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Aquaculture development and role in the global food system
Fish have been a regular part of the human diet as long as 40 millennia ago and represent an important food commodity today. Fish currently account for 17% of animal derived protein and 6.5% of total human protein consumption globally (FAO 2014). Fish products, valued at 129 billion US$ in 2012, comprise one of the most widely traded segments of the world food economy (Troell, Naylor et al. 2014). During the past five decades, world per capita fish consumption displayed an impressive increase from an average of 10 kg in the 1970s to 19.2 kg in 2012 (FAO 2014), and around 20 kg per capita in 2015 (Figure 1.1). Indeed, fish are currently estimated to feed more than 2.9 billion people (FAO 2014). Fish do not only represent an excellent source of protein, but are also a unique and rich source of long chain polyunsaturated n-3 fatty acids (LC-PUFAs n-3), especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Both fatty acids are key dietary components for human health. Indeed, these n-3 fatty acids have well-known and almost universally accepted beneficial effects on neuronal development in young children and in the prevention of a range of human pathologies, including cardiovascular and inflammatory disease, as well as neurological disorders (NRC 2011). Fish represent an important source of micronutrients (vitamins and minerals) and some less well-known nutrients such as taurine and choline. This unique blend of nutrients make fish a truly unique and important food for human consumption (Tacon and Metian 2013, FAO 2014).
Global fish production has grown steadily at an average annual rate of 3.2% over the last five decades (Figure 1.2 a – b), outpacing world population growth by 1.6% (FAO 2014). This increase in food fish supply has been facilitated by the dramatic increase in aquaculture production. Although capture fisheries provided most of the fish supply during the 1960-1970s, global capture production has leveled off since the end of the 1980s. In order to maintain a consistent supply of high-quality and sustainable seafood for a growing human population, aquaculture production has expanded by almost 12-fold since the 1990s (FAO 2014). In 2012, aquaculture industry set an all-time record, providing 67 million tons fish products, which accounts for about 49% of the fishery output destined for human consumption (Figure 1.2 c) (FAO 2014). Aquaculture is the fastest growing food production sector, and it continues to expand alongside terrestrial crop and livestock production (Troell, Naylor et al. 2014). As a result of the impressive growth of aquaculture, the demand in terms of aquafeeds has also rapidly increased over the last years (Figure 1.3 a). Thus, global industrial compound aquafeeds production has increased almost four-fold from 7.6 million tons in 1995 to 29.2 million tons in 2008, and is expected to reach 71 million tons by 2020 (FAO 2014). This trend clearly indicates that the availability of quality aquafeeds, and therefore quality feed ingredients, is a major challenge to support the growing aquaculture production.
Aquaculture feeds have been traditionally based on fish meal (FM) and fish oil (FO) derived from industrial feed-grade fisheries (also called reduction fisheries) of small pelagic species such as anchovies, sardines, herring and mackerel. FM and FO supply essential amino acids and polyunsaturated fatty acids of the n-3 series (EPA and DHA), respectively, and are preferentially used because they mimic the food consumption of fishes in their natural habitat. However, reduction fisheries have reached their sustainable limits (Figure 1.3 b), and there is no realistic prospect of FM and FO production being increased in the future. Therefore, the strictly limited supply and ever-rising demand resulting in increasing prices, as well as the necessity to preserve wild fish populations, make it crucial to find efficient alternatives to FM and FO for aquaculture to continue to expand (Naylor, Goldburg et al. 2000, Tacon and Metian 2008, FAO 2014).
Towards a sustainable aquaculture
In the future, the amount of global aquaculture productions is expected to continue to expand to meet the growing consumer demands for seafood (Hardy 2010, Hixson 2014). To improve aquaculture sustainability, many issues require to be addressed, in particular those related to fish nutrition. As previously described, FM and FO have represented the most important ingredients for aquafeeds during the last decades. However, both are now a limited resource, and their prices, which have continually risen in the past three decades, are likely to increase further with continued growth in demand. Thus, a reduction in the use of FM and FO represents a major challenge for sustainable aquaculture.
