The rise of fish oil: From feed to human nutritional supplement

ABSTRACT

This article tells the story of how fish oil has gone from an inferior to a premium oil product. With the growth of aquaculture, fish oil became a valuable feed component. Its value increased further as the belief in the health benefit of Omega-3 opened the market for direct human consumption through nutritional supplements. Strong demand, limited supply growth and weaker substitution between Omega-3 and Omega-6 has led to diverging trends between fish oil and vegetable oil prices, with fish oil now a premium oil product. We also investigated how the salmon aquaculture industry has responded to the challenge of rising fish oil prices. Concerted effort in nutrition and feed research is shown to have allowed a reduction in the share of fish oil in feed from around 25% in the early 2000s to around 12% today. This substitution effort has allowed salmon production to grow without using substantially more fish oil.     

Fish oil substitution by soybean oil in Sharpsnout seabream Diplodus puntazzo: Performance,fatty acid profile,and liver histology

The present study was conducted to determine the effect of soybean oil on the performance and liver histology in Sharpsnout seabream (Diplodus puntazzo) (initial body mass 35 g). Four experimental diets were formulated containing 0%, 25%, 50%, and 75% of soybean oil substituting fish oil. Fish were fed for 84 days. Increasing the level of soybean oil had no significant effects on growth and feed efficiency parameters. Biometrics, body composition, protein efficiency, and energy efficiency were not affected by the fish oil replacement. Muscle and liver fatty acids reflected fish oil substitution. Moreover, histology did not show statistical differences among treatments. Sharpsnout seabream juveniles can be fed with diets in which fish oil has been replaced with soybean oil at up to 75% of total lipid in the diet for a period of 84 days without affecting growth, feed efficiency, biometric parameters, and body composition.     

豆油代替鱼油对哲罗鱼稚鱼生长和体成分的影响

研究不同比例豆油取代粗制鱼油对哲罗鱼（Hucho taimen）稚鱼生长和体成分的影响。实验分4个组，每组设3个重复，每个重复100尾稚鱼（体质量7～8g）。第1组（G1）为对照组，添加15％鱼油，第2组（G2）、第3组（G3）和第4组（G4）分别用5％、10％和15％豆油取代鱼油。实验进行8周。结果表明，不同处理成活率和平均日增重差异不显著（P〉0．05），但随着豆油比例的增加，特定生长率和肥满度都得到提高，G3和G4组特定生长率显著高于G1组（P〉0．05）；G4组肥满度显著高于G1组（P〉0．05）。G3、G4组饵料系数显著降低（P〈0．05）。不同处理体成分中水分、粗蛋白质和粗脂肪含量均差异不显著（P〉0．05）。实验表明，豆油作为脂肪源可以完全代替鱼油用于哲罗鱼稚鱼饵料生产。     

Restoration of the fatty acid composition of red seabream (Pagrus auratus) using a fish oil finishing diet after grow-out on plant oil based diets

This study evaluated the potential for manipulating the fatty acid composition of juvenile red seabream, Pagrus auratus. Prior to the start of the study, three groups of fish had been reared for 3 months on a fish oil based diet or diets where the added fish oil had been replaced with either canola or soybean oil. In the present study, fish that had previously been fed either the canola or soybean oil diets were fed a fish oil based diet. Three additional treatments included fish being maintained on their original diets of fish oil, canola oil or soybean oil. Fish were fed their respective diets twice daily to apparent satiety for 32 days. Samples of fish from each treatment were collected after 0, 1, 2, 4, 8, 16 and 32 days. Composition and growth of the fish were determined at each sample point. Most treatments showed no differences in growth performance, although fish fed a fish oil diet after previously being fed a soybean oil diet showed slightly better growth. No significant differences among treatments were observed in proximate composition of the fish, although there was a significant increase in total fat and individual fatty acid (g kg^?1 live‐weight) content of the fish from all treatments over the period of the study. No significant changes in the relative fatty acid composition (% of total fatty acids) over time were observed in the three treatments where fish were maintained on their original diets. In contrast, fish that were previously fed either the canola or soybean oil diets and were then fed a fish oil diet had significant changes in both the relative (% of total fatty acids) and absolute (g kg^?1 live‐weight) fatty acid content. Key changes observed included a decrease in the relative levels of polyunsaturated fatty acids (PUFA) such as 18 : 2n ? 6 and 18 : 3n ? 3. Increases in the relative levels of the long‐chain polyunsaturated fatty acids (lcPUFA) 20 : 5n ? 3 and 22 : 6n ? 3 were also observed in both treatments. The rates of absolute (g kg^?1 live‐weight) change/accumulation of these fatty acids followed an exponential equation that differed for each fatty acid in each treatment. Examination of the retention efficiency of specific fatty acids also showed marked differences between fatty acids within treatments and also differences between treatments. Biologically important fatty acids such as 20 : 5n ? 3 and 22 : 6n ? 3 had only moderate retention efficiencies and these were unaffected by treatment. In contrast, the retention efficiencies of 18 : 2n ? 6 and 18 : 3n ? 3 suggested selective retention of these fatty acids when fed fish oil diets, but moderate catabolism when fed the plant oil diets. There were also high retention efficiencies of most saturated and monounsaturated fatty acids suggestive of active retention and/or active synthesis of these fatty acids by the fish. The results of this study, particularly the increases in lcPUFA, support the usefulness of a fish oil based finisher diet for fish raised predominantly on plant oil based diets.     

