 1989).

In general, warmwater omnivorous or herbivorous fish species such as common carp, channel catfish and eel have been found to be more tolerant of high dietary carbohydrate levels. Furuichi and Yone (1980) compared the utilization of carbohydrates by common carp, red seabream, and yellowtail. Growth retardation and low feed efficiency were noticed in common carp fed diets containing over 40 percent dextrin, red seabream fed over 30 percent dextrin, and yellowtail fed over 20 percent dextrin. Studies of common carp (Takeuchi et al., 1979) and channel catfish (Garling and Wilson, 1977) have shown that carbohydrate levels up to about 25 percent of the diet are utilized as effectively as lipids as an energy source.

All studies on finfish to date have shown that they need the same essential amino acids as most other animals (Table 3). The requirements for individual amino acids are fairly consistent between species, although variability is apparent both between species and between studies on the same species. A large part of the variability may be explained by differences in the methods used by various workers. Luquet (1989) pointed out the rather close agreement between amino acid requirements for coldwater fish (rainbow trout) and those of warmwater fish (channel catfish) when expressed in absolute terms and not as the percentage of the protein content.

 

Luquet (1989) further suggested that extensive research on the determination of quantitative amino acid requirements does not seem to be a priority, as indirect approaches provide a rather accurate estimate of requirements. Table 4 summarizes the range of essential amino acid requirements that have been determined for a variety of species of finfish.

Dietary lipids provide essential fatty acids (polyunsaturated fatty acids, PUFAs) that fish, like all animals, cannot synthesise but require for the maintenance of cellular function. Plant oils are generally rich sources of linoleic series fatty acid (n-6) and with the exception of linseed, conopher seed and hempseed oils, contain little or no linolenic series fatty acid (n-3) (New, 1987; Tacon, 1990; Table 5). Linolenic series fatty acids are found in terrestrial animal fats only in trace amounts and are common only in marine oils, highly unsaturated fatty acids HUFAs (20:5n-3, 22:5n-3, 22:6n-3) are virtually restricted to this source (Steffens, 1989; Tacon, 1990; Table 6).

Dietary essential fatty acid (EFA) requirements of the most commonly cultivated fish species are presented in Table 7.

 

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It is apparent from this table that marine fish (i.e. red seabream, sea bass, yellowtail, turbot, plaice) have exclusive requirement for HUFAs (20:5n-3 and 22:6n-3), while freshwater or anadromous species require a higher proportion of C18 fatty acids within the n-3 series.

In general, coldwater freshwater (i.e. salmonids, ayu) fish have an exclusive requirement of n-3 series PUFA (18:3n-3, 20:5n-3, 22:6n-3) in their diet, while warmwater freshwater fish require both n-3 series and n-6 series PUFA (i.e.carps, eel) or the n-3 or n-6 series (tilapia).

  All fish studied to date appear to require EFA (n-3 or n-6) at about one to two percent of the diet by dry weight. At present, there is no firm quantitative information on the dietary EFA requirement of marine shrimp or freshwater prawns. The information available at present is more suggestive than conclusive, however, as with fish, it is believed that n-3 series fatty acids have a higher EFA activity than n-6 series fatty acids in shrimp and prawn (NRC, 1983; Tacon, 1990).

 

 
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Determination of dietary mineral requirement of aquatic animals has been complicated by the fact that they have the ability to absorb minerals from the surrounding water in addition to the food ingested. The dietary requirement of a fish and shrimp species for a particular element thus depends to a large extent upon the concentration of the element in the water body (Steffens, 1989; Hepher, 1990). At present, there is little information concerning the contribution of waterborne elements to the total mineral balance of fish or shrimp. Further, aquatic animals’ absorption of minerals is largely influenced by variations in response to salt regulation or osmotic pressure. As a result, freshwater fish have greater demands for adequate mineral supplies than marine fish and shrimp.

Minerals available in feed ingredients are not always sufficient to meet the fish’s requirements. Some of the minerals may be leached during the processing of the food (Hepher, 1990). Moreover, many of the feed ingredients may be rich in particular minerals and deficient in others, or only partially available to the animal consumers. Dietary mineral requirements of five fish species are presented in Table 8. However, the information on mineral requirement is not complete and sometimes highly variable. Cho and Schell (1980, adapted from Hepher, 1990) summarized the requirement of 16 minerals (Table 9) for fish. Apparently the summary was prepared from available data for different fish species and should thus be considered as suggestive. Since much of the mineral requirement is supplied by the food, partial mineral supplementation may be sufficient to meet dietary needs.

Although the pathologies related to vitamin deficiency in fish are well investigated, quantitative dietary vitamin requirements of fish and aquatic animals are probably the least studied area in fish nutrition. The vitamin requirements of the majority of species of fish in culture have not been determined

  (De Silva and Anderson, 1995). As a result, the data obtained from studies of salmonids, common carp or channel catfish (Table 10) are usually applied to other species while formulating the complete diet for intensive culture. While natural food is usually rich in vitamins, this may not be the case with supplementary feed. Vitamin deficiency mainly appears, therefore, in intensive culture systems, where supplementary feed is the major, if not the only, source of feed (Hepher, 1990) or where formulated complete feed is the only source of feed.

