Elsevier

Aquaculture

Volume 468, Part 1, 1 February 2017, Pages 184-192
Aquaculture

Retro-engineering the protein sparing effect to preserve n-3 LC-PUFA from catabolism and optimise fish oil utilisation: A preliminary case study on juvenile Atlantic salmon

https://doi.org/10.1016/j.aquaculture.2016.10.013Get rights and content

Highlights

  • A ‘retro-engineering’ of the protein sparing effect for the potential preservation of n-3 LC-PUFA from catabolism was evaluated.

  • There were no differences in fish performance, but trends were apparent in relation to the sparing of shorter chain fatty acids.

  • This study provides insight into new strategies for the potential optimisation of fish oil use in aquaculture.

Abstract

Protein has historically been the more expensive macronutrient in aquafeeds. In consideration of this, the implementation of high energy diets for increased protein utilisation is common in modern day aquafeed formulations, evoking what is widely coined as the “protein sparing effect”. However, market forces are changing the cost and availability of raw materials, and oils are now as expensive, if not more expensive, than most protein sources. In light of this, the current study looked to retro-engineer the “protein sparing effect” to test the hypothesis that dietary lipids (in particular omega-3 long chain polyunsaturated fatty acids; n-3 LC-PUFA) could be spared from catabolism by the provision of a high protein diet, manifesting in a more efficient retention/utilisation of these health promoting fatty acids. As such, a case study was implemented in which Atlantic salmon (Salmo salar) were fed a series of iso-energetic diets containing a sliding ratio of dietary protein to lipid, and a fixed concentration of n-3 LC-PUFA. Specifically, these experimental diets were achieved by keeping a constant inclusion of marine derived raw materials (fish meal and fish oil), and varying the relative inclusion of animal by-products (meat and bone meal, blood meal and tallow). Growth performance and feed utilisation parameters as well as tissue fatty acid composition and apparent fatty acid metabolism were assessed. Despite the varying proximate composition of the experimental diets, no significant differences were evident in growth or tissue proximate parameters. Likewise, no conclusive differences were apparent between treatments in relation to the concentration of n-3 LC-PUFA in fillet and whole body samples. However, a remarkable sparing of saturated fatty acids (SFA) and monounsaturated fatty acids (MUFA) from catabolism was apparent and most certainly attributable to 1) the reduced dietary supply of these nutrients, and 2) the preferential utilisation of excess dietary protein for energy production. The results achieved by this study were not conclusive, and the hypothesis cannot be accepted or rejected, but some promising indications have been obtained, ultimately requiring further in-depth investigation.

Statement of relevance

This study documents the effects of feeding a sliding protein to lipid ratio in diets for juvenile Atlantic salmon on potential n-3 LC-PUFA sparing. Essentially, given the rising costs of oil sources in aquafeeds we have ‘retro-engineered’ the protein sparing effect and developed a new approach for consideration in sustainable feed development.

Introduction

Protein has historically been the more expensive macronutrient incorporated into aquafeed formulations. Commercial formulations that guarantee high protein deposition rates are favoured, as the catabolism of dietary protein for energy is wasteful and costly, while simultaneously detrimental to water quality (Cho and Bureau, 2001, Cowey, 1995, Watanabe, 2002). As such, the efficient use of protein-rich raw materials has been, and still is, of high priority for the aquafeed industry. Resultantly, strategies to reduce protein catabolism via the incorporation of high levels of comparatively inexpensive lipid sources have been developed (Cho, 1992, Kaushik and Médale, 1994). These formulations were coined “high energy diets” and permitted a more efficient deposition of protein into tissue via a preferential catabolism of dietary lipid for energy, that in effect, spared dietary protein (Beamish and Medland, 1986, Einen and Roem, 1997, Hemre and Sandnes, 1999, Hillestad and Johnsen, 1994, Kaushik and de Oliva Teles, 1985). This phenomenon is now well known, referred to as the “protein sparing effect”, and an established practice in modern-day commercial aquafeed formulation.

Almost three decades after the initial development of this nutritional approach, markets, raw material availability, nutritional knowledge and consumer expectation have all changed dramatically. The aquafeed sector is now severely impeded by a limited supply of fish oil. This precious resource is not only the subject of high internal demand, but also highly contested and under further pressure by other industry sectors (i.e. the nutraceutical industry). Simultaneously, the well documented health promoting properties of omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA) has stimulated a generation of savvy consumers with an expectation for high concentrations of these fatty acids in aquaculture products (Cleveland et al., 2012, Lands, 2014, Tacon and Metian, 2008, Turchini et al., 2011b, Williams and Burdge, 2006). Combined, each of these aspects has contributed to a drastic increase in the price of this raw material and a widespread understanding and acceptance within the aquaculture community that past practices relating to the inefficient utilisation of this resource cannot continue.