Replacing palm oil for soybean oil in tilapia, Oreochromis niloticus (L.), feed

Four isonitrogenous and isocaloric semi-purified feeds containing various levels of palm oil (0,1, 2 and 3%) as a replacement for soybean oil were fed to three replicate groups of tilapia, Oreochromis niloticus (L.), fingerlings, mean initial weight 5.5 g, in 45-l aquaria for 5 weeks under controlled laboratory conditions. Weight gain, feed conversion ratio, protein efficiency ratio and the chemical composition of the fish bodies did not vary significantly (P > 0.05). These results indicate that palm oil could replace soybean oil in feeds for O. niloticus fingerlings without any negative effect on the fish growth or body composition under these conditions.     

Evaluation of canola oils as alternative lipid resources in diets for juvenile red seabream, Pagrus auratus

This study examined three potential oil resources, crude and refined canola oil and refined soybean oil as replacements for added dietary fish oil in diets for juvenile red seabream. These oil resources were evaluated for their potential to replace added fish oil (40 g kg^?1) in fishmeal based (600 g kg^?1) diets, with 100 g kg^?1 of total lipids. Each of the three plant oils was used to replace 25%, 50%, 75% or 100% of the added dietary fish oil. Each of the three plant oils showed potential as a replacement for dietary fish oil, although a significant reduction in growth and feed utilisation was observed with the complete (100%) replacement of added fish oil by crude canola oil. No other significant effects of oil type or inclusion level on growth were apparent. A negative control (no added fish oil or plant oil, 60 g kg^?1 of total lipid) yielded poorer growth than all treatments except the diet containing 40 g kg^?1 of added crude canola oil (100% replacement). This observation confirmed that the added oils were utilized by the fish. A positive control diet containing 80 g kg^?1 of added fish oil (140 g kg^?1 total dietary lipid) sustained the best growth in the study, confirming that the 13 experimental diets were energy limiting as planned. Notably, few effects of the alternative oils were seen on the proximate composition of the fish. However, the influence of the alternative oils on the tissue fatty acid composition was considerable, irrespective of plant oil type or processing grade. Particularly notable was the overall increase in the level of polyunsaturated fatty acids in the tissues of the fish fed the plant oil diets, with these trends becoming more apparent with the greater levels of fish oil replacement. Minimal reductions in the levels of the long‐chain polyunsaturated fatty acids of eicosapentaenoic (20:5n‐3) and docosahexaenoic (22:6n‐3) acid were observed from any of the plant oil treatments. Sensory assessment, by an Australian taste panel, of the fish fed the fish oil reference, or the 100% replacement by refined canola or refined soybean diets showed a preference in order of canola oil > soybean oil > fish oil fed fish. Clearly, both canola and soybean oils have considerable potential as replacements of fish oils in diets for this species.     

