alternative lipid sources in aquafeeds
TRANSCRIPT
May | June 2010
Feature title: Alternative Lipid Sources in Aquafeeds
The International magazine for the aquaculture feed industry
International Aquafeed is published five times a year by Perendale Publishers Ltd of the United Kingdom.All data is published in good faith, based on information received, and while every care is taken to prevent inaccuracies, the publishers accept no liability for any errors or omissions or for the consequences of action taken on the basis of information published. ©Copyright 2009 Perendale Publishers Ltd. All rights reserved. No part of this publication may be reproduced in any form or by any means without prior permission of the copyright owner. Printed by Perendale Publishers Ltd. ISSN: 1464-0058
The global aquaculture industry is one of the fastest growing food production sectors
with farmed seafood currently accounting for about 50 percent of all fish consumed in the world.
Aquafeeds and fish oilsIt is estimated that aquaculture
produces about 65 million tonnes of seafood valued at more than US$78 billion annually.
Aquaculture is anticipated to play an increasingly important role in meeting the seafood demand of a growing human population. The rapid increase in aquaculture production world-wide has been fueled by the use of industrially manufactured aquafeeds.
Conventionally, marine fishmeal and fish oil are used as the major feed ingredients in the formulation of commercial aquafeeds to supply dietary protein and lipid, respectively. It is estimated that aquafeeds currently con-sume about 90 percent of the global supply of fish oil and many have predicted that the demand for fish oil from the aquaculture industry will imminently out strip supply.
Marine fish oil production has not increased beyond 1.5 million tonnes for the past quarter of a century and in order to further expand, the global aquaculture industry cannot continue to rely solely on this source of lipid.
The high demand, impending short sup-ply and often times high prices makes
dietary fish oil a bottle-neck in the farming of aquatic animals, and there is currently great urgency within the global aquafeed industry in finding suitable alternatives to replace marine fish oils.
This article will give an overview of the
various alternative lipid sources, grouped according to their main chemical charac-teristics. Their unique potential advantages and challenges for use in aquafeeds will be highlighted. The physiological effects of various lipid sources and their components on growth, lipid metabolism, health and post-harvest qualities of the farmed fish are
briefly discussed.
Alternative lipid sources
Oils and fats are characterised by their unique fatty acid composition.
The major vegetable oils have one common characteristic; none contain n-3 long chain-polyunsatu-rated fatty acids (LC-PUFA).
In contrast, marine fish oils have a high content of n-3 LC-PUFA. In consideration of the fact that the dietary fatty acid composition is mirrored in farmed fish fillet, the inclusion of alternative lipid sources in aquafeed can have sig-nificant impacts on the nutritional
qualities of farmed seafood products. The n-3 LC-PUFA are known to impart health-promoting benefits to human consumers.
Saturated fatty acid (SFA)-rich plant oils include palm oil, palm kernel oil and coconut oil.
Alternative Lipid Sources in Aquafeedsby Dr Wing–Keong Ng, Fish Nutrition Laboratory, School of Biological Sciences, Universiti Sains Malaysia, Dr Giovanni Turchini, School of Life and Environmental Sciences, Deakin University, Australia, Dr Douglas Tocher, Institute of Aquaculture, University of Stirling, Scotland
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Figure 2: The annual world production (1995-2008) of the three major vegetables oils as compared to fish oil
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Global production of crude palm oil (CPO) exceeded 43 million tonnes and together with about nine million tonnes of coconut and palm kernel oils, constitutes a highly available and sustainable source of lipids for the aquafeed industry.
When freshly extracted, CPO is the richest known natural source of β-carotene and is also a rich source of vitamin E, consisting of tocopherols and tocotrienols. Several studies have shown that various palm oil fractions can be successfully used either singly or in combination with other plant oils in the aquafeeds of commercially farmed species. The limited PUFA content, combined with the presence of natural antioxidants (in the case of CPO), has been reported to impart enhanced pellet and fillet oxidative stability.
Furthermore, the overall fatty acid modi-fication of the fish fillet is less detrimentally affected by SFA-rich oils, when compared to other alternative lipid sources.
Nevertheless, concerns have been expressed on its potential negative effect on nutrient digestibility, particularly when fed to cold water fish species during the winter season.
High quality sources of dietary energy
Soybean, corn, safflower, cottonseed and sunflower oils are the main n-6 PUFA-rich (namely linoleic acid, 18:2n-6) oils produced.
When incorporated into aquafeeds, these n-6 PUFA-rich plant oils have been reported to be high quality sources of dietary energy and fatty acids during the grow-out cycle in most fish tested to date.
However, a major concern of using these oils is that linoleic acid is abundantly and preferentially deposited in the fish fillet. Since our human diets already contain too much n-6 PUFA, some scientists believe that a good fish oil substitute should limit the deposition of these less desirable fatty acids in fish fillets.
Once deposited, linoleic acid is also known to be selectively retained in fish fillets and resistant to ‘dilution’ even after switching to a fish oil finishing diet. This may be problematic in the context of using fish oil finishing diet strategies to restore beneficial n-3 to n-6 PUFA ratios in farmed fish fillets.
