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Seasonal changes in water temperature affect the utilization of dietary fatty acids in Atlantic salmon (Salmo salar L.). Furthermore, fatty acid profiles of terrestrial oils dictate their suitability in terms of provision of metabolic energy and final product quality. An on‐farm, growth trial of Atlantic salmon was conducted in Tasmania, Australia over the final year of grow‐out (323 days), consisting of a 'summer phase' and a 'winter phase'. Poultry by‐product oil, canola oil and tallow were fed at high dietary lipid inclusion level (80%) to assess growth, fillet fatty acid composition and sensorial attributes. In the summer phase, the tallow diet appeared to provide added substrate for metabolic energy, potentially enhancing the deposition of n‐3 LC PUFA into the fillet, despite lower final weight and a reduced apparent lipid digestibility. Subsequent winter phase results suggested all diets adequately provided metabolic energy and fillet n‐3 LC PUFA concentrations were comparable. Additionally, this study highlights the importance of a well‐considered experimental design and subsequent statistical interpretation, for commercial scale, on‐farm feeding trials. Ultimately, this study demonstrates the importance of seasonally tailored diets for Atlantic salmon, using high terrestrial oil inclusion, under challenging Australian farming conditions. [ABSTRACT FROM AUTHOR]

Nutritional strategies are currently developed to produce farmed fish rich in n-3 long-chain PUFA (LC-PUFA) whilst replacing fish oil by plant-derived oils in aquafeeds. The optimisation of such strategies requires a thorough understanding of fish lipid metabolism and its nutritional modulation. The present study evaluated the fatty acid bioconversion capacity of rainbow trout (Oncorhynchus mykiss) fry previously depleted in n-3 PUFA through a 60-d pre-experimental feeding period with a sunflower oil-based diet (SO) followed by a 36-d experimental period during which fish were fed either a linseed oil-based diet (LO) (this treatment being called SO/LO) or a fish oil-based diet (FO) (this treatment being called SO/FO). These treatments were compared with fish continuously fed on SO, LO or FO for 96 d. At the end of the 36-d experimental period, SO/LO and SO/FO fish recovered >80 % of the n-3 LC-PUFA reported for LO and FO fish, respectively. Fish fed on LO showed high apparent in vivo elongation and desaturation activities along the n-3 biosynthesis pathway. However, at the end of the experimental period, no impact of the fish n-3 PUFA depletion was observed on apparent in vivo elongation and desaturation activities of SO/LO fish as compared with LO fish. In contrast, the fish n-3 PUFA depletion negatively modulated the n-6 PUFA bioconversion capacity of fish in terms of reduced apparent in vivo elongation and desaturation activities. The effects were similar after 10 or 36 d of the experimental period, indicating the absence of short-term effects. [ABSTRACT FROM PUBLISHER]

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 replacement of fish oil with vegetable based oils is a strategy which is increasingly being adopted by the aquafeed industry as an essential component to reduce the reliance on a limited supply of a natural resource, which in turn also provides cost benefits. The aim of the present investigation was to evaluate the lipid and fatty acid digestibility of a mix blend vegetable oil in juvenile Murray cod; an emerging species of increasing interest in the Australian aquaculture sector. Five semi purified experimental diets were formulated with 17% lipid originating from fish oil (FO) which was substituted in 25% increments by blended vegetable oil (VO), formulated using olive oil (12%), palm oil (43%) and linseed oil (45%) to match the major fatty acid classes of the FO. Significant differences between the dietary treatments with regard to the lipid and individual fatty acid digestibility were recorded. This study clearly demonstrated that in Murray cod fatty acid digestibility decreases with chain length, and inversely increases relative to the degree of unsaturation. The combination of chain length, degree of unsaturation and the melting point of individual fatty acids resulted in the digestibility of PUFA>MUFA>SFA and short chain>longer chain fatty acids. Therefore, the inclusion of a mixed blend vegetable oil resulted in significantly reduced lipid digestibility in juvenile Murray cod. [Copyright &y& Elsevier]

