The effect of marine and non-marine phospholipid rich oils when fed to juvenile barramundi (Lates calcarifer)
Introduction
The phospholipids form the structural bilayer of cell membranes providing integrity and fluidity (Hazel and Williams, 1990, Tocher et al., 2008). Central to their biological importance is their structure with lipoproteins that assist in the extracellular transport of lipids thus improving parameters such as growth, survival and health throughout the organism (Tocher et al., 2008). However, the total lipid content in fish is mostly composed of neutral lipid in the form of triacylglycerol (TAG) which is a more readily available energy source (Glencross, 2009).
There is evidence to suggest that most larval and early juvenile fish have a dietary requirement for intact phospholipids as endogenous biosynthesis is not sufficient (Coutteau et al., 1997). Coutteau et al. (1997) reported that the phospholipid requirement of fish and crustaceans varied depending on the life stage and history. Freshwater fish generally have lower dietary requirements, of around 2% whereas marine fish generally had higher requirement ranging up to 7% however that gradually reduced as fish grew. Early studies found that in both rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar) the phospholipid requirement of first swim-up sized fish (< 0.2 g) was 4% supplied in the form of soybean lecithin (Poston, 1990a, Poston, 1990b). However, larger salmon (~ 7.5 g initial) showed no improvement in terms of growth suggesting that endogenous synthesis of phospholipid is sufficient to support the requirement of the fish and that high dietary levels had a negative effect on survival (Poston, 1990a). It should also be noted that the latter study, and possibly others, refer to a requirement of phospholipid containing ingredients rather than the precise phospholipid content which is often unclear.
With very few exceptions, provision of marine derived phospholipid to cultured fish is limited. Moreover, there are few studies on the effect of dietary phospholipids in juvenile fish greater than 5 g as it is generally accepted that they don't have a requirement based on the historical evidence presented for Atlantic salmon (Poston, 1990a). Recently, the influence of dietary phospholipid from either krill oil or soybean lecithin was investigated in Atlantic salmon from first feeding up to smolt (0 to 70 g range) (Taylor et al., 2015). These authors demonstrated a range of improvements among the parameters tested and concluded that Atlantic salmon have a dietary requirement for intact phospholipid particularly in early development. Therefore, with the continual reduction of fish meal (FM) and fish oil (FO) in commercial feeds and the complex biochemistry of the phospholipids particularly in juvenile fish, further investigation is warranted. Moreover, the preferential incorporation and retention of phospholipid fatty acids are important in maintaining phospholipid quality and also to fulfil other downstream roles of the phospholipid classes (Linares and Henderson, 1991).
Recent in- and ex-vivo methods have so far demonstrated that barramundi or Asian seabass (Lates calcarifer) are not capable of any measureable long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis (Alhazzaa et al., 2011a, Mohd-Yusof et al., 2010, Tu et al., 2012). However, some notable effects on the phospholipid composition of tissues were identified, which may suggest that juvenile barramundi have a requirement for intact phospholipids in order to prevent the onset of deficiency (Alhazzaa et al., 2011b, Tu et al., 2013). It appears that when dietary PL are not sufficient then very selective retention of tissue phospholipids occurs in barramundi and other species, until depletion, this being a mechanism to prevent the onset of PL deficiency and secondary pathologies as a result (Skalli and Robin, 2004, Tocher et al., 2008, Tu et al., 2013).
Most phospholipid requirement studies to date have used soybean lecithin containing high levels of n − 6 PUFA, while others have used egg lecithin or various other marine sources such as fish roe lecithin (Cahu et al., 2009). Recent studies have clearly demonstrated the potential of marine derived phospholipid sources to improve larval and juvenile fish performance (Betancor et al., 2012, Taylor et al., 2015). To date, information is scarce on the effect phospholipid in juvenile barramundi diets. Therefore, an experiment was designed to compare the metabolic effect of marine and non-marine neutral lipid (NL) and polar phospholipid (PL) sources using a two-by-two factorial approach in juvenile barramundi. The biochemical and molecular mechanisms underpinning the role of phospholipids were also investigated.
Section snippets
Ingredient and diet preparation
The diets were formulated to provide digestible protein at ~ 55%, lipid at ~ 12% with a digestible energy value of ~ 19 MJ/kg. The dry ingredients were passed separately through a hammermill (Mikro Pulverizer, type 1 SH, New Jersey, USA) such that the maximum particle size was less than 750 μm. All ingredients were then thoroughly mixed in using an upright commercial mixer (Bakermix, Model 60 A–G, NSW, Australia). The chemical composition of the main dietary ingredients is presented in Table 1. A
Growth and feed utilisation
In the present study, the two levels of omega status were defined as n − 3 and n − 6 and the two levels of lipid class were defined as neutral lipid and phospholipid. There was no difference in the initial weight of the fish among the treatments; however there were significant differences in the growth and feed utilisation parameters (as final weight, feed intake and FCR) upon termination of the 56 d growth assay (Table 5). There were significant interaction terms indicating that the effectiveness
Discussion
It is well established that larval and early juvenile fish have a dietary requirement for intact phospholipids that can lead to long term improvements in many growth performance parameters (Coutteau et al., 1997, Tocher et al., 2008). Historically, most phospholipid studies were conducted with commercial phospholipid preparations of commonly available emulsifying agents such as lecithin from soybeans or corn (Tocher et al., 2008). However, in some cases the use of these products has potentially
Conclusions
Studies have consistently demonstrated a range of advantages when using phospholipid-rich lipid sources in diets for larval and juvenile fish. In this study, we report for the first time the use of phospholipid-rich krill oil and also soybean lecithin compared to neutral lipid sources including fish oil and soybean oil. In support of the vast majority of studies, we demonstrated that the inclusion of either marine or non-marine phospholipid maintains performance of juvenile barramundi
Acknowledgements
The authors wish to acknowledge the technical assistance provided by Bruno Araujo, David Blyth, Natalie Habilay, Simon Irvin, Kinam Salee and Richard Thaggard of the Bribie Island Research Centre (BIRC), Queensland, Australia. The authors wish to acknowledge the CSIRO Agriculture for financial support.
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