The current situation of aquaculture has therefore forced a change from marine resources towards more sustainable ingredients in fish diets. Terrestrial plant ingredients represent a good alternative, mainly due to their abundance and the competitive market price (Gatlin, Barrows et al. 2007, Hardy 2010). Significant progress has been made during the last years in partially replacing marine FM and FO by plant ingredients in nutritionally well-balanced diets. Nevertheless, the incorporation of plant products at high levels in fish diets is recognized to have some disadvantages, particularly related to the differences in amino acid (AA), carbohydrate, cholesterol and fatty acid (FA) composition compared to marine resources, but also to the presence of anti-nutritional factors in vegetables. Consequently, the replacement levels have to be adapted depending on the species and the developmental stages. These differences in composition can have metabolic and physiological consequences that deserve further investigation to provide adequate background for successful greater use of plant feedstuffs in aquafeeds.
The presented research was undertaken to identify the potential effects of a combined and total replacement of FM and FO by plant proteins and vegetable oils during the whole life cycle of rainbow trout (Oncorhynchus mykiss). Therefore, the following sections will firstly introduce basic concepts of fish nutrition, underlining the importance of lipid and protein metabolism in fish. Subsequently, the effects of FO, FM or combined FM and FO replacement in fish metabolism and physiology, at both the biochemical level and molecular-transcriptional level, will be discussed.
Lipid nutrition and metabolism
Lipids, together with proteins and carbohydrates, comprise the major macronutrients that are required to provide both essential nutrients for energy production and building blocks for cell and tissue development. In this context, they contribute to growth and maintenance of homeostasis in all organisms (Bell and Koppe 2011). Specifically, dietary lipid supply is crucial for different reasons in fish. First of all, lipids are vital to supply essential fatty acids (EFA). EFAs are defined as not being independently synthesized by the organism, but are necessary for the proper function of cellular metabolism (synthesis of prostaglandins and similar compounds), as well as for the maintenance of membrane structure integrity (via their fluidity). Secondly, they also mediate the intestinal absorption of different compounds, such as liposoluble vitamins and carotenoid pigments. Lastly, lipids play a major role in energy supply, a role which is of upmost importance in fish, since several fish, including rainbow trout, poorly digest carbohydrates (Guillaume 2001).
Lipids and essential fatty acid requirements in fish
Dietary lipid requirement is complicated to define for any fish species, because it is influenced by a variety of different factors. The factors include the chemical nature (ability to react in chemical meaning) and the different functional roles of lipids, the competition with other macronutrients (proteins and carbohydrates) as dietary energy sources, and the environmental factors, such as temperature (NRC 2011). Due to all these factors, the definition of the exact lipid requirements for fish is not particularly meaningful. However, it has long been accepted, that dietary lipid amounts ranging from 10-20% (dry weight basis) are sufficient to allow protein to be effectively used for fish growing, without resulting of the deposition of excessive lipids in fish tissues (Cowey and Sargent 1979, Watanabe 1982, Sargent, Henderson et al. 1989, Corraze 2001).
Another important aspect affecting the requirement of dietary lipid is linked to the fact that dietary lipids are necessary to supply EFAs. The fatty acids (FA) that are commonly termed ‘essential’ are not synthesized de novo in the organism, and must therefore be supplied by the diet (Glencross 2009). EFAs are important components of phospholipids, which are themselves the major constituents of cell membranes and transport lipoproteins. EFA are also implicated in the synthesis of a whole family of molecules, which have a hormonal function in sensu lato: the prostaglandins and similar compounds, such as leukotriens and thromboxanes (Sargent, Tocher et al. 2002). Moreover, some of these EFAs are known to be the precursors of an important class of signaling molecules named docosanoids. These molecules, which are made by oxygenation of twenty-two-carbon essential fatty acids (EFAs), especially docosahexaenoic acid (DHA), have been mainly studied in mammals and are known to possess both anti-inflammatory and protective properties.