Replacement of fish oil in plant‐based diets for Pacific white shrimp,Litopenaeus vannamei,by stearine fish oil and palm oil

Stearine fish oil (SFO) and palm oil (PO) have emerged as promising alternatives for the replacement of fish oil (FO) in aquafeeds. This study evaluated the replacement of FO with alternative oils in practical diets for Litopenaeus vannamei. In a clear brackish water study (14.1 g/L) utilizing shrimp (0.29 ± 0.02 g, initial weight), FO was replaced by SFO at inclusion ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 (FO:SFO) and PO as 90% of FO. After 55 days, no significant differences (p < 0.05) in final weight, growth, or survival of shrimp were observed. A second trial (8 weeks) in low‐salinity water (2.1 g/L) with shrimp (0.92 ± 0.02 g, initial weight) evaluated diets with 100% FO, 100% SFO, 90% PO, 90% soybean oil (SO), or 90% flaxseed oil (FXO) as a replacement for FO and four commercially produced diets with 2% of FO, SO, PO, or FXO. One treatment received half rations of the commercial FO diet, and one treatment was based entirely on natural productivity. Results show that the fatty acid profiles of the tail muscle conformed to the lipids of the feed, and highly unsaturated fatty acids (HUFAs) were preserved. Following 8 weeks of culture, there were no significant differences in production performance.     

Growth, digestibility and nutrient utilization of rainbow trout (Oncorhynchus mykiss) and European sea bass (Dicentrarchus labrax) juveniles fed different dietary soybean oil levels

A shortage of marine raw ingredients, such as fish oil, is predicted in the near future. The use of suitable alternative lipid sources, such as vegetable oils, is crucial for sustainable growth of the aquaculture sector. Three isonitrogenous (50% Crude Protein) and isoenergetic (19 kJ g⁻¹) diets, in which fish oil was gradually replaced by soybean oil (0, 25 and 50% of the oil fraction) were tested for 12 weeks, using European sea bass (Dicentrarchus labrax) and rainbow trout (Oncorhynchus mykiss) juveniles. No dietary effects (p>0.05) on growth performance, feed conversion, digestibility or body composition were observed with either species. The results obtained suggest the inclusion of soybean oil up to 50% of the dietary lipid is possible, in diets for sea bass and/or rainbow trout juveniles.     

豆油替代鱼油对中华绒螯蟹幼蟹生长、非特异性免疫和抗病力的影响

以酪蛋白和明胶为蛋白源配制5组等氮等能的实验饲料, 分别为全鱼油(F1)、25%(F2)、50%(F3)和75%(F4)豆油替代组和全豆油组(F5), 每组4个重复, 投喂初始体质量为(0.27±0.03) g的中华绒螯蟹(Eriocheir sinensis)幼蟹6周, 评价豆油替代鱼油的饲料对幼蟹生长、非特异性免疫力和抗病力的影响。结果发现, 各组幼蟹的存活率差异不显著(P>0.05); F4组幼蟹的增重率和特定生长率显著高于F1组(P<0.05), 但与其他各组差异不显著(P>0.05)。F4组幼蟹的血清酸性磷酸酶和碱性磷酸酶活性均显著高于其他各组(P<0.05), F5组最低。幼蟹的血清中超氧化物歧化酶活性以F4组最高, 显著高于F3组(P<0.05), F3和F4组均显著高于其他各组(P<0.05), 其他F1、F2和F5三组间差异不显著(P>0.05)。F1组幼蟹的血清丙二醛含量最高(P<0.05), 其余各组之间则差异不显著(P>0.05); F4组幼蟹的血清酚氧化酶活性最高, 而F5组最低(P<0.05); 各组幼蟹的溶菌酶差异不显著(P>0.05)。实验结束后, 采用嗜水气单胞菌(Aeromonas hydrophila)攻毒10 d, 发现F5组幼蟹最早出现死亡, 且该组幼蟹的累积死亡率显著高于F1和F4组(P<0.05)。结果提示, 采用豆油完全替代饲料中的鱼油对幼蟹的生长影响不显著, 但会降低其抵抗感染疾病的能力; 用75%豆油替代鱼油可在明显提高幼蟹免疫机能和抗病力的情况下获得良好的生长性能。

     

Dietary fish oil replacement by soybean oil: Effect on plasma vitellogenin,sex steroids and ovarian steroidogenesis in Chinese strip‐necked turtles (Mauremys sinensis)

