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In consideration of economical and environmental concerns, fish oil (FO) substitution in aquaculture is the focus of many fish nutritionists. The most stringent drawback of FO replacement in aquafeeds is the consequential modification to the final fatty acid (FA) make-up of the fish fillet. However, it is envisaged that a solution may be achieved through a better understanding of fish FA metabolism. Therefore, the present study investigated the fate of individual dietary FA in rainbow trout (Oncorhynchus mykiss) fed a FO-based diet (rich in 20 : 5n-3) or a linseed oil-based diet (LO; rich in 18 : 3n-3). The study demonstrated that much of the 18 : 3n-3 content from the LO diet was oxidised and, despite the significantly increased accretion of Delta-6 and Delta-5 desaturated FA, a 2- and 3-fold reduction in the fish body content of 20 : 5n-3 and 22 : 6n-3, respectively, compared with the FO-fed fish, was recorded. The accretion of longer-chain FA was unaffected by the dietary treatments, while there was a greater net disappearance of FA provided in dietary surplus. SFA and MUFA recorded a net accretion of FA produced ex novo. In the fish fed the FO diet, the majority of dietary 20 : 5n-3 was accumulated (53.8 %), some was oxidised (14.7 %) and a large proportion (31.6 %) was elongated and desaturated up to 22 : 6n-3. In the fish fed the LO diet, the majority of dietary 18 : 3n-3 was accumulated (58.1 %), a large proportion was oxidised (29.5 %) and a limited amount (12.4 %) was bio-converted to longer and more unsaturated homologues.

A more efficient utilisation of marine-derived sources of dietary n -3 long-chain PUFA (n -3 LC PUFA) in cultured Atlantic salmon (Salmo salar L.) could be achieved by nutritional strategies that maximise endogenous n -3 LC PUFA synthesis. The objective of the present study was to quantify the extent of n -3 LC PUFA biosynthesis and the resultant effect on fillet nutritional quality in large fish. Four diets were manufactured, providing altered levels of dietary n -3 substrate, namely, 18 : 3 n -3, and end products, namely, 20 : 5 n -3 and 22 : 6 n -3. After 283 d of feeding, fish grew in excess of 3000 g and no differences in growth performance or biometrical parameters were recorded. An analysis of fatty acid composition and in vivo metabolism revealed that endogenous production of n -3 LC PUFA in fish fed a diet containing no added fish oil resulted in fillet levels of n -3 LC PUFA comparable with fish fed a diet with added fish oil. However, this result was not consistent among all treatments. Another major finding of this study was the presence of abundant dietary n -3 substrate, with the addition of dietary n -3 end product (i.e. fish oil) served to increase final fillet levels of n -3 LC PUFA. Specifically, preferential β -oxidation of dietary C18 n -3 PUFA resulted in conservation of n -3 LC PUFA from catabolism. Ultimately, this study highlights the potential for endogenous synthesis of n -3 LC PUFA to, partially, support a substantial reduction in the amount of dietary fish oil in diets for Atlantic salmon reared in seawater. [ABSTRACT FROM AUTHOR]

Abstract There is a growing trend of 'replacing' long-chain omega-3 polyunsaturated fatty acid (n-3 LC PUFA) rich oils with C 18 shorter-chain omega-3 polyunsaturated fatty acid rich oils in Atlantic salmon aquafeed formulations. n-3 LC PUFA, including 20:5n-3 and 22:6n-3, play contrasting physiological roles and are metabolised differently in comparison to C 18 PUFA. Accordingly, the present study recorded the effect of replacing n-3 LC PUFA rich dietary fish oil with C 18 n-3 PUFA rich camelina oil at two inclusion levels in commercial-like diets fed to market-sized Atlantic salmon. This assessment was achieved by an analysis of industry relevant production parameters including growth performance, fatty acid composition and metabolism, nutrient digestibility and consumer acceptance (liking and attribute analysis of fillet). The trial was conducted over the final 150 days of an on-farm grow-out period in seawater. The dietary replacement of n-3 LC PUFA with C 18 n-3 PUFA resulted in a significant decrease in fillet n-3 LC PUFA and a poorer growth performance. However, in the absence of fish oil, the inclusion of camelina oil at high levels (40%) contributed to an improved n-6/n-3 ratio and partially ameliorated low dietary n-3 LC PUFA by providing added substrate for endogenous n-3 LC PUFA synthesis in comparison to a 20% camelina oil inclusion. Furthermore, consumer acceptance of Atlantic salmon was unaffected by the dietary addition of camelina oil. Highlights • The dietary replacement of n-3 LC PUFA with C18 n-3 PUFA resulted in a decrease in fillet n-3 LC PUFA and poorer growth. • Camelina oil at a diet lipid level of 40% provided an improved n-6/n-3 ratio in the fillet compared to a 20% inclusion level. • High dietary provision of camelina oil provided added substrate for endogenous n-3 LC PUFA synthesis in Atlantic salmon. • Consumer acceptance including overall taste of Atlantic salmon fillet was unaffected by the dietary addition of camelina oil. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: This study aimed to gain a better understanding of the metabolic fate of dietary fatty acids in rainbow trout, with a specific focus on the effect of varying total C18 PUFA level. Fish were fed a control fish oil based diet or one of five experimental fish oil deprived diets formulated with a constant 1/1 ratio of 18:3n-3/18:2n-6 and varying total C18 PUFA levels for a period of 7weeks. The transcriptional changes of the Δ-6 desaturase and elongase enzymes in direct comparison to in vivo fatty acid bioconversion, estimated using the whole-body fatty acid balance method, were analysed. The main findings were that i) the efficiency of Δ-6 desaturase was negatively affected by C18 PUFA availability, but the total apparent in vivo enzyme activity was directly proportional to C18 PUFA substrate availability; ii) Δ-6 desaturase had a greater affinity towards n-3PUFA than n-6PUFA; iii) excessive C18 PUFA substrate availability could limit the availability of Δ-6 desaturase to act on C24 fatty acid; iv) the elimination of dietary n-3LC-PUFA (enzyme products) up-regulated the transcription rate of Δ-6 desaturase; but v) the total apparent in vivo enzyme activity was directly and positively affected by substrate availability, and not product presence/absence nor the extent of the enzyme transcription rate. [Copyright &y& Elsevier]