In fish, the precise nature of EFA and their absolute dietary requirements are difficult to determine, and a high variability exists among species, in particular between freshwater and marine fish (Tocher 2003, Glencross 2009, NRC 2011). This difference is essentially related to the capacity of fish to bio-convert C18 FA into LC-PUFAs, and therefore to the distinct activity of two classes of enzymes implicated in this conversion process: elongases and desaturases. Elongases are responsible for the condensation of activated FAs with malonyl-CoA in the FA elongation pathway, while desaturases introduce a double bond in the fatty-acyl chain at the C6 or C5 position from the carboxyl group (Tocher 2003). This ability for elongation and desaturation is considered to be more effective in freshwater fish than in marine fish (Bell and Koppe 2011). The difference between marine and freshwater fish can generally be accounted for by considering the natural diets of the different species. Indeed, while marine food webs are generally characterized by high levels of LC-PUFAs of the series n-3 (n-3 LC-PUFA, which mainly came from the presence of microalgae and plankton), namely eicosapentaenoic acid (EPA, 20:5 n-3) and docosahexaenoic acid (DHA, 22:6 n-3), freshwater food webs are not. On the other hand, freshwater food webs are rich in alpha-linolenic acid (18:3 n-3) and linoleic acid (18:2 n-6). Therefore, the widespread ability of freshwater fish to convert C18 PUFAs to the biologically active C20 and C22 PUFAs may be the result of an evolutionary adaptation and a high evolutionary pressure to maintain the capacity to endogenously produce LC-PUFA in freshwater species (Tocher 2003). Generally speaking, one can say that the only EFAs which are truly essential in freshwater fish are the two C18 (alpha-linolenic and linoleic acid), while the principally important EFAs for marine fish are PUFAs and, in particular EPA and DHA. However, it is important to keep in mind that this classification must be considered as a highly simplified classification, and that differences may exist even within the two classes. Indeed, although it is generally accepted that for freshwater fish 18:3 n-3 is the only EFA, the specific dietary requirements of this EFA can vary between 0.5 and 1% between species. EFA requirements can be variable, but requirement ranges for C18 PUFAs and LC-PUFAs for a certain number of species have been established, and the most important findings are summarized in Table 1.1.
Table of contents :
Chapter 1 Introduction and review of literature
1.1.1 Aquaculture development and role in the global food system
1.1.2 Towards a sustainable aquaculture
1.2 Lipid nutrition and metabolism
1.2.1 Lipids and essential fatty acid requirements in fish
1.2.2 Endogenous synthesis and bioconversion of fatty acids
1.2.3 Storage of lipids
1.2.4 Mobilization and catabolism of lipids
1.2.5 Lipids and fatty acids in fish reproduction.
1.3 Protein nutrition and metabolism in fish
1.3.1 Protein and amino acids requirement
1.3.2 Utilization of dietary protein
1.3.3 Proteins and amino acids in fish reproduction
1.4 Replacement of fish oil (FO) and fishmeal (FM) by plant-ingredients in aquafeeds
1.4.1 FO replacement by vegetable oils
18.104.22.168 Characteristics of vegetable oils
22.214.171.124 Consequences of dietary FO replacement by VOs
1.4.2 FM replacement by plant proteins
126.96.36.199 Characteristics of plant proteins
188.8.131.52 Consequences of dietary FM replacement by plant proteins
1.4.3 Combined replacement of FO and FM by plant sources
1.5 Objectives of the thesis and major questions
Chapter 2 Material and methods
2.1 Experimental trials
2.1.1 Animals, feeding trial and sampling procedures
2.2 Analytical methods
2.3 Statistical analysis and data mining
Chapter 3 Results
Chapter 4 General discussion
4.2 Conclusions and perspectives