In order to investigate the effects of dietary fish oil replacement, the turtles (Mauremys sinensis) were fed four experimental diets for 10 months: FO (100% fish oil), FSO (70% fish oil and 30% soybean oil), SFO (30% fish oil and 70% soybean oil) and SO (100% soybean oil), sampled at pre‐vitellogenesis, vitellogenesis and post‐vitellogenesis. The results showed that plasma gonadotropin‐releasing hormone (GnRH) levels were the highest at pre‐vitellogenesis, which promoted the secretion of gonadotropin and sex steroids. Therefore, plasma luteinizing hormone (LH) and estrogen (E₂) levels were significantly increased at post‐vitellogenesis (p < 0.05), while follicle‐stimulating hormone (FSH) levels increased at vitellogenesis (p < 0.05). The FO and FSO groups had significantly higher GnRH and E₂ levels than the other two groups (p < 0.05). In addition, plasma vitellogenin (Vtg) levels significantly increased at vitellogenesis and post‐vitellogenesis (p < 0.05), which were significantly higher in the groups of FO and FSO than SO (p < 0.05). Moreover, the expression levels of hepatic estrogen receptor α (Erα) mRNA were significantly increased at vitellogenesis and post‐vitellogenesis while ovarian Cyp19α1α mRNA were significantly increased at post‐vitellogenesis (p < 0.05), and both were the lowest in SO. Taken together, the replacement of fish oil with 66.7% soybean oil is feasible.     

Effect of different oil cake sources on growth, nutrient retention and digestibility, muscle nucleic acid content, gut enzyme activities and whole-body composition in silver barb, Puntius gonionotus fingerlings

Six iso‐nitrogenous (30% crude protein) and iso‐energetic (15 kJ g⁻¹) diets were prepared using different oil cake sources, viz. groundnut, soybean, sunflower, sesame, mustard and mixed oil cakes as major ingredients, and protein sources along with a minimum of 5% fish meal in each diet and were fed to silver barb Puntius gonionotus fingerlings (16.20±0.11 g) ad libitum four times a day close to an apparent satiation level for a period of 60 days to determine the effect of diets on growth, nutrient utilization, apparent digestibility coefficient (ADC) of the nutrients in the diets, gut enzyme activity, muscle nucleic acid content and whole‐body chemical composition of fish. Significantly higher (P<0.05) weight gain, specific growth rate, protein efficiency ratio, nutrient retention, ADC of nutrients in the diets, DNA:RNA ratio, protease and amylase activity with lower (P<0.05) feed:gain values were recorded in fish‐fed groundnut and soybean oil cake‐based diets than other diets tested. Among the dietary treatment groups, significantly higher (P<0.05) whole‐body protein, lipid and energy were also found in groundnut oil cake‐ and soybean oil cake‐based diets. The study suggests that the groundnut and soybean oil cake‐based diets, which led to significantly higher (P<0.05) growth and nutrient utilization than the other oil cake‐based diets in P. gonionotus fingerlings, may be used for pond culture of this species.     

Comparison of effects of vegetable oils blended with southern hemisphere fish oil and decontaminated northern hemisphere fish oil on growth performance, composition and gene expression in Atlantic salmon (Salmo salar L.)