Given the large variety of species under culture, it is not practical to undertake extensive studies on nutrient requirements and utilization of each species. If we critically examine the existing database on the nutrient requirements of different fish species, a tolerable generalization of nutritional requirement for different species groups can be made. The available literature indicates that the some of the nutrient requirements of major cultivable fish species can be generalized into four broad groups:

  • marine carnivores (e.g. yellowtail, red seabream, sea bass, salmon, trout, grouper);
  • freshwater carnivores (e.g. snakehead, eel, goby);
  • freshwater omnivores (e.g. tilapia, catfish, common carp, Indian major carps, shrimp); and
  • freshwater herbivores (e.g. grass carp, silver carp).

 

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The nutrient requirements shared among different species generally include protein, lipid, amino acid and water-soluble vitamins. Further, carbohydrate utilization by closely related species or species groups falls within a narrow range. Similarly, the differences in the requirements of most of the micronutrients, such as amino acids, vitamins, minerals and fatty acids show marginal variation between cultured species (De Silva and Anderson, 1995).   Therefore, general nutritional principles can be applied, and reliable data from closely related species can be utilized as and when appropriate. The use of such nutritional strategies will strengthen sustainability of the production system as a whole.

A large proportion of aquaculture in many developing countries is carried out in rural areas, where farmers adopt extended extensive or semi-intensive farming practices.

 

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Although information concerning the nutritional requirement of many of these cultivated fish species is well established, most has been generated from laboratory-based controlled feeding trials and, hence, is useful mainly for the formulation/production of nutritionally complete feeds for intensive culture systems. Such data may be less applicable to farming conditions where nutrition is from natural food supplies or supplemental artificial feeds. Natural productivity of the water body generally plays an important role in these production processes.
  Nutrition and feeding of finfish and crustaceans in semi-intensive pond-farming systems are complex and poorly understood. Little or no information exists on the dietary requirements under farming conditions for many of the species cultured. To a large extent, this is due to the difficulties of quantifying the contribution of naturally available food organisms to the overall nutritional budget of pond-raised finfish or crustaceans (Tacon, 1993). In order to maximize cultivation production, there is an ongoing need to develop basic understanding of nutrient dynamics, specifically the role of fertilization and natural productivity.

 

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Such under-standing will allow us to ensure that cost-effective diets are developed that take into account nutritional requirement differences between species, natural productivity of the water bodies and the location-specific availability of inputs.

There is evidence that even minor modifications of different inputs into semi-intensive systems can bring about major changes in terms of growth, reproductive performance and overall productivity of the system (De Silva and Davy, 1992; Veerina et al., 1993; Bjerkan, 1996; Tacon and De Silva, 1997; Miaje et al., 1999). Since feed and fertilizers represent about 60-80 percent of the total cost of aquaculture production, understanding feed management strategies and their implementation is of major importance. Although there are many examples of how improved feeding strategies can be successfully implemented using simple indigenous techniques (Tacon and De Silva, 1997), further studies and research are necessary.

 

Method of feed presentation, feeding rates and frequency are three areas where much improvement can be made. In many semi-intensive systems, supplementary feeding is based on fish biomass. Environmental factors and natural foods, which are known to influence food consumption and fish growth, are seldom considered. In addition, no precise schedule(s) or table(s) for most cultured fish are available.

 

 
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Feeding rates or ration size need to be determined by pond ecology (which varies considerably with season), in addition to fish biomass. Maximum benefits from supplemental feeding can only achieved if the diet is ingested in its entirety, and supplied to fish or shrimp at a rate compatible with the quantity and quality of natural food available in the pond. It has often been advocated that feeding regimes should reflect the feeding habit of the species under wild conditions (Tacon, 1993). The majority of feeding tables recommended by feed manufacturers for use within semi-intensive farming systems are hypothetical and set irrespective of dietary composition, pond fertilization rate, natural food availability, stocking density and standing crop (Tacon, 1993). The benefits of increasing feeding frequency have also been well documented Villalon, 1991; Sumundra, 1992). These authors reported reduced leaching and feed loss, improved growth and feed efficiency for shrimp through increased feeding frequency. Table 11 presents a summary of experimental data available on feeding frequencies at which optimal growth was observed in different finfish species. The variability in the feeding frequencies shown in the table is probably more indicative of uncertainty of results, rather than of the biological variability per se (De Silva and Anderson, 1995). Nevertheless, it emphasises the importance of feeding frequency in aquaculture. The optimum feeding rate and frequency of presentation must, therefore, be determined for individual feeds and farms by carefully monitoring feed consumption, growth and feed efficiency over several growing seasons (Tacon, 1993).
  In most semi-intensive systems, particularly small-scale rural operations, supplemental feeds are dispensed in powdered form (De Silva, 1993). There is increasing doubt about the efficiency of this form of feeding, since there appears to be significant wastage and individual fish face difficulties ingesting sufficient quantities of each of the constituent ingredients to obtain a nutritionally balanced meal. In cyprinid polyculture, farmers often use “feeding bags”, which are suspended at a number of locations, the perforated bottom touching the water surface (Tacon and De Silva, 1997) to increase the feeding efficiency. Although there has been no scientific evaluation of the efficacy of this feeding method, most farmers believe the returns are higher than with hand broadcasting.