Extensive research into the incorporation of alternative lipid sources (i.e. vegetable oils and animal fats) has successfully reduced the level of fish oil incorporated into aquafeed formulations whilst concurrently maintaining optimal growth performance (Turchini et al., 2009). However, this strategy also has issues of concern. As with fish oil, vegetable oils and animal fats now experience high demand from competing industries (including direct utilisation in human nutrition), and therefore, achieve higher market prices. In addition, it is well demonstrated that when these alternative sources are incorporated into aquafeeds, the fillet fatty acid concentration reflects that of the dietary lipid source (Caballero et al., 2002, Senadheera et al., 2011, Thanuthong et al., 2011a, Torstensen et al., 2005, Turchini et al., 2013). Alternative lipid sources generally contain very little n-3 LC-PUFA, and in effect, high incorporation levels negatively impact fillet content of n-3 LC-PUFA, consequently reducing fillet quality and potential health benefits for consumers (Hallund et al., 2010, Midtbø et al., 2013, Midtbø et al., 2015, Pickova and Mørkøre, 2007, Rosenlund et al., 2011, Seierstad et al., 2005).

Atlantic salmon is a widely cultured species, and as a sector, likely the largest consumer of fish oil. In recent years, salmonid diets have seen major reductions in fish oil inclusion levels, with aquafeed producers relying on alternative lipid sources for the production of high energy diets (Tacon and Metian, 2008). However, the sector is now in clear need of new strategies to combat the aforementioned challenges for the optimisation of n-3 LC-PUFA utilisation, the maintenance of economic viability and a guaranteed provision of sufficient quantities of n-3 LC-PUFA in farmed products to meet consumer expectations.

It is known that salmonids possess a high capacity to digest and utilise high concentrations of dietary protein with no impact on apparent protein or lipid digestibility, with excess protein being catabolised for energy production (Einen and Roem, 1997). Accordingly, it is conceivable that the “protein sparing effect” could be re-conceptualised and retro-engineered to use protein to spare n-3 LC-PUFA. Considering the increasing costs associated with dietary lipid sources and subsequent concerns surrounding the future practicalities of high alternative lipid inclusion levels, the utilisation of animal by-product meals (i.e. meat and bone meal and blood meal) that are relatively cheap and in low demand from competing industries may represent a more cost effective solution in comparison to the provision of oil for utilisable energy in growing fish. Accordingly, a case study was implemented using Atlantic salmon (Salmo salar) to test the hypothesis that a higher dietary protein to lipid ratio, in iso-energetic diets with a fixed n-3 LC-PUFA content, could improve the utilisation efficiency (retention into fish tissues) of n-3 LC-PUFA. Specifically, these experimental diets were achieved by keeping a constant inclusion of marine derived raw materials (fish meal and fish oil), and varying the relative inclusion of animal by-products (meat and bone meal, blood meal and tallow). Growth performance and feed utilisation parameters as well as tissue fatty acid composition and apparent fatty acid metabolism were assessed.

Section snippets

Ethics statement

All animals and procedures used in this experimentation were approved by the Deakin University Animal Welfare Committee (Number B18-2012). All possible steps towards minimizing animal suffering were taken.

Animals, experimental design and sampling

Juvenile Atlantic salmon (Salmo salar) sourced from a commercial producer (Mountain Fresh Trout and Salmon Farm, Harrietville, VIC, Australia) were transported to the Deakin Aquaculture Futures facility (Deakin University, Warrnambool campus, VIC, Australia). Fish were acclimated to the

Diets

All objectives pertaining to the compositional characteristics of the experimental diets were achieved. The protein to lipid ratio of the three experimental diets (D38/22, D43/20 and D48/17) increased from 1.7 to 2.1 to 2.8, with increasing protein contents of 382.1 to 431.2 to 479.5 mg g diet 1, and decreasing lipid contents of 228.8 to 204.7 to 172.2 mg g diet 1, respectively, whilst maintaining a constant energy content (~ 22.0 kJ g diet 1) (Table 1). The SFA concentration, comprised mostly of 16:0

Discussion

Over the duration of the 9 week grow-out period feed acceptance was high. Fish under all dietary treatments exhibited good growth performance, gaining ~ 300%, and growing from ~ 185 g to ~ 730 g, with no differences between treatments in biometric parameters and overall feed efficiency. However, treatments differed significantly with respect to dietary protein and lipid utilisation parameters. PER and %NPU decreased with increasing P/L ratio, demonstrating a trend towards reducing protein deposition

Acknowledgements

This research was supported under the Australian Research Council’s Discovery Project funding scheme (DP1093570). The views expressed herein are those of the authors and are not necessarily those of the Australian Research Council. The authors also express their gratitude to Dr Richard Smullen (Ridley Agriproducts) and the Midfield Group for kindly donating the raw materials used for experimental feed preparation.

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