Replacement of fish oil with sustainable alternatives, such as vegetable oil, in aquaculture diets has to be achieved without compromising the nutritional quality, in terms of n-3 highly unsaturated fatty acid (HUFA) content, of the product. This may be possible if the level of replacement is not too high and oil blends are chosen carefully but, if high levels of fish oil are substituted, a fish oil finishing diet prior to harvest would be required to restore n-3HUFA. However, a decontaminated fish oil would be required to avoid increasing undesirable contaminants. Here we test the hypotheses that blending of rapeseed and soybean oils with southern hemisphere fish oil will have a low impact upon tissue n-3HUFA levels, and that decontamination of fish oil will have no major effect on the nutritional quality of fish oil as a feed ingredient for Atlantic salmon. Salmon (initial weight ~ 0.8 kg) were fed for 10 weeks with diets in which 60% of fish oil was replaced with blends of soybean, rapeseed and southern hemisphere fish oil (SVO) or 100% decontaminated northern fish oil (DFO) in comparison with a standard northern fish oil diet (FO). Decontamination of the oil was a two-step procedure that included treatment with activated carbon followed by thin film deodorisation. Growth performance and feed efficiency were unaffected by either the SVO or DFO diets despite these having lower gross nutrient and fatty acid digestibilities than the FO diet. There were also no effects on the gross composition of the fish. Liver and, to a lesser extent flesh, lipid levels were lower in fish fed the SVO blends, due to lower proportions of neutral lipids, specifically triacylglycerol. Tissue lipid levels were not affected in fish fed the DFO diet. Reflecting the diet, flesh eicosapentaenoic acid (EPA) and total n-3 fatty acids were higher, and 18:1n-9 lower, in fish fed DFO than FO, whereas there were no differences in liver fatty acid compositions. Flesh EPA levels were only slightly reduced from about 6% to 5% although docosahexaenoic acid (DHA) was reduced more severely from around 13% to about 7% in fish fed the SVO diets. In contrast, the liver fatty acid compositions showed higher levels of n-3 HUFA, with DHA only reduced from 21% to about 18% and EPA increased from under 8% to 9–10% in fish fed the SVO diets. The evidence suggested that increased liver EPA (and arachidonic acid) was not simply retention, but also conversion of dietary 18:3n-3 and 18:2n-6. Increased HUFA synthesis was supported by increased hepatic expression of fatty acyl desaturases in fish fed the SVO diets. Flesh n-3HUFA levels and desaturase expression was significantly higher in fish fed soybean oil than in fish fed rapeseed oil. In conclusion, partial replacement of fish oil with blends of vegetable oils and southern hemisphere fish oil had minimal impact on HUFA levels in liver, but a greater effect on flesh HUFA levels. Despite lower apparent digestibility, decontamination of fish oil did not significantly impact its nutritional quality for salmon.     

Palm oil-laden spent bleaching clay as a substitute for marine fish oil in the diets of Nile tilapia, Oreochromis niloticus

A 8‐week feeding trial was conducted to determine the effect of substituting fish oil with palm oil‐laden spent bleaching clay (SBC), a by‐product from crude palm oil (CPO) refining, on growth, feed utilization, fatty acid composition and heavy metal accumulation in the muscle of Nile tilapia, Oreochromis niloticus. Four isonitrogenous and isolipidic practical diets were formulated to contain 0, 100, 200 or 300 g kg^?1 SBC. Growth performance of Nile tilapia was significantly better in fish fed the 100 g kg^?1 SBC diet compared with fish fed the 0, 200 or 300 g kg^?1 SBC diet. Growth and feed utilization efficiency of fish fed 200 or 300 g kg^?1 SBC were similar to fish fed the control diet without added SBC. Whole‐body composition, body‐organ indices and haematocrit of tilapia were not affected by dietary treatments. Fatty acid compositions in the muscle lipid of Nile tilapia were strongly influenced by dietary treatments with progressively elevated levels of total saturates and n‐6 PUFA because of the dietary influence of these fatty acids from residual CPO adsorbed onto SBC. A gradual decrease in total n‐3 PUFA concentrations were also observed with the ratio of n‐3 to n‐6 fatty acids in muscle lipids decreasing from 4.75 to 4.41, 3.23 or 2.37 after 8 weeks on the 0, 100, 200 or 300 g kg^?1 SBC diet, respectively. The arsenic, cadmium and lead concentrations in the experimental diets increased with increasing dietary levels of SBC but the concentrations of these heavy metals in the whole body and bone of Nile tilapia were not significantly different among fish fed the various diets. The present 8‐week study showed that in fishmeal‐based diets for Nile tilapia, palm oil‐laden SBC can totally replace added fish oil. The use of this presently discarded waste product from palm oil refining in tilapia diets will greatly contribute to reducing the impact of rising feed costs in the culture of tilapia in many tropical countries. Other potential benefits may include acting as a feed binder, removal of mycotoxins in fish feeds as well as adsorbing toxic substances present in the culture water.     