Similar but somewhat modified feeding methods are used by the farmers of Andhra Pradesh in India. They keep the powdered feed mixture in perforated polyethylene bags suspended by wooden poles at a number of points around their ponds. Fish browse on the feed through the perforations and within two to three hours most of the feed in the bag is utilized. This method results in minimum wastage of feed and helps the farmers to apply medication effectively through feed (Veerina et al., 1993). In carp farming systems in Bangladesh, supplemental feeds are dispensed in both wet dough balls and in powdered dry mixture form. More recently, Miaje et al. (1999) demonstrated that both pelleted and dough forms of supplemental feed, comprised of mustard oil cake, rice bran and wheat bran, appear to be more suitable than powdered feed for Indian major carps and Java barb in polyculture.

 

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Several other supplementary feeding techniques, ranging from manual feeding by placing floating feed items into a floating or fixed-surface bamboo frame to simple demand feeders, are practised in different parts of the world. However, in many cases, hard data are lacking on the efficiency of these feeding techniques (Tacon and De Silva, 1997). In semi-intensive aquaculture, there is a need for further research in these directions.

Several other techniques have been reported to maximize use of supplementary feed in semi-intensive farming systems. De Silva (1985) advocated the adoption of a mixed feeding schedule with alternate high and low protein diets. Adoption of such mixed feeding techniques using feeds already available appears more cost effective and feasible than developing new feeds (De Silva, 1993). Furthermore, protein-rich diets (more expensive) diluted with carbohydrate-rich diets (less expensive) (Hepher and Pruginin, 1981) as well as use of a limited number of diets with improved nutritional value (Hepher, 1990; Sumagaysay et al., 1990) may offer significant economic advantage by reducing the overall feed cost in intensive aquaculture practices.


Nutrition and health

A large number of research studies have been carried out to quantify the nutrient requirements of fish and shellfish (Alliot et al., 1974; Jauncey, 1982; Daniel and Robinson, 1986; Akand et al., 1989, 1991a,b); De Silva et al., 1989; Borlongan and Parazo 1991; Ellis and Reigh, 1991; Borlongan, 1992; Koshio et al., 1993; Castell et al., 1994; Habib et al., 1994; Hasan et al., 1994; Mourente et al., 1995; Querijero et al., 1997; Hossain and Furuichi, 1999, 2000; Ngamsnae et al., 1999). Another area of research that has received significant attention in aquaculture nutrition is the use of plant and animal by-products as fishmeal substitutes in fish feed (Atack et al., 1979; Dabrowski and Kozak, 1979; Higgs et al., 1979; Capper et al., 1982; Jackson et al., 1982; Tacon et al., 1984; Wee and Wang, 1987; Davies et al., 1989; Wee and Shu, 1989; Fowler, 1990; Gallagher, 1994; Kaushik et al., 1995; Habib and Hasan, 1995; Stickney et al., 1996; Brunson et al., 1997; Hasan et al. 1997a, b) Unfortunately, the major emphasis of these studies was on optimizing growth, feed efficiency and general health condition.

  Thus although these studies have generated a significant amount of information on pathology in relation to nutrient imbalance and the presence of toxic and antinutritional factors in feed ingredients, the possible effects of macro- and micronutrients on immunological parameters have mainly been overlooked. Nutrition and farm management strategies play critical roles in fish health and disease outbreaks within intensive farming systems and, to lesser extent, in semi-intensive farming systems (Tacon, 1997a). However, it must be emphasized that nutrition and farm management should not only satisfy the dietary nutrient requirements of the farmed species for maximum growth but also for increased immunocompetence and disease resistance. There is a growing need to take immunological parameters into account in nutritional studies on aquatic animals. This is especially important, since fish appear to depend more heavily on non specific defence mechanisms than mammals (Kaushik, 2000).

The effects of vitamins C and E are well documented, but several other nutrients and feed additives, including other vitamins (vitamin A), trace elements (Zn, Cu, Se, Mn, Fl), essential fatty acids and carotenoids have also been reported to play important roles in the immune response of fish (Devresse et al., 1997). While the application of vitamins, essential fatty acids and other micronutrients has shown conclusive evidence of a role in sustaining the immune function of fish in laboratory trials and in intensive commercial aquaculture operations, their influence on the defence mechanisms of fish reared under semi-intensive culture conditions (where ecological factors also influence diet) remains to be determined. It is perceived that natural food production in semi-intensive culture conditions should supply enough of these nutrients to meet the fish immune response requirements (Dickson, 1987; Castell et al., 1988; Castille and Lawrence, 1989; Hepher, 1990; Trino et al., 1992; Tacon, 1993).

