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The following are summaries of papers on fish oil replacement published in peer-reviewed journals recently.

1. Total replacement of fish oil by soybean or linseed oil with a return to fish oil in turbot (Psetta maxima) - 1. Growth performance, flesh fatty acid profile, and lipid metabolism. C. Regost, J. Arzel, et al. 2003. Aquaculture 217:465-482. (Corresponding author Dr. Sadasivam Kaushik, E-mail: kaushik@st-pee.inra.fr)

The study investigated the replacement of fish oil by vegetable oils and the effects of a washout with a return to fish oil on growth performances and lipid metabolism. Three diets with identical protein and lipid levels were formulated containing either 9% of added fish oil (FO), soybean oil (SO), or linseed oil (LO). Each diet was fed to marketable size turbot for 13 weeks. After that all groups were fed with FO diet for 8 weeks. The inclusion of vegetable oils in the diets resulted in a slight decrease in growth compared to those fed with fish oil-based diet. Feed conversion and body composition were not affected by dietary lipid sources. Fatty acid composition of liver and muscles reflected the dietary fatty acid composition. Fish fed SO diet were rich in 18:2w6 (linoleic acid). Fish fed LO diet were rich in 18:3w3 (linolenic acid). Fish fed the vegetable oils had lower levels of 20:5w3 (EPA) and 22:6w3 (DHA) when compared to those fed with FO. The fatty acid composition of fish fed previously with SO and LO diets was still different to that of fish fed with FO diet even after being on FO-based diets for 8 weeks. The levels of 18:2w6 and 18:3w3 declined after the transfer, but still higher than those of fish fed with FO diet. The study demonstrated that replacement of fish oil by vegetable oils is possible with negligible impact on growth performance of turbot, however, a duration of 8 weeks of feeding with fish oil is not sufficient to restore the fatty acid profile of turbot back to that of fish fed with fish oil for 19 weeks.

2. Total replacement of fish oil by soybean or linseed oil with a return to fish oil in Turbot (Psetta maxima) - 2. Flesh quality properties. C. Regost, J. Arzel, et al. 2003. Aquaculture 220:737-747. (Corresponding author, Dr. Sadasivam Kaushik, E-mail: kaushik@st-pee.inra.fr)

This study evaluated the flesh quality of fish produced from the previous study (see above). It found that the inclusion of vegetable oils did not affect the gutted and fillet yields. However, the sensory qualities, notably odor, color and texture were influenced by the oils. The dorsal fillet of the turbot fed soy oil had a more pronounced potatoes odor and a lower fat texture. However, these differences disappeared after the washout period, demonstrating that sensory properties due to the intake of vegetable oils can be reduced with a return to a fish-oil-based diet.

3. Rapeseed oil as an alternative to marine fish oil in diets of post-smolt Atlantic salmon (Salmo salar): changes in flesh fatty acid composition and effectiveness of subsequent fish oil ''wash out.'' J.G. Bell, F. McGhee, et al. 2003. Aquaculture 218:