Effects of dietary fish oil substitution with mixed vegetable oils on growth and fillet fatty acid composition of juvenile Caspian great sturgeon (Huso huso)

The objective of this study was to determine the effects of fish oil replacement with dietary vegetable oils on growth performance, chemical composition and fatty acids profiles in fillets of farmed Caspian great sturgeon juveniles Huso huso (26.97 ± 0.49 g). Five isonitrogenous and isolipidic diets were formulated, containing 10 % of added oil. The diet with 100 % kilka fish oil (Caspian tyulka, Clupeonella caspia) was the control. Fish oil was substituted by 50 % of vegetable oils consisting of an equal share of sunflower and soybean oils (diet A), sunflower and canola oils (diet B) and soybean and canola oils (diet C). In diet D, 100 % of fish oil was replaced with vegetable oil (1:1:1 ratio of sunflower oil, soybean oil and canola oil). Significant differences (P > 0.05) were not detected during 60 days feeding trial in final body weight, weight gain, condition factor, specific growth rate, feed conversion rate, protein efficiency ratio and the chemical composition of fillet (crude protein, crude lipid, moisture and ash). Generally, the fatty acids composition of fish fillets was reflective of the dietary lipid sources. These results indicate the feasibility of substituting fish oils with the mixture of vegetable oils in diets of juvenile H. huso without negative influence on growth providing optimum ratios of n-3/n-6 and n-3/18:1n-9 is met in the diet.     

Vegetable oil blend as alternative lipid resources in diets for gilthead seabream, Sparus aurata

The recent decreasing worldwide supplies of marine oils have forced the aquaculture industry to investigate alternative lipid sources for use in marine fish feeds. The aim of this study was to determine the impact of dietary replacement of fish oil by vegetable oils on gilthead seabream (Sparus aurata) growth performance, nutritive utilization, body composition, and fatty acid profile as well as feed cost. Two dietary vegetable oil (VO) mix blends (VO1 and VO2) in which: sunflower (SO), cottonseed (CO) and linseed (LO) for VO1 or soybean oil (SBO) for VO2, were tested as 60% fish oil (FO) substitutes versus the 100% FO control or reference diet (FO). Three iso-proteic (46% CP) and iso-lipidic (18%) experimental diets were hand fed, twice a day, 6 days a week to apparent visual satiety to triplicate groups of seabream growers (average initial weight, 130.9 ± 3.44 g), until fish reached market size (300–400 g/fish) after 20 weeks at mean ambient temperature 27.0 ± 1.8°C. All experimental diets were well accepted by seabream growers regardless of the different lipid sources used, as overall mean feed intake (FI) and daily intake (DFI) were not significantly different (P > 0.05) among dietary treatments. In terms of growth performance, fish fed VO1 diet (with LO) exhibited a relatively lower, but significant (P < 0.05), total weight gain (WG) than fish fed all FO diet (FO). However, mean value of WG of fish fed either vegetable oil-tested diet was nonsignificantly different. Feeding seabream growers vegetable oil (VO) diets (VO1 or VO2) had no significant effect on specific growth rate (SGR), daily weight index (DWI), or feed conversion ratio (FCR) among dietary treatments. Consumption of VO for 20 weeks did not significantly alter the major nutrient composition of fish, but the muscle fatty acid (FA) profile was significantly altered compared to the reference FO diet. Comparatively reduced levels of eicosapentaenoic acid (EPA) and docosahexaenoic acids (DHA), as well as elevated levels of linoleic and linolenic acids (LA and LNA) compared with fish fed the FO were noticed. In terms of economics, 17 or 20% reduction in Kg feed cost was obtained for diets VO1 or VO2, respectively. In terms of growth performance and cost, VO2 diet showed slight relative superiority over VO1 diet. However, in terms of liver structure morphology, VO1 diet (with LO) has resulted in less fat-infiltration and altered hepatic cells than VO2 (with SBO). As these traits do not affect yield or the price paid for the fish, VO2 diet has therefore been considered better than VO1 as complementary lipid sources for gilthead seabream grower diets.     