The dynamics of fatty acid composition modifications were examined in tissues of Murray cod fed diets containing fish oil (FO), canola oil (CO) and linseed oil (LO) for a 25-week period and subsequently transferred to a FO (finishing/wash-out) diet for a further 16 weeks. At the commencement of the wash-out period, following 25 weeks of vegetable oil substitution diets, the fatty acid compositions of Murray cod fillets were reflective of the respective diets. After transfer to the FO diet, differences decreased in quantity and in numerousness, resulting in a revert to the FO fatty acid composition. Changes in percentages of the fatty acids and total accumulation in the fillet could be described by exponential equations and demonstrated that major modifications occurred in the first days of the finishing period. A dilution model was tested to predict fatty acid composition. In spite of a general reliability of the model ( Y=0.9234 X+0.4260, R2=0.957, P<0.001, where X is the predicted percentage of fatty acid; Y the observed percentage of fatty acid), in some instances the regression comparing observed and predicted values was markedly different from the line of equity, indicating that the rate of change was higher than predicted (i.e. Y=0.4205 X+1.191, R2=0.974, P<0.001, where X is the predicted percentage of α-linolenic acid; Y the observed percentage of α-linolenic acid). Ultimately, using the coefficient of distance ( D), it was shown that the fatty acid composition of fish previously fed the vegetable oil diets returned to the average variability of the fillet fatty acid composition of Murray cod after 70 or 97 days (LO and CO respectively). [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]

A considerable body of literature exists concerning the use of marine-derived raw materials in aquaculture feed formulations and the resulting modifications of the health promoting qualities of farmed fish. However, these investigations and reports are fundamentally limited to fatty acids and little is known regarding the other nutritional characteristics of cultured fish and seafood products. This knowledge gap, often coupled with inaccurate or incomplete media reports, generates consumer confusion and mistrust in relation to farmed fish quality. The aim of the present study was therefore to characterise not only the fatty acid composition, but also the resulting protein, amino acid and micronutrient composition of rainbow trout fed diets containing fish oil or vegetable oils. To achieve this, triplicate groups of rainbow trout ( Oncorhynchus mykiss ) were subjected to two distinct vegetable oil- based diets (one rich in alpha-linolenic acid, 18:3n-3, ALA and the other rich in linoleic acid, 18:2n-6, LNA) and a traditional n-3 long chain polyunsaturated fatty acid (LC-PUFA) rich, fish oil-based diet over a 126 day period. Growth performance and feed utilisation parameters as well as tissue fatty acid composition, amino acid composition, vitamin, mineral and trace element concentration in fish fillets were subsequently assessed. Expectedly, major differences were evident between treatments in relation to fatty acid composition, where for the most part fillet composition reflected that of the diet. Notably, no differences were observed regarding fillet amino acid composition, as was also the case for the vast majority of vitamins, minerals and trace elements. Therefore, with the exception of the fatty acid profile and vitamin A concentration, other beneficial nutritional qualities of fish are fundamentally maintained and unaffected when alternative oil sources are implemented in aquaculture feed formulations, maintaining cultured rainbow trout a nutritious and nourishing product for human consumption. [ABSTRACT FROM AUTHOR]

Abstract: The complete and successful replacement of fish oil in aquafeed without compromising fillet omega-3 long chain polyunsaturated fatty acid (n−3 LC-PUFA) concentrations has yet to be achieved. As means of overcoming this issue, the potential of in vivo n−3 LC-PUFA biosynthesis in cultured fish has been widely investigated and may permit the future economical and sustainable development of aquaculture. Two of the key enzymes involved in this process include the fatty acid Δ-6 and Δ-5 desaturases, where, in the terminal protein of each complex, iron is present in the active di-iron centres. To the best of authors'' knowledge, no investigations have thus far been attempted to elucidate the potential roles of dietary iron on the fatty acid metabolism of cultured fish. In this study, graded inclusions of dietary iron were tested in rainbow trout fed vegetable oil based diets. This study generated preliminary evidence suggesting that dietary iron concentration can have a positive effect on LC-PUFA biosynthesis in fish and data are herein reported. In particular, dietary iron level was shown to affect the final n−3 LC-PUFA content of fish tissues and also to modulate the activity of fatty acid desaturase enzymes. This activity was insufficient to compensate for a diet lacking in LC-PUFA but does highlight potential strategies to maximise this activity in cultured fish fed fish oil free diets. [Copyright &y& Elsevier]