In recent years, probiotics (non pathogenic, opportunistic bacteria of the genera Bacillus, Lactobacillus, Streptococcus) and natural immunostimulants (yeast, glucans) have shown promise for increasing disease resistance in fish and shellfish. As a result, there has been a growing interest in the use of immunostimulants as prophylactic agents to minimize the risk of disease outbreaks. The large number of commercial immunostimulants on the market clearly reflects this interest; however, the results obtained to date have not always been consistent and these products are still less effective than vaccines (Devresse et al., 1997).

 

 
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There is, therefore, a clear need to improve the stability of immunostimulants, micronutrients and oral vaccines, especially under subtropical and tropical culture conditions, as well as nutritional information related to effective use of vaccines and/or chemotherapy, before adopting new immunostimulation techniques (Lall pers. comm). Further, improving health through proper nutrition would not only reduce the need for chemotherapeutants, but also help avoid major disease outbreaks.

Toxic and antinutritional factors (blocking effective nutrient assimilation) present in plant ingredients, nutritional imbalances of formulated feed, adventitious toxic factors and toxic compounds formed during feed storage and processing etc., can all severely affect the health status of cultured species and lead to increased susceptibility to disease. Although information on these aspects is documented, and appropriate precautions during feed formulation and processing can minimize the risk (Devresse et al., 1997; Lall pers. comm), there is a need for further research to develop better strategies to minimize such toxicity effects.


Nutrition and fish quality

Fish is a highly nutritious food, containing high amounts of proteins with high biochemical value for humans. In addition, it is a very good source of polyunsaturated fatty acids (PUFA) known to be beneficial in preventing cardiovascular diseases, breast and colon cancer, psoriasis etc. (Kaushik, 2000). Highly unsaturated fatty acids (HUFA) present in marine fish oil are medically proven to be beneficial against inflammatory disorders and ischaemic heart disease by modifying arachidonic acid/prostaglandin pathways (Sargent, 1992). Fish also contain micronutrients such as iodine, selenium and fat-soluble vitamins (A and D) that have positive effects on human health. In many developing countries of the world, small fish are eaten whole and thus contribute calcium, phosphorus and iron to the human diet.

Improvement of feed and nutrition in aquaculture may give us the opportunity to further improve the nutritional quality and benefits of the fish consumed. Nutritional value, colour and appearance, smell and taste, texture and storing capacity may all be affected by the quality of nutrition and feed provided during culture.

  Recent work on salmonids showed that product quality can be tailored by modifying the dietary composition, and a more nutritious fillet can be produced (Kaushik, 2000). However, more research has to be done on this field, giving adequate consideration not only to nutrient bioavailability but also to postharvest quality control.


Aquafeed and the environment

Given that feed is the biggest source of nutrient loading in fish and shrimp aquaculture production, clear understanding of its impact is essential for sustainable development, either intensive or semi-intensive. This will help reduce negative impacts and improve predictability of environmental effects. Present knowledge and understanding of the environmental impacts of aquafeed needs further refinement; however, it is generally acknowledged that these impacts can be reduced by feeding fish with more environmentally friendly diets, developing better feeding strategies and by a sound farm management. Interrelationships among various factors and strategies in dealing with environmental pollution and aquafeed are schematically shown by De Silva and Anderson (1995). The authors advocated a holistic approach and noted that fish nutritionists can no longer be formulators of nutritionally wholesome diets, but need to consider fresh strategies in diet development and feed cost reduction.

In developed countries, where intensive farming of carnivorous fish species is primarily dependent on a supply of nutritionally complete, formulated diet, mitigation of negative impacts of aquafeed through development of more environmentally friendly diets is considered to be a major challenge. Potential pollutants from aquafeed are phosphorus and nitrogen, as well as organic matter. Alvarado (1997) describes the flux of nutrients from a gilthead seabream farm, where fish under intensive production were fed commercially extruded bream diets (Fig. 1). It was shown that 180 kg of solids, 13 kg phosphorus and 105.4 kg of nitrogen were released to the environment through excretion and by uneaten feed to produce 1000 kg of fish. Thus lowering the amount of nitrogen and phosphorus in feed as far as possible will be one of the most efficient ways to reduce pollution effects.

Further, more environmentally friendly diets can be produced by developing diets with reduced food conversion ratios (FCR), e.g. by improving palatability and digestibility of raw ingredients.

 

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The precise requirements for protein, amino acid and energy for each species and stage of development, as well as strain, need to be refined. It is also acknowledged that nutrient requirements change as the intensity of culture changes. The digestibility of nutrients is not precisely defined in many commercial feeds, and current research shows that feed performances and digestibility can be increased with the use of enzymes that enhance plant protein use, and by use of extrusion technology. Therefore, continued research on processing techniques and additives and enzymes for improving feed performances and digestibility are required, for example, on optimization of protein/energy ratios and amino acid profiles to reduce nitrogen excretion.

Improved knowledge of feeding strategies has also helped improve feed utilization and reduce the FCR and waste, thereby reducing negative environmental impacts (Alvarado, 1997). Examples of improved feeding strategies range from:

 
  • sophisticated computer-controlled feeding devices for intensive commercial production of high value marine fish;
  • increased feeding frequency;
  • adjustment of feeding rate based on pond productivity; and
  • use of supplemental feed in pelleted or dough form, as opposed to powdered form, in semi-intensive aquaculture systems.