Evaluation of fish-meal free diets for rainbow trout, Oncorhynchus mykiss

Eight experimental diets were formulated for rainbow trout using agricultural byproducts as major ingredients. Each experimental diet contained varying amounts of corn grain, corn gluten meal, corn gluten feed and one of the following: 200 g kg^?1 peanut meal, 200 or 400 g kg^?1 soybean meal (SBM), 390 g kg^?1 low-allergen soy flour, 310 g kg^?1 soy protein concentrate, 300 g kg^?1 low-allergen soy protein concentrate or 200 g kg^?1 SBM + 110 g kg^?1 blood meal. One diet contained 200 g kg^?1 SBM and canola oil as the main lipid source. The remaining diets contained 95 g kg^?1 menhaden oil. Fish fed a commercial trout diet exhibited significantly greater weight gain (322%), and a lower feed conversion ratio (0.89) but significantly lower protein efficiency ratio (2.18) than fish fed the experimental diets. Within the experimental diets, fish fed the 400 g kg^?1 soy flour diet and the 400 g kg^?1 soybean meal diet had significantly higher weight gains (276% and 268%) and protein efficiency ratios (2.58 and 2.52), and lower feed conversion ratios (1.02 and 1.03) than fish fed other experimental diets. Fillet flavour varied between treatments. Most notable was the lower fishy flavour and higher chicken flavour of fish fed the diet that contained canola oil rather than menhaden oil. Microscopic evaluation of the liver and five sections of the gastrointestinal tract failed to demonstrate any differences between treatment groups. The ingredient costs of several experimental diets were lower than the estimated cost of a standard commercial trout diet. However, the superior feed conversion ratios of fish fed the control diet resulted in lower feed costs per unit of fish produced.     

Potential of local agricultural by-products for the rearing of African catfish Clarias gariepinus in Rwanda: effects on growth, feed utilization and body composition

This study was performed to evaluate the effect of replacing fish meal with local by‐products on Clarias gariepinus growth performance, feed utilization and body composition. A control diet contained 50% of fish meal. In four other diets, fish meal was partially replaced by vegetable and animal protein blend composed of sunflower oil cake, soybean oil cake, groundnut oil cake, bean meal, chicken viscera and blood meal. The study was conducted in a recirculating water system at a mean temperature of 23.6°C. The five test diets were compared with a commercial diet developed for African catfish. All diets were balanced to be equal in gross energy (19 kJ g⁻¹) and crude protein (40%). The experimental groups were fed in triplicate for 8 weeks, increasing fish weight from about 6.2 g at start to 52.3 g in the end. Best specific growth rate (SGR=3.4), feed efficiency (FE=1.3) and protein efficiency ratio (PER=3) were obtained with the control diet (diet 50% fish meal), although there were no significant differences between the group of fish fed the control diet and those fed diets based on groundnut oil cake or bean meal, whereas SGR (2.17), FE (0.85) and PER (1.95) were significantly (P<0.01) lower in fish fed diet containing sunflower oil cake. No significant differences (P<0.05) were found in fish fed commercial diet and diets containing bean meal or groundnut oil cake. Groundnut oil cake or bean meal can thus replace at least 50% of fish meal in the diet of Clarias fingerlings without amino acid supplementation. Because of its economic importance and its potential in animal nutrition sunflower oil cake is still an interesting feed ingredient, but its efficiency should be improved by various processing techniques. African catfish can utilize efficiently a diet with low percentage of animal protein without growth reduction.     

Effects of totally replacing dietary fish oil by linseed oil or soybean oil on juvenile hybrid sturgeon,Acipenser baeri Brandt♀×A. schrenckii Brandt♂

The growth performance, body composition, fillet fatty acid content, serum hepatic enzymes and postprandial changes in serum lipid metabolism of hybrid sturgeon (70.8 ± 0.5 g) were investigated to determine the effects of total replacement of 80 g kg^‐1 fish oil (diet A) with linseed oil (diet B) and soybean oil (diet C), respectively. No significant differences in weight gain rate and specific growth ratio were observed among all fish groups (p > .05). Diet A fish group had the highest, but diet B fish group had the lowest feed intake (p < .05). Feed efficiency of fish fed diet A was significantly lower than the other two fish groups (p < .05). Fish fed diet A had the lowest fillet and liver lipid contents (p < .05). Serum lactate dehydrogenase, alanine aminotransferase and aspartate aminotransferase activities of fish fed diet A were significantly lower than those of fish fed diets B and C (p < .05). The contents of linoleic acid (C18:2n6) and linolenic acid (C18:3n3) in fillets showed a significantly positive linear correlation with the diets. Serum glucose and non‐esterified fatty acid were just affected by the time point (p < .05). The lipid source, time point and interaction of both factors had significant effects on serum triglyceride, high‐density lipoprotein cholesterol and low‐density lipoprotein cholesterol (p < .05). Serum total cholesterol was only affected by interaction of time point and lipid source (p < .05), and ketone body was not affected by lipid source, time point or interaction of both factors (p > .05). In summary, total replacement of fish oil with linseed oil or soybean oil had no significant adverse effects on hybrid sturgeon growth during 84‐day period, and linoleic acid and linolenic acid in fillet were modified by dietary treatments.     