Abstract: The aim of this study was to evaluate the effects of varying total amounts of dietary C18 PUFA on growth performance and tissue fatty acid composition in rainbow trout (Oncorhynchus mykiss). The experimental feeding trial was implemented over an entire grow-out period with a subsequent finishing period up to commercial size. Overall performance was unaffected by the dietary treatments, whilst fillet fatty acid make-up was mirroring that of the diet. The different dietary C18 PUFA levels had different effects on the fillet fatty acid productive value (i.e. fatty acid retention rates) during the grow-out and finishing periods. Specifically, during the grow-out period, diets rich in C18 PUFA resulted in increased n−3 LC-PUFA content in fish fillets. However, from an n−3 LC-PUFA restorative viewpoint, during the subsequent finishing period, the high C18 PUFA content of fillets of fish previously fed with high C18 PUFA diets was detrimental to the success of the finishing strategy. Therefore, depending on the strategy to be implemented (maximal fatty acid bioconversion or a fish oil restorative period), dietary C18 PUFA inclusion can be viewed from both a positive or a negative perspective. [Copyright &y& Elsevier]

Abstract: Recommendations to endorse the sustainability of wild fish stock utilisation, supporting the health of marine ecosystems, are clashing with those to increase omega-3 fatty acids (n−3 LC-PUFA) consumption and promoting human health. The objective of this study was to evaluate the role of salmonid aquaculture as a user or supplier of n−3 LC-PUFA, as a means of understanding the potential of the sector in conserving or depleting wild fisheries. A case-study feeding trial was implemented on rainbow trout up to commercial size, in which fish were fed a fish oil- or a linseed oil-diet. Harvested fish were analysed for fatty acid composition and difference and liking using consumers. The n−3 LC-PUFA input/n−3 LC-PUFA output ratio was computed. Consumers showed no preference, but were able to distinguish between samples. The fatty acids of the fillets were significantly modified by the diets. On the input side, for the production of 100g of fish fillet, it was necessary to use 8.6g of n−3 LC-PUFA to produce an output of 1.9g of n−3 LC-PUFA in the fish oil-fed fish; in contrast it was only necessary to use 270mg of n−3 LC-PUFA to produce 560mg of these fatty acids in the linseed oil-fed fish. It was showed that the substitution of fish oil with linseed oil in aquafeed is an easily implemented tool to transform salmonids farming from a consumer into a net producer of health promoting n−3 LC-PUFA and accomplish its role in conserving wild fisheries in the future. [ABSTRACT FROM AUTHOR]

Abstract: With the salmonid industry currently exploiting the vast majority of globally available fish oil, there is the need to optimise fish oil utilisation by increasing its efficiency in terms of transferring the health-promoting long chain omega-3 fatty acids (n−3 LC-PUFA) into farmed fish flesh. The aim of this study was to evaluate if dietary fatty acid deposition is affected by the time of feeding, and hence identify possible innovative feeding strategies towards more efficient use of dietary fish oil. Over a period of 12weeks, three diets with different lipid sources, canola oil (CO), fish oil (FO) or a 50/50 blend of the two oils (Mix), were alternated daily and fed to rainbow trout (Oncorhynchus mykiss). Six treatments were administered to fish, reference treatment (REF, continuously fed FO), control treatment (CT, continuously fed Mix), am canola oil ration (amCOR), pm canola oil ration (pmCOR), am canola oil satiation (amCOS) and pm canola oil satiation (pmCOS). Fish received either the CO diet in the am or pm feeds and received the FO diet at the opposite time. A significant increase in growth and feed consumption was noted in the pmCOS treatment. Fillet fatty acid profile was modified by associated feeding schedules and was generally reflective of dietary fatty acid profile. No significant increases in n−3 LC-PUFA deposition were observed. However, both linoleic acid (18:2n−6) and α-linolenic acid (18:3n−3) contents were significantly higher in pmCOR compared to amCOR and CT. The results of the present study suggest the existence of cyclical circadian patterns in fatty acid deposition in rainbow trout. [Copyright &y& Elsevier]