In addition to the potential for environmental degradation by waste aquafeed, therapeutant misuse should not be overlooked. Medicated feeds are often used indiscriminately during disease outbreaks in hatcheries, nurseries and farms (Hasan and Ahmed, 2001). Although some of these drugs are unstable in water and do not cause any major problem, others are very stable and can precipitate development of bacterial strains that are resistant to these drugs. Furthermore, use of multiple, related, drugs can result in development of bacterial strains that are highly resistant to a wide range of drugs (Alvarado, 1997).

     

 

 
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Thus several factors and strategies have to be carefully monitored, and alternative strategies developed, while dealing with aquafeed and environmental pollution.


Broodstock and larval nutrition

It is well recognized that adequate nutrition has an important role to play in the reproductive success of all animals, including fish. There are a number of aspects of reproduction that can be affected by nutritional status:

  • the time of first maturity;
  • the number of eggs produced (fecundity);
  • egg size; and
  • egg quality as measured by chemical composition, hatchability and larval survival (De Silva and Anderson, 1995).

During the last decade, increased attention has been paid to the role of different components of broodstock diets. It has been shown that essential fatty acids, vitamins (A, E and C), trace minerals, -carotene and other carotenoids can affect fecundity, egg quality, hatchability and larval quality (Kaushik, 1993; De Silva and Anderson, 1995; Izquierdo and Fernandez-Palacios, 1997), and that the amino acid requirements of broodstock are apparently similar to those for optimal growth (De Silva and Anderson, 1995). Results of these studies also indicate that there exists great species diversity in nutritional requirements affecting reproduction. Apart from common carp, most of these studies have been carried out for marine carnivorous fish species (De Silva and Anderson, 1995; Izquierdo and Fernandez-Palacios, 1997); thus relatively little is known about broodstock nutrition of freshwater omnivorous/herbivorous fish species. Therefore, there is an immediate need to learn about the nutritional requirements for broodstock maintenance and reproduction for most of the commercially important freshwater fish species. Clearly defined broodstock nutrition is not only necessary for high-value fish cultivated under intensive aquaculture, but could also significantly enhance production success of species grown under semi-intensive farming conditions. Another aspect that has received little attention to date is the nutrition of male broodstock. Possible improvement of sperm quality through dietary manipulation deserves further consideration.

In a study continued over a period of eight months prior to spawning, Watanabe et al. (1984a) found that EFA (n-3 PUFA)-deficient diets in red seabream produced eggs with significantly lower survival and high levels of larval deformity.

 

Some interesting short-term effects of nutrients on broodstock have also been described in red seabream. It has been reported that specialized diets given immediately prior to, or during, spawning of red seabream affected the composition of the eggs. Pigments such as -carotene, canthaxanthin or astaxanthin resulted in marked improvement in the percentage of buoyant eggs. Therefore, there is a need to define different stages of broodstock nutrition for appropriate management. Nutritional requirements of broodstock can further differ depending upon the phase of reproductive period. These periods are generally distinguished as:

  • the period from commercial size to broodstock size;
  • immediately prior to, or during, spawning; and
  • post-spawning.

Formulation of complete diets should, therefore, take into account the stage-specific, as well as species-specific nutritional requirements of the broodstock. Nutritionists and the feed industry should also consider the options for developing three types of broodstock diets:

  • conditioning diet,
  • reproduction diet, and
  • recovery/maintenance diet.

The broodstock conditioning diet should be formulated as an optimized growout diet to meet the full nutritional requirements of the species from commercial to broodstock size in maximal synergy with the environment. The reproduction diet used before or during spawning should meet the needs for maximal reproductive performance (spawning success and fecundity), gamete quality, and vertical transfer of nutrients and biologically active substances to offspring. The recovery/maintenance diet should assist recovery from reproductive exhaustion and reconditioning for the next reproductive cycle.

The nutrient requirements of all animals vary throughout their life cycle. Complex morphological and physiological changes invariably modify feeding and nutritional requirements. Finfish, nutrition during the embryonic stage is provided by the yolk sac and oil globules. The transition from an endogenous to an exogenous food supply, which marks the onset of the larval stage, is one of the most critical phases of the lifecycle and is the period when much of the mortality of hatchery-reared stock occurs (De Silva and Anderson, 1995). In spite of the clear importance of nutrition in influencing the survival, growth and development of larvae, however, relatively little is known about the absolute nutrient requirements of these stages of aquatic animals.

 

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It is generally acknowledged that the feeding behaviour of larvae has a dominant role in larval nutrition, e.g. the larvae of many fish species will not take an artificial diet. There are also species that will take an artificial diet, but this must be supplemented with live zooplankton to satisfy all nutritional requirements. In some species, provision of microparticulate diets in addition to live food enhances both growth and survival rates (Kanazawa, 1991a).