Several selected cultivars of these oilseeds have been recently developed to contain significantly lower concentrations of linoleic acid.
The major monounsaturated fatty acid
(MUFA)-rich oil produced is rapeseed (canola) oil with olive, peanut and rice bran oils making up the rest of this class of lipids. Oleic acid (18:1n-9) and other MUFA are readily digested and β-oxidized by fish to produce energy and have been reported to have no known adverse effect on fish growth performance.
Depending on market prices, rape-seed oil is cur-rently one of the commonly utilized lipid alternatives in c o m m e r c i a l a q u a f e e d s , especially those formulated for cold water and temperate species.
Plant oils rich in n-3 PUFA [namely α-linolenic acid, ALA (18:3n-3), and steari-donic acid, SDA (18:4n-3)] has generated much research interest due to the ability of these fatty acids to be bio-converted into the longer chain, more un s a t u r a t ed ,
physiologically important n-3 LC-PUFA by many farmed species, albeit mostly at lim-ited capabilities.
Despite encouraging evidence of poten-tial bio-conversion of ALA and SDA into n-3 LC-PUFA, the inclusion of these oils (i.e. linseed/flaxseed, camelina, perilla and echium) in aquafeeds is limited, as they are
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A new generation of omega-3 lipids with a broader spectrum of health bene� ts.
- High DHA contents, preferably in easily digestible and highly bio available form for aquaculture use.
- Numerous bene� ts on improving the immune response, better weight gain and physical conditions of land animals.
Marine phospholipids
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Figure 3: Palm oil is the most produced and fractionated oil in the world and many fractions have been successfully evaluated in aquafeeds
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Impact of lipid sources on farmed fish
As mentioned above, dietary fatty acid com-position directly influ-ences flesh fatty acid composition, the extent of which depends on the level of substitution of fish oil, the duration of feeding and the pre-cise fatty acid composi-tion of the substituting oils.
In general, substitution with vegetable oils results in increased proportions of C18 fatty acids (18:1n-9, 18:2n-6 and 18:3n-3), and decreased propor-tions of n-3 LC-PUFA
(EPA, 20:5n-3 and DHA, 22:6n-3). In choosing substituting oils, we should
aim to minimise these effects and so, ideally, the replacing oil should satisfy some general criteria. The oil should have a high MUFA content, not only to provide a good energy source, but also to reduce the level of C18 PUFA, which should be relatively low.
For this reason, oils with high C18 PUFA, particularly 18:2n-6, should be used sparingly.
The replacement oil should contain 18:3n-3, not simply for potential conversion to EPA and DHA, but also because its inclusion will help to balance the n-3/n-6 ratio and limit 18:2n-6 inclusion. Some researchers suggest that oil blends consist-ing of several plant oils are better in terms of health and welfare of the fish when used in aquafeeds.
Dietary fat influencesDietary fatty acid composition also
influences various aspects of lipid and fatty acid metabolism. These include digestibility, lipogenesis, lipid transport and uptake, fatty acid catabolism and fatty acid desaturation and elongation that can all influence tissue fatty acid composition.
For example, the amount of dietary SFA influences the digestibility of lipids especially at low water temperatures. LC-PUFA synthesis from 18:3n-3 has been shown to be increased in fish fed diets with fish oil substituted by vegetable oils through up-regulation of desaturase and elongase gene expression and consequently
New lipid sourcesIn recent years, new lipid sources con-
taining n-3 LC-PUFA are the subject of intense research interest. These include oils derived from marine invertebrates such as copepods, krill and amphipods.
Given the significantly large biomass of marine invertebrates and projecting a ‘safe’ level of harvest, it has been estimated that these new sources have the potential to produce more marine oils than current global fish oil production.
However, there are some technical con-cerns on the use of such oils, such as har-vesting technologies and the large content of waxes and phospholipids together with great variability in fatty acid composition.
One good n-3 LC-PUFA-rich source are oils derived from by-catch and fishery or aquaculture by-products. With better management and utilisation, it is estimated that the total quantities of fish meal and fish oil coming from aquaculture and fishery derived waste and by-products are most likely in the range of several million tonnes.
Single cell oils (from microalgae) and genetically modified oilseeds represent novel n-3 LC-PUFA-rich oils.
Nutritionally, single cell oils are likely to be the best alternative to fish oil as they contain even higher amounts of beneficial n-3 LC-PUFA, but their very high produc-tion costs and limited availability make their use almost prohibitive. Oils from genetically modified oilseeds are not yet a commercial commodity and legislative issues may need to be addressed before such oils can be used in aquafeeds.
currently relatively expensive and limited in supply.
However, they can be useful in oil blend formulations to adjust dietary n-3 PUFA levels.
