Diet and feed effect on body composition:

The amount of EPA and DHA are not the same in all types of fish, it depends on several factors such as species, sex, size, diet, water temperature and season (Abbas et al., 2009) and among these factors diet plays an important role.

Food intake and efficiency have been improved in farmed salmon by 40% and 20%, respectively (Thodesen et al., 2001). Farmed fish cannot catch their feed and therefore feed composition depends on the feed offered by fish farmer. One of the advantages of farmed fish over wild fish is the capability to control the environment to some extent which is good for getting high amount of EPA and DHA in feed.

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Salmon have the enzymes to elongate the shorter chain of omega-3 to the longer chain but the capacity is limited (Miller et al., 2007) thus they should introduce very long chain omega-3 into their diets. Nowadays companies producing salmon feeds use energy dense diets which consist of low protein and high lipid ratios (Hemre and Sandnes, 1999 ).

Salmon is carnivorous which means that they do not tolerate high concentration of carbohydrates in their diets. The feed should cover all the fish requirements to provide a healthy fish with fast growing performance and also low mortality.

Low levels of the two essential fatty acids such as 18:2n-6 and 18:3n-3 in fish diets shows deficiency problems. According to Glencross, (2009) for instance low feed efficiency, fatty liver, poor growth, and high hepatosomatic index, increased water content in whole body or muscle and high accumulation of 20:3n-9 in the tissue. Another study in salmon showed that a short period of dietary deficiency of EFA leads to substantial change in the fatty acid composition of liver and blood but not the carcass lipid (Ruyter et al., 2000).

2.6.1 Fish meal and fish oil as a feed for Salmon

It has been more than 15 to 20 years of changes of the composition and formulation of substitution for these commodities. The initial reason is the shortage of sustainable fish meal (FM) and fish oil (FO), the cost of these commodities are increasing despite the fact that the production remains constant (Tacon and Metian, 2008). In addition they may contain contaminants and also organic pollutants, like dioxins and also PCBs.

The fish meal production was 4.83 million tons worldwide according to IFFO, (2011) and also some by-products in fish meal production has been increased and reached 22% of Norwegian production (Chamberlain, 2011). Fish meal substituted by plant protein in some causes caused reduction in growth or also feed conversion, which may be due to the presence

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of anti-nutritional factors that mostly influenced feed intake and gut function (Kaushik et al., 1995, de Francesco et al., 2004, de Francesco et al., 2004 and Espe et al., 2006). Substitution of fish meal with soybean protein concentrate up to 80% or 100% for halibut (Berge et al., 1999) and rainbow trout Oncorhynchus mykiss (Kaushik et al., 1995) showed no negative on growth performance or nutrient utilization. However, according to Gomes et al. (1995) and Espe et al., 2006) studies on rainbow trout and also salmon showed that the total alteration of fish meal with plant protein has influence on growth performance.

According to Kaushik et al. (2004) and Espe et al. (2006) lipid retention declines by applying high levels of plant proteins in salmon feed. Soybean is considered as a good alternative dietary source for fish meal due to the fact that both dehulled and solvent-extracted soybean meal contain high protein and sufficient balance of essential amino acids (Carter et al., 2000).

Fish oil is the optimal oil for salmon due to the fact that it contains high levels of the very long chain omega-3 fatty acids. Fish oil production was 1.6 million tons in 1990 but has been decreasing since 2005 (Silva, 2010). At present salmon industry uses about 60% fish oil in the salmon diet (Nasopoulou and Zabetakis, 2012).

2.6.2 Vegetable oil as a feed for Salmon

Vegetable oil is a cheap and abundant source in comparison to fish oil. The production has increased significantly and has been evaluated to be used as an alternative source for fish oil Vegetable oil consists of a mixture of saturated, monounsaturated and polyunsaturated fatty acids (Bell et al., 2002). Vegetable oil contains more shorter chain fatty acids in comparison to fish oil, for instance linoleic (18:2 ω−6) and oleic acid (18:1 ω−9), and are devoid of the very long chain (Caballero et al., 2002).To compensate for this companies are trying to produce gene modified (GM) rapeseed oil which is high in the very long chain omega-3.

According to Tartibian et al. (2010) it has been suggested that approximately 40-50%

vegetable oil can be used in salmon diet, but studies by Torstensen et al. (2000), Bell et al.

(2001) and Bell et al. (2002) indicated that when vegetable oil inclusion level reached to 50%

and above, there was a significant accumulation of 18:2n-6 and reduction of 20:5n-3 and 22:6n-3 in the fish flesh.

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There are different kinds of vegetable oil such as rapeseed oil, soybean oil, palm oil and also olive oil. Soybean oil is the cheapest vegetable oil with 211 million tons of production in 2009(Soystats, 2010). Salmon fed soy bean oil has higher slaughter yields in comparison to the ones fed fish oil (Regost et al., 2004). Rapeseed oil, which is particularly rich in 18:1n−9, has become an attractive substitute for fish oil in salmon diets. By diluting or washing out the vegetable oil derived fatty acids and also using fish oil finishing diet omega-3 long chain polyunsaturated can be restored ( Bell et al., 2004 and Bell et al., 2005 Torstensen et al., 2004 and Torstensen et al., 2005).

Olive oil as monounsaturated fatty acid can be consumed partially as substituted for fish oil in salmon (Torstensen et al., 2004). Olive oil used partially as substitution for fish in European sea bass culture, during growth out phase (Mourente et al., 2005). Studies by Torstensen et al. (2008) showed that when salmon were fed vegetable oil at low temperature the growth and protein utilization were improved in salmon. The ratio of n-3/n-6 in vegetable oils is 0–0.3:1 and in fish oils is 5–6:1(Linderborg et al., 2013).

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Figure 5- Structure of Alpha-linolenic acid (ALA) derived from plant sources and also Long-chain n-3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) adapted from Mozaffarian et al.

(2011)

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