In recent years, a considerable amount of research has been devoted to study of the nutritional requirements of marine fish larvae. Comparison of the biochemical composition of eggs and larvae at different stages, pattern of loss and conservation of nutrients during starvation and feeding experiments that control live-prey or microdiet nutrient composition are some of the most frequently used methods to study nutritional requirements of marine fish larvae (Izquierdo and Fernandez-Palacios, 1997). Live foods constitute the main diet for marine fish larvae, but a single live food species is often unable to satisfy the complete nutritional requirement of the species under culture. Since many finfish are reared on a single type of live food at any one time, a number of studies have been conducted to investigate the effect of enriching live food organisms with various nutrients. Results indicate that larval fish require diets with a high protein content (Kanazawa, 1991b, Kissil, 1991; Cho and Cowey, 1991; Luquet, 1991) and sufficient amounts of essential fatty acids (Lavens et al., 1991). The live foods that have been most intensively investigated, with respect to their nutritive suitability are brine shrimp (Artemia spp.) and rotifer (Brachionus plicatilis). Artemia is low in the essential fatty acids eicosapentaenoic acid (C20:5n-3, EPA) and docosahexaenoic acid (C22:6n-3, DHA); thus simple methods of bioencapsulation have been developed to incorporate particulate products into brine shrimp nauplii. The nauplii consume particles of a desired composition prior to being offered as prey for fish larvae. The nutritive value of rotifers is made suitable by culturing them a suitable medium such as -yeast, and by feeding with a mixture of homogenized lipids and baker’s yeast or marine alga (Chlorella spp.), all of which are rich in n-3 polyunsaturated fatty acids. While it is generally considered that eicosapentaenoic and docosahexaenoic acids are important fatty acids in the nutrition of larval fish, the specific fatty acids required vary between species (De Silva and Anderson, 1995).

 

Culture of most marine fish and shrimp larvae, at least during early ontogenesis, still relies heavily on the supply of live food items (brine shrimp, rotifers and microalgae). This dependence on live food is already causing concern, due to the lack of resources and increasing production costs. Efforts are underway to find alternatives, such as other sources of live prey, partial replacement/supplement of live prey and development of microparticulate diets for larvae. Much research effort is also being invested in establishing nutritional limiting factors, early use of artificial diets and reduction of the weaning period, for many marine fish larvae (Alami-Durante and Meyers, 1993; De Silva and Anderson, 1995; P. Lavens pers. comm.)

For most hatchery-reared marine and freshwater fish and shrimp larvae, development of complete artificial diets for rearing, or reduction of length of weaning from live food, is of immense importance for further development of aquaculture of these species. Determination of absolute nutrient requirements of fish larvae of commercial importance is also an essential prerequisite before any attempt is undertaken to formulate/develop an artificial diet. The necessity to utilize diets with optimum stability and good physical characteristics in water, along with enhanced attractability, are clearly recognized. Attractiveness is an especially critical factor for optimum ingestion of the diet and is a crucial component for accurate evaluation of the nutritional value of the particular formulation. Other aspects that need to be addressed for the development of larval diets are improvement of digestibility of microparticles and diet quality through supply of requisite nutrients, e.g. exogenous digestive enzymes. Another approach is incorporation of feeding stimulants, especially amino acids, to dry diets. Research on the physical behaviour of particles in the water column and controlled leaching of components also deserves further attention. Although much of the potential for manipulation of natural productivity to ensure production of good quality larvae under semi-intensive farming systems remains to be determined (Kaushik, 2000), the problems associated with nutrient quality of live food do not necessarily occur when larvae are reared under natural pond conditions. For example, where planktonic growth is stimulated by fertilization/natural productivity, as in the case of rearing of Chinese carps (De Silva and Anderson, 1995), the range and suitability of natural foods enable the cultured organisms to obtain all nutrient requirements without further supplement.

 

 
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Demand and supply of marine resources for aquafeeds

It is envisaged that world annual fishmeal production will remain static at 6.5 million mt over the next decade. World annual fish oil production will remain around 1.24 million mt during next decade, although this is expected to fluctuate due to El Niño. To keep pace with global aquaculture production, a marked increase in use and production of formulated feed is foreseen for the next 25 years (Tables 12 and 13, Fig. 2) (Barlow, 2000). High quality fishmeal and fish oil are the major dietary ingredients for the production of formulated feed. It is, therefore, predicted that the requirement for these will increase from 2,115 to 3,262 million mt for fishmeal and from 0,708 to 1,308 million tonnes for fish oil between 2000 and 2025, to support today’s intensive aquaculture industry (Table 14, Figs. 3 and 4) (Barlow, 2000).

While the demand for fishmeal for the aquaculture industry will increase, it is projected that there will be a drastic reduction in the use of fishmeal for the poultry industry (Barlow, 2000) (Fig. 4) and, as a result, aquaculture will have sufficient fishmeal to 2020 and beyond. It is also predicted that sufficient fish oil will be available to year 2010, although fluctuations caused during El Niño may create temporary shortages. However, beyond this period, there will be a shortfall of marine oil for aquaculture feed (Table 14, Fig. 4).

