Invited Speakers

Dr. Sadasivam Kaushik

Current knowledge of protein and amino acid nutrition and metabolism in fish

Kaushik, S.J.¹; Seiliez, I.¹
¹ INRA, UMR Nutrition, Aquaculture & Genomics Unit, France

Abstract:
Although fish are generally considered better converters of dietary protein, compared to terrestrial vertebrates, there is an increasing body of evidence that still further improvements in dietary protein utilisation can be achieved by optimising dietary supply in tune with the requirements. Given the large number of species of farmed finfish and shrimp, it would be difficult to establish the quantitative requirements for all the ten indispensable amino acids (IAA) for each of them. Analysis of reliable data available however strongly points towards much homogeneity between the different species. The ideal protein appears to be that reflecting the whole body IAA profile of the corresponding species with an indispensable / dispensable amino acid ratio of close to one to one. But, it is also as yet difficult to make a clear distinction between IAA needs for maintenance and growth components for all species. Optimising amino acid supply in tune with the requirements and improving protein utilisation for body protein growth with limited impacts on the environment in terms of nutrient loads is a generic imperative in all animal production systems. With the continued high annual growth rate reported for global aquaculture, our commitments should be to make sure that this growth is indeed reflected in provision of protein of high biological value for the humans. The limited availability of fish meal has led to some concerted efforts in fish meal replacement, analysing all possible physiological or metabolic consequences. The rising costs of plant feedstuffs make it necessary to strengthen our basic knowledge on amino acid availability and utilisation. Regulation of muscle protein accretion bears great significance with strong practical implications. In fish, despite low muscle protein synthesis rates, the efficiency of protein deposition appears to be high. Exploratory studies on amino acid flux, inter-organ distribution and particularly of muscle protein synthesis, growth and degradation and the underlying mechanisms as affected by dietary factors are warranted. Research on specific signalling pathways involved in protein synthesis and degradation have been initiated in order to elucidate the reasons for high dietary protein / amino acid supply required and their utilisation.

Dr. Ashild Krogdahl

Important antinutrients in plant feedstuffs: Biological effects and knowledge gaps

Krogdahl, A.¹; Chikwati, E.²; Refstie, S.³; Bakke-McKellep, A.M.¹; Penn, M.H.²
¹ Norwegian School of Veterinary Science, Norway; ² Aquaculture Protein Centre (APC), Norwegian School of Veterinary Science, Norway; ³NOFIMA, Norway

Abstract:
Antinutrients (ANFs), although not necessarily toxic per se, are compounds decreasing the nutritional value of feeds. Legumes, beans in particular, contain the greatest variety of ANFs, but their presence is also of importance in cereals, rape seeds, sunflower and other common plant feeds. Among the best known ANFs are non-starch polysaccharides which interfer with key components of digestion and thereby reduce utilisation of nutrients; phytic acid which binds divalent ions and thereby impairs mineral absorption; enzyme inhibitors which reduce digestion of protein, lipid and starch; lectins which bind to enterocyte receptors and thereby may stimulate gut growth, insulin production and metabolism in general; saponins which interfere with cholesterol absorption and metabolism and thereby with lipid metabolism in general; goitrogens which inhibit iodine transport in the thyroid gland and thereby increase iodine requirement; and phytoestrogens which may interfere with effects of endogenous oestrogen. Our own investigations on effects of soybean ANFs in salmon indicate that there may be components with antinutritive effects yet to be described. Carnivorous fish seem to differ from warm-blooded animals in several manners with respect to their response to antinutrient. For example, salmonids are much more sensitive to protease inhibitors, and considerable differences also occur among different fish species. Hence, knowledge regarding one species may not be applicable for another. Until now efforts to gain understanding regarding effects of ANFs in fish have been based on results of studies employing whole ingredients or crude extracts containing mixtures of several ANFs, known as well as unknown, present at levels which mostly have been unknown. Investigations with purified ANFs are needed to sort out which are of significant importance and if interactions between them are essential for the responses. A recent study with Atlantic salmon focused on ANFs that might affect lipid digestion and metabolism: soy saponins, phytosterols, chitosan and a lipase inhibitor. The study showed that soy saponins interfered with intestinal structure and function of absorptive membranes. However, no signs of enteritis in the distal intestine were observed. The lipase inhibitor reduced lipid digestibility, but the phytosterols were most potent in reducing the digestibility of lipid.

Dr. Ronald Hardy

Changing paradigms for alternate proteins for aquafeeds: challenges and opportunities associated with biofuels.

Hardy, R.¹
¹ Hagerman Fish Culture Center, United States of America

Abstract:
Over the past year, the duel demands of producing crops for food and fuel have reached a point of surpassing supplies, and prices for agricultural commodities and marine resources have left their 30-year trading ranges. These changes have increased the costs of feeds and presented unique challenges for the livestock and aquaculture industries. Further, they are having profound effects on the alternatives feed producers have to replace marine resources in fish feeds. Although demand for grains for ethanol production have disrupted the feed industry, there remain opportunities to capture and utilize protein from ethanol factories. Novel approaches to ethanol production can actually increase global protein supplies for feeds, but extensive research will be required to develop methods to produce these proteins and to use them in livestock and aquafeeds. Aquaculture nutrition researchers need to design their research efforts in the context of global macroeconomic changes that are likely to occur over the next decade.