Fish oil (FO)- and canola oil (CO)-based diets were regularly alternated in a daily cycle (amCO: alternation of CO in the morning and FO in the afternoon, and pmCO: alternation of FO in the morning and CO in the afternoon) or in a series of weekly cycles (2W: alternation of 2 weeks on CO and 2 weeks on FO, 4W: alternation of 4 weeks on CO and 4 weeks on FO), over a 16-week period in juvenile Murray cod ( Maccullochella peelii peelii). No significant differences were observed between any of the treatments in relation to the final weight. However, fish subjected to the 2W schedule were larger ( P>0.05) than all other treatments (37.2 ± 0.30 vs. 34.3 ± 0.58 in the control treatment). Fish receiving the 2W treatment had a significantly lower total net disappearance of eicosapentaenoic acid 20:5n-3 (EPA) and docosahexaenoic acid 22:6n-3 (62.1% and 24.0% respectively) compared with the control treatment (fish continuously fed a blend of 50% FO and 50% CO). Likewise, Murray cod receiving the amCO daily schedule had a significantly lower total net disappearance of EPA in comparison with the CD and pmCO treatments. These data point towards the existence of cyclical mechanisms relative to fatty acid utilization/retention. [ABSTRACT FROM AUTHOR]

Abstract: The Murray cod, an Australian native freshwater fish, supports a relatively small but increasing aquaculture industry in Australia. Presently, there are no dedicated commercial diets available for Murray cod; instead, nutritionally sub-standard feeds formulated for other species are commonly used. The aim of the present investigation was to assess the suitability of two plant based lipid sources, canola oil (CO) and linseed oil (LO), as alternatives to fish oil for juvenile Murray cod. Five iso-nitrogenous, iso-calorific, iso-lipidic semi-purified experimental diets were formulated with 17% lipid originating from 100% cod liver oil (FO), 100% canola oil, 100% linseed oil and 1:1 blends of canola and cod liver oil (CFO) and 1:1 blends of linseed and cod liver oil (LFO). Each of the diets was fed to apparent satiation twice daily to triplicate groups of 50 Murray cod with initial mean weights of 6.45±1.59 g for 84 days at 22 °C. Final mean weight, specific growth rate and daily feed consumption were significantly higher for the FO and LFO treatments compared to the LO treatment. Feed conversion and protein efficiency ratios were not significantly different amongst treatments. Experimental diets containing vegetable oil and vegetable oil blend(s) had significantly higher concentrations of n-6 fatty acids, predominantly in the form of linoleic acid (LA), while n-3 fatty acids were present in significantly higher concentrations in LO and LFO treatments. The fatty acid composition of Murray cod fillet was reflective of the dietary lipid source. Fillet of fish fed the FO was highest in EPA (20:5n-3), ArA (20:4n-6) and DHA (22:6n-3). Fish fed the CO diet had high concentrations of oleic acid (OlA) (192.2±10.5 mg g lipid−1), while the fillet of Murray cod fed the LO diet was high in α-linolenic acid (LnA) (107.1±6.7 mg g lipid−1). The present study suggests that fish oil can be replaced by up to 100% with canola oil and by up to 50% with linseed oil in Murray cod diets with no significant effect on growth. [Copyright &y& Elsevier]

Fish are the main source of long-chain polyunsaturated fatty acids (LC-PUFA, >C 18) for human consumption. In general, it has been widely observed that the fatty acid (FA) profiles of farmed fish are reflective of the diet. However, the degree of tissue FA "distortion" based on incorporation of different dietary FA into fish tissues varies greatly depending on FA type, fish species and environmental factors. In terms of fish FA composition, this variation has not been comprehensively reviewed, raising the question: "Are fish what they eat?". To date, this remains unanswered in detail. To this end, the present review quantitatively summarized the 'diet-fish' FA relationship via an analysis of FA composition in diets and fish tissues from 290 articles published between 1998 and 2018. Comparison of this relationship among different fish species, tissue types or individual FA was summarized. Furthermore, the influence of environmental factors such as temperature and salinity, as well as of experimental conditions such as fish size and trophic level, feeding duration, and dietary lipid level on this relationship are discussed herein. Moreover, as a means of restoring LC-PUFA in fish, an emphasis was paid to the fish oil finishing strategy after long-term feeding with alternative lipid sources. It is envisaged that the present review will be beneficial in providing a more comprehensive understanding of the fundamental relationship between the FA composition in diets, and subsequently, in the farmed fish. Such information is integral to maintaining the quality of farmed fish fillets from the perspective of FA composition. • The 'diet-fish' FA relationship was systematically reviewed. • The 'diet-fish' FA relationship was analyzed by quantitative approaches. • Factors influencing the 'diet-fish' FA relationship were analyzed and discussed. • An emphasis was paid to the fish oil finishing strategy in fish farming. • Dietary FA incorporation in fish tissues is affected by many non-dietary factors. [ABSTRACT FROM AUTHOR]