Effects of fish oil and partial fish meal substitution with oilseed oils and meals on growth performance, nutrient utilization and health of the rainbow trout Oncorhynchus mykiss

A feeding trial was conducted to investigate the effects of the replacement of dietary fish meal and fish oil with oilseed meals (soybean or canola) and canola oil on growth, nutrient utilization, body composition, diet digestibility and hematological parameters of rainbow trout Oncorhynchus mykiss. Seven diets were used; the control diet (designated FM) contained fish meal and fish oil as the main protein and lipid sources. For the experimental diets, 40% of fish meal protein was substituted with soybean meal, canola meal or a soybean/canola meal mixture, and these diets (designated SM, CM and SCM, respectively) contained fish oil as the lipid source. Three additional diets (SM?+?CO, CM?+?CO and SCM?+?CO) were formulated with the same vegetable protein meals but with fish oil replaced by canola oil. Fish were fed twice daily to apparent satiation for 11?weeks. The growth of fish fed the CM?+?CO diet was significantly lower than that of fish fed the FM, SCM, SM?+?CO and SCM?+?CO diets. The feed conversion ratio (FCR) was significantly better in fish fed the FM and SCM?+?CO diets than in fish fed the CM and CM?+?CO diets. Furthermore, feed intake was significantly lower for fish fed the CM?+?CO diet than in fish fed the SCM diet, and lipid digestibility of the CM?+?CO diet was significantly lower than that of all other diets. No significant differences of body composition were observed. Circulating leukocyte levels, leukocyte ratios and serum lysozyme activity remained unaffected by dietary treatment. However, it was observed that fish fed the CM?+?CO diet displayed hematocrit levels significantly lower (P?<?0.05) than that of fish fed the other diets. The results indicate that when diets contain either fish oil or canola oil, canola meal and soybean meal can be incorporated into rainbow trout feeds at a combined 32% inclusion level (replacing 40% of fish meal protein) without inducing significant negative effects on growth, nutrient utilization or health.     

Cottonseed oil as a complementary lipid source in diets for gilthead seabream Sparus aurata juveniles

The efficacy of using cottonseed oil (CSO) as a fish oil (FO) substitute in gilthead seabream (Sparus aurata) juveniles feed was evaluated. Fish (BW_i 4.0 ± 2.9 g) were fed one of four isoproteic (~48% CP) and isolipidic (~18% L) diets for 9 weeks. Added oil was either FO (control diet, CTRL) or CSO, replacing 50% (CSO50 diet), 60% (CSO60 diet) and 70% (CSO70 diet) of dietary FO. Results indicated that FO replacement by CSO up to 60% level had no detrimental effects on growth or nutritive utilization and composition in fish muscles. Higher CSO intake (CSO70 diet, 56 g kg^?1) led to a 16% reduction in weight gain, 14% in feed utilization (FCR) and 57% in muscle n‐3 long‐chain polyunsaturated fatty acids (lc PUFA) as compared with CTRL and to abundant accumulation of lipid within the hepatocytes. Use of CSO altered fatty acid (FA) profiles of muscle and liver. Data suggested utilization of linoleic acid (LOA) by fish and retain of docosahexaenoic acid (DHA) in muscles. Therefore, limits of CSO inclusion as the main source of supplementary dietary lipid, with no negative effects on fish performance or nutritive composition and utilization in muscles, are: 40–48 g kg^?1 feed for gilthead seabream juveniles.