Dr. Gordon G. Bell

Dietary oil sources: current position and future challenges

Bell, J.G.¹; Tocher, D.R.¹
¹ Institute of Aquaculture, University of Stirling, United Kingdom

Abstract:
Due to reduced availability and increasing prices of marine fish oil (FO) and meal (FM) new sustainable alternatives are required, including vegetable oils (VO) and meals, as a matter of urgency. While fish thrive on the n-3 highly unsaturated fatty acids (HUFA) and balanced amino acid components of marine FO and FM there is now considerable evidence supporting significant replacement of these traditional materials with sustainable terrestrial alternatives. However, while many fish species can be cultured, for the majority of the growth cycle, on diets with high levels of VO or plant meals with no reduction in growth, reduced flesh concentrations of n-3 highly HUFA decreases the nutritional value of the product for consumers. Thus, any substitution of feed raw materials should ensure that cultured fish should have n-3 HUFA levels close to those in wild fish and should not be compromised in terms of growth. This requires careful selection of feed raw materials as well as enhanced knowledge of the mechanisms controlling lipid and fatty acid metabolism in fish. Target values for reduction of FO and FM in aquafeeds have been proposed by the EU FP 6 project-AQUAMAX and suggest that very significant reductions in both FO and FM should be achievable for most cultured species. However, it may not be in the best interests of fish health to replace FO over the whole production cycle. Therefore, FO should be retained in feeds for broodstock and early larvae to meet developmental requirements for n-3 HUFA during these important periods. Probably the best phase to use high VO diets is during ongrowing from juvenile to pre-market. VO rich in monoenoic fatty acids and low in saturates may be better digested than some FO, especially at low water temperatures. The use of FO-rich finishing diets may also be useful in increasing n-3 HUFA in pre-market fish. In addition, careful choice of FO should allow best use of this valuable resource such that high energy oils, rich in monoenoic fatty acids, should be used during the rapid ongrowing phase while high HUFA oils should be used to restore n-3 HUFA in the pre-market phase. A wide range of VOs can be used as substitutes although the large volume products, including rapeseed, palm and soya are likely to dominate formulations, novel oils, including GM products should be considered either alone, or as part of a VO blend, with a balanced saturate, monoene, polyunsaturated (PUFA) and HUFA composition, similar to the prey of the cultured species being optimal. There is evidence that oils with very high levels of PUFA may compete with HUFA at the sn-2 binding sites on cellular phospholipids, and thereby reduce HUFA levels, and, thus, oils with a low PUFA/high monoene content should be chosen to reduce this loss. In most fish species, the correlation between dietary fatty acids and tissue fatty acids is very strong although the correlation is highest in oil-rich tissues. Thus, the fatty acids accumulated in tissues is controlled by dietary input and the metabolic processes involved in digestion and absorption, conversion (desaturation and elongation), oxidation and deposition. Using both conventional biochemical and molecular techniques, considerable advances have been made in recent times to elucidate the mechanisms that regulate lipid metabolism in fish. However, these mechanisms are crucial to the development of sustainable aquafeeds in the coming years and much work remains to be done for both new and existing aquaculture species. In recent years the presence of undesirable substances in food, and especially seafood, has attracted considerable attention. In oily fish species, like salmonids, the accumulation of persistent organic pollutants (POPs) including dioxins, PCBs, PBDEs and other organohalogen compounds has implications for food safety. The majority of the POPs in fish comes from the diet and use of VO can reduce POPs levels at the expense of reduced n-3 HUFA. Therefore, use of decontaminated fish oils could reduce POPs levels and simultaneously preserve beneficial HUFA. Oil sources for aquafeeds in the future should consider products that are currently underutilised, as well as GM oilseeds, while striving to enhance knowledge of lipid metabolic processes and selective breeding techniques. This should ensure more efficient use of precious lipid resources to maintain and enhance aquaculture produce for future generations.

John Sweetman

Enhancing natural defences and barrier protection of aquaculture species

Sweetman, J.¹; Torrecillas, S.²; Davies, S.³ ; Izquierdo, M.S.²
¹ Alltech Aqua, Greece. ²Grupo de Investgacion en Aquicultura (GIA), ICCM & IUSA, Spain. ³Fish Nutrition and Health Group, School of Biological Sciences, University of Plymouth, UK.