Animal fatsTerrestrial animal fats include tallow,
poultry by-product fat and lard. About 12 million tonnes of rendered animal fats are manufactured every year around the world and are generally more eco-nomical than fish and plant oils. Animal fats represent a very diverse group of products but are generally rich in SFA although some can be rich in MUFA and contain PUFA.
Their fatty acid composition is largely influenced by the diet of the livestock.
For example, poultry by-product fat in Australia is enriched with MUFA as chick-ens are commonly fed a rapeseed-based diet, while in the USA, it has relatively higher n-6 PUFA as birds are commonly fed a soybean-based diet.
Recent studies have reported that these lipid sources are well digested and utilised by most fish species. Growth performance of aquaculture species are generally not negatively impacted by dietary animal fats as long as the diets are formulated to contain sufficient amount of MUFA and PUFA to facilitate the digestion of SFA and meet the essential fatty acid requirements of the aquatic animal.
Terrestrial animal fats are increasingly recognized as safe and cost-effective lipid sources when properly used in aquafeeds.
Figure 4: Farmed seafood fed alternative lipid sources remains a good dietary source of health-promoting omega-3 fatty acids for the human consumer
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Seafood from farmed animals fed diets with high fish oil replacements remains a good source of n-3 LC-PUFA that, although reduced, are still higher than in any alterna-tive meat or food item and so contribute positive health benefits.
Impacts of dietary lipid source on physical quality aspects are few, but lower oxidation values in flesh during shelf life in fish fed vegetable oils com-pared to fish oil is the most consistent. Other influences include some limited effects on flesh colour, texture and gap-ing, and liquid holding, but not freshness, during shelf life.
In most farmed species studied, taste panelists were able to discriminate amongst some specific attributes and quality param-eters of fillet of fish fed different lipid sources.
However, these differences are relatively subtle and consumers have been reported not to show any specific preferences for differently fed farmed fish. Effects on fil-let organoleptic properties are somewhat subjective and variable, and are dependent on the dietary oil blends used. The product quality fac-tors can, to a large extent, be restored through the use of finishing diets rich in fish oil.
In conclu-sion, in the current era of increased con-sumer demands for food safety, t r a c e a b i l i t y and quality, the challenge for the aquaculture industry is to maintain the r e c o g n i s e d benefits of seafood con-sumption for human health, especially when a l t e r n a t i v e lipids are used in aquafeed for-mulations, while m a i n t a i n i n g susta inabi l i ty and profitability of the industry.
increased activity of the desaturation/elon-gation pathway.
Unfortunately this is restricted to cer-tain species and does not occur in marine fish and crustaceans. Irrespective of species, increased synthesis of LC-PUFA is not able to compensate for the lack of dietary n-3 LC-PUFA.
Lipids and fatty acids are now known to be highly metabolically active, involved in controlling and regulating cell metabolism and animal physiology through mechanisms involving gene expression and several lipid signalling pathways.
Therefore, modification of tissue fatty acid compositions can have wide ranging effects.
Among the most studied are the eicosa-noids (for example, prostaglandins, leukot-rienes and resolvins) that are metabolically active derivatives of LC-PUFA with impor-tant roles in mediating inflammatory and immune responses to a variety of stresses.
The n-6-derived eicosanoids are pro-inflammatory whereas n-3-derived eicosanoids are either less potent or anti-inflammatory.
Thus, substitution of n-3-rich fish oil by n-6-rich vegetable oils will alter the eicosanoids produced resulting in effects on inflammatory and immune responses, which can be potentially detrimental or beneficial depending upon the particular stress.
The above illustrates one mechanism whereby altered dietary fatty acids, espe-cially n-3/n-6 balance, can affect the health and welfare of fish.
However, substitution of fish oil with vegetable oils affects the immune system in several ways, including both cellular and humoral immunity, although these effects do not always alter resistance to disease.
Other aspects of health status of fish that may be affected by dietary fatty acids include welfare, through altering the cortisol response to stress, tissue morphology (for example, liver and intestine) that may or may not affect organ functionality, skeletal development, cataracts and development of atherosclerosis and cardiac lesions.
However, many factors can affect stress, immunity and pathogen resistance in fish, including the type, level and duration of veg-etable oil feeding, other dietary nutrients, fish species and environmental conditions.
Product quality encompasses physical aspects such as freshness and appearance, and organoleptic properties, as well as nutritional quality, which is largely defined by the n-3 LC-PUFA content.
About the Authors:Wing–Keong Ng of the Fish
Nutrition Laboratory, School of Biological Sciences, at Universiti Sains Malaysia in Penang, Malaysia (Email: [email protected]); Dr Giovanni M. Turchini, is at the School of Life and Environmental Sciences, Deakin University, Warrnambool, VIC 3280 in Australia and Douglas R. Tocher is at the Institute of Aquaculture, University of Stirling in Stirling, Scotland.
Author’s note:The subject matter of this article
will be the topic of an upcoming book entitled ‘Fish Oil Replacement and Alternative Lipid Sources in Aquaculture Feeds’ edited by the authors and published by CRC Press - Taylor and Francis Group: http://www.crcpress.com/
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