 

 

Since marine fish oil is rich in n-3 PUFA and the major source of unsaturated fatty acids in compound aquafeeds - an essential dietary nutrient for all marine carnivorous finfish and crustacean species (NRC, 1993) - the feed industry should look for possible substitutes.The demand for fish oil could be reduced by using vegetable oil or a blend of fish oil and vegetable oil as a source of unsaturated fatty acids (n-3 and n-6). With the exception of strictly carnivorous fish species, fish are able to use C18:2n-6 or C18:3n-3 and convert them into corresponding HUFA: C20:4n-6 in the case of n-6 series, and C20:5n-3 or C22:6n-3 in case of n-3 series. Gene research on marine carnivorous fish to acquire the capability to elongate C18:3n-3 deserves further consideration (see Dunham et al., this volume).


Alternative protein sources: plant and animal by-products

Fishmeal and fish oil are the most widely used dietary components of commercially produced high quality fish/shrimp feed throughout the world. Fishmeal and oil are preferred for commercial feed production because of their unique balance of protein (amino acids) and lipids (long chain n-3) in a highly digestible energy dense form. Substitution with other ingredients, especially those of plant origin, is likely to compromise nutrient balance and fail to match the energy concentrations achieved using fishmeal and oil. Nevertheless, the high costs of these ingre-dients have severely restricted their use, especially in semi-intensive aquaculture systems.

     

 

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Agricultural (animal and plant) by-products and wastes of agro-processing industries are widely available in most parts of the world and many are traditionally used as feed for farm animals. Of these (Table 15), the major conventional feed ingredients used as protein and energy supplements for fish, to date, are:

  • meat meal, meat and bone meal mix, poultry by-product meal, blood meal (animal origin); and
  • oilseed cakes, pulses, cereals and cereal by-products (plant origin).

Considering the increasing cost of fishmeal and doubt concerning its long-term availability, much research has been carried out on the use of plant and animal by-products as fishmeal substitutes (Dabrowski and Kozak, 1979; Jackson et al., 1982; Tacon et al., 1984; Wee and Wang, 1987; Davies et al., 1989; Fowler, 1990; Gallagher, 1994; Kaushik et al., 1995; Stickney et al., 1996; Brunson et al., 1997; Hasan et al. 1997a). Although some animal by-products have shown similar nutritional values to those of fish meal for many fish species, it is generally felt that there is currently no realistic alternative to fish meal or fish oil, because of the high levels of high quality protein and lipid required for cultivation of the fast growing carnivorous fish species. Replacement of fishmeal with plant by-products does not generally achieve the desired level of growth and other performances required for culture of fish and other aquatic animals (Higgs et al., 1979; Tacon et al., 1984; Stickney et al., 1996; Alexis, 1997; Brunson et al., 1997; Tacon, 1997b).

  Lower performances are attributed to toxic and nutrient-uptake blockers in plant ingredients and the inherent essential amino acid deficiencies of most plant proteins (Kaushik, 1989; NRC, 1993; Hasan et al., 1997b; Tacon, 1997b). Nevertheless, opportunities exist for use of these by-products as fish meal for most fish and shrimp grown in less intensive systems.

Considering the improved extensive and semi-intensive farming systems practised by most small-scale fish farmers of Asia and other developing countries, where fish and other aquatic animals are able to fulfill part of their nutritional requirements from naturally available food in the system (Tacon, 1993), ingredient quality similar to that of fish meal is probably not necessary for most farmers.

 

 
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More relevant research emphasis would be on the “appropriate utilization of these ingredients as supplemental feed” rather than “evaluation of these ingredients as fishmeal substitutes” (De Silva, 1993, 1999). Improved information is needed on:

 
  • nutritional composition (quantity, quality and bioavailability of protein, lipid, carbohydrate, vitamin and mineral) of feed ingredients;
  • their suitability for incorporation into practical diets; and
  • results of experimental work on the use of these ingredients in fish feed.

Further, there is evidence that the use of many of these ingredients can be enhanced by simple processing techniques. For example, cooked starch is more useful to many omnivorous and herbivorous fish than raw starch. Many shrimp farmers in Andhra Pradesh, India cook their ingredients before use. Cooking has been reported to increase the digestibility and increase feed efficiency (Hasan and Amin, 1997). So information on simple processing techniques and palatability to the target species, along with storage and preservation, seasonal and regional variability in proximate composition etc., will all lead to more efficient utilization of these feed ingredients.

In contrast to semi-intensive aquaculture practices, demands for fish meal and agricultural by-products of high quality are expected to increase, keeping pace with the dynamic developments in intensive aquaculture of carnivorous fish species in industrialized countries of the world. This will require greater availability of quality feeds. Quality of alternative protein/energy sources, be it animal or plant, has to be ensured by appropriate evaluation and screening, starting from safe and careful collection, processing using optimum technologies and maintaining efficient quality control systems. Likewise, consumers expect effective tracking and safety control systems in their final choice of foods.

Mention may be made about the spread of bovine spongiform encephalopathy (BSE), the dreaded mad cow disease. The World Health Organization (WHO) has recently concluded that BSE may be spreading worldwide through international trade in animal feed. Meat and bone meal are two of the animal by-product meals widely used as fishmeal substitutes for fish farming in different parts of the world. It is, therefore, imperative that production, sale and use of all plant and animal by-products be based on sound science, true documentation, realistic risk analyses and clear labelling, to ensure food safety. Appropriate documentation and clear labelling will also improve consumer confidence and freedom of choice.

 

Development of regional database for aquaculture development

The costs of feed ingredients and farm input are increasing, while market costs for the major cultivated finfish and crustacean species have remained static or are decreasing.

 

 
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It is, therefore, likely that increased aquaculture production will be from herbivorous/omnivorous aquatic animals in developing countries of Asia and other parts of the world. Aquaculturists could reduce current dependence on natural marine resources to farm carnivorous finfish and marine shrimp through the use of low-cost, locally available, alternative feed ingredients. Research into the appropriate utilization of these feed supplements in semi-intensive aquaculture, however, is still required. There is little information on their availability and abundance, especially in many developing countries. In addition, development of intensive poultry, dairy and other livestock industries is likely to intensify the competition for nutrients and feed resources. A reliable database of agricultural feed resources is thus an essential prerequisite for planning sustainable aquaculture development. A national agricultural feed survey would create a database of feed resources and enable national aquaculture development strategy planning. Further, the establishment and/or strengthening of national and regional fertilizer and feed ingredient databases, and information systems, will enable projection of major agricultural by-products throughout the world that may benefit the aquaculture feed industry.


Conclusions

Over the last decade, the world has witnessed spectacular growth in the aquaculture industries of many developing countries. As a result, aquaculture has been contributing significantly to food security and poverty alleviation. It is anticipated that global aquaculture production will continue to increase and further contribute to these needs. Nutrition and feeding play a central role in sustainable aquaculture and, therefore, fertilizers and feed resources continue to dominate aquaculture needs. Much of the increased aquaculture production in developing countries of Asia and Africa will likely be achieved through expansion of semi-intensive, small-scale pond culture, thus feed and fertilizer resource availability, as well as cost, could be the major bottlenecks for such development.

In intensive aquaculture of marine carnivorous species, fish meal and fish oil will continue to be the major ingredients in the near future, although there may be scope for some use of animal by-products as alternative protein sources.

  Recognizing the current importance of fish meal and fish oil within industrially compounded aquafeeds, while lack of fish meal is not foreseen in the next 25 years, there is a risk that a lack of marine oils may occur in the short term (5-10 years). With the expansion of intensive aquaculture, aquaculturists must carefully assess the impact of nutrient loading in the aquatic environment and use both science and judgement for reducing such impacts. Furthermore, a careful balance between environment, health/disease resistance and feed use should be maintained, so that the system does not deteriorate and negatively impact market value and consumer confidence.

Use of formulated feed and fishmeal has no clear future in semi-intensive aquaculture in developing countries of the world. Nevertheless, further intensification of commercial aquaculture will take place, even in developing countries, for shrimp and carnivorous freshwater species, with the same potential as mentioned above for an overall shortage of conventional feed ingredients. Alternative feed ingredients should be sought at the same time as improvement of pond management and manipulation of pond productivity. Use of nutritionally complete formulated diets will, however, continue to play a dominant role in hatchery and nursery production.


Recommendations

Improvement of nutrition and feeding for sustainable aquaculture development can be achieved thorough:

  • increased understanding of the dietary nutrient requirements of cultured species, including their application to practical culture conditions;
  • developing species-specific broodstock diets that allow complete domestication and maximal reproductive and larval quality;
  • better understanding of larval nutritional requirements, in order to develop suitable compound diets, which will further reduce the need for live food;
  • improving the understanding of the aquaculture farming systems (extensive, semi-intensive or intensive; closed or open culture systems) and the potential nutrient loads and losses to the environment, to maximize nutrient retention efficiency;
  • understanding and monitoring the dynamics of nutrient flows and sinks within pond-based farming systems;

 

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  • improving the efficiency of resource use in aquaculture through use of appropriate agricultural and fishery by-products/wastes and nonfood-grade feed materials;
  • developing feeding strategies based on renewable feed ingredient sources where production can keep pace with the growth of the sector;
  • better understanding of nutrient bio-availability and interactions of commonly used feed ingredients;
  • better understanding of nutrient modulation of disease resistance;
  • improved strategies to minimize toxicity of nutrients and other compounds of feed origin;
  • promotion of “good aquaculture feed manufacturing practice” and “good on-farm feed management”;
  • recognizing the importance of feed and consumer concerns over food safety issues (irrespective of the culture system), including the need for traceability of feed materials and production methods; and
  • considering the effect of diet on product quality and the positive nutritional characteristics of the final product in terms of human nutrition, e.g. omega-3 fats, iodine, selenium, vitamins A and D.


Acknowledgements

The author gratefully acknowledges Sadasivam J. Kaushik, Mali Boonyaratpalin, Santosh P. Lall, Oyvind Lie, Torbjorn Asgard, Patrick Lavens, Stewart Barlow, Albert G.J. Tacon, Freddy Ib and Chawalit Orachunwong for their generous contributions. The information generated during the Nutrition Session of the Conference has been duly incorporated into the manuscript.


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1 mrhasan@citechco.net

 
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