Abstract:
The promotion of nutritional strategies that optimise natural defence mechanisms in fish are of critical importance in producing robust juveniles and adult fish. These animals are more capable of minimising the impact of opportunistic pathogen attack so promoting improved liveability and performance characteristics.The importance of the piscine gastro-intestinal tract as a major endocrine and osmo-regulatory organ is noted as well as its key function as a defensive barrier to aquatic pathogen invasion, innate and specific immunological activity and induction of tolerance will be considered. The effect of diet composition on gut integrity, microbial profile, tissue damage (enteritis) and inflammation is explored with the application of prophylactic measures based on prebiotic additives. Investigations using the inclusion of a specific structural form of mannan oligosaccharide (MOS), (Bio-Mos®, Alltech Inc., USA) and a yeast extract product (NuPro®, Alltech Inc., USA) have been shown to improve the immune status, gut morphology and improve the important aspect of barrier protection through the enhancement of mucal production in a number of aquaculture species. The evaluation of the dose dietary inclusion of 2‰, 4‰ and 6‰ MOS in a commercial diet on the rearing performance of on-growing European sea bass (Dicentrarchus labrax) and its implications in growth performance, liver metabolism, certain immune parameters, digestibility and flesh quality will be considered.

Dr. Viviane Verlhac

Nutrition and immunity: an update

Verlhac, V.¹
¹ NRD/CA - DSM Nutritional Products France, Research Centre of Animal Nutrition and Health, France

Abstract:
Immunity encompasses all mechanisms and responses used by the organism to defend itself against bacteria, virus and other disease-causing organisms. Adequate supply and balance of nutrients are required for a proper efficiency of every aspect of immunity and host defense. Research has identified dietary factors which affect the immune response of human and animals such as amino-acids, fatty acids, oligo-elements, vitamins. Some of these nutrients have proven specific actions on the immune system when provided at pharmacological doses. This paper will review these nutrients and their current use in aquaculture. The immune system is an efficient but also complex system. Its complexity has made the assessment of the effects of diets on immune functions difficult. Nevertheless, the standardization of methodology between laboratories as well as the use of new techniques at the cell or gene level should help to better understand the mechanisms of the modulation of the immune response. This paper will review the major functions of the immune system of fish and shrimp and the methodology used to assess them. Both cellular and humoral functions including cytokines, as important mediators of the immune response, will be discussed in relation to potential means to modulate them and the underlying mechanism. A better understanding of mechanisms of modulation of the immune functions should help in the discovery of new dietary factors to improve the immune status of the animal, leading to better disease resistance.

Dr. Marisol S. Izquierdo

Importance of dietary nutrients for fish larvae production and quality

Izquierdo, M.S.¹; Hemre, K.²
¹Grupo de Investigación en Acuicultura (GIA - ICCM & IUSA), Spain ²National Institute of Nutrition and Seafood Research (NIFES), Norway

Abstract:
Despite the importance of high quality fry production for marine fish aquaculture success, nutrition of fish larvae is not yet well understood. Studies regarding larval nutrition are complicated due to the restricted acceptance of dry diets by first feeding larvae. Moreover, rotifers and Artemia fed to fish larvae seem to be incomplete in several nutrients, even if they are enriched on essential fatty acids, such as balanced proteins and critical vitamins and minerals. Several nutritional factors have been found to directly or indirectly affect survival, growth and poorly developed digestive system, muscle and skeleton of first feeding fish larvae. Recently developed histological, behavioral and molecular tools are bringing new information to better understand larval nutrition. Essential fatty acids, long recognize to be one of the most important nutritional factors affecting growth, survival and neural tissue development in fish larvae, have been recently found to affect behavior, muscle formation and skeletogenesis, as well as stress related genes expression such as glucocorticoid receptor and heat shock proteins (HSP70). Fatty acids, particularly DHA, have been also found to be determinant of flatfish correct pigmentation, their requirements to advoid pseudoalbinism being higher than those needed to obtain the best growth and survival in halibut. Dietary phospholipids have been found to markedly affect dietary lipids utilization, being also related to bone formation and skeleton deformities prevention. But interrelations among nutrients must be also considered. For instance the elevation of dietary vit E allows reducing the n-3 HUFA requirements in larval seabream. Moreover, imbalances in n-3 HUFA/vitamin E ratios have been found to cause dystrophic changes in skeletal muscle of sea bass larvae, in relation to an increased susceptibility of the muscle membranes to oxidative stress, contributing to appearance of skeletal deformities. Interrelation between vit C and vit E are shown in seabream larvae where the elevation of any of them spares the requirements for the other. Among fat-soluble vitamins, a special interest has been paid to vitamin A for its central role in modulating embryonic and larval development. Halibut juveniles, and probably most marine fish larvae, are able to convert astaxanthin and cataxanthin to vitamin A, although at a low rate. Hence, the concentration of these carotenoids in Artemia and copepods would be sufficient to cover the vitamin A requirements of the larvae. On the contrary, un-enriched rotifers may have very low carotenoids content and must be enriched with vitamin A. Among minerals, iodine, as part of thyroid hormone, plays a role in modulating larval development and metamorphosis. Rotifers and Artemia iodine contents can be several hundred times lower than in copepods, the marine fish larvae natural prey. Supplementing cod larvae through rotifers with iodine and selenium up to copepod levels, markedly increased survival. Fast growing fish larvae have a high demand for protein. But live preys eaa profiles seems to be deficient in certain eaa depending on the fish larvae species. Since marine fish larvae lack a functional stomach partially hidolyzed protein sources have been frequently used in larval dry diets, showing not only growth improvements in several species but also a reduced deformities degree. Besides, a marked effect on muscle development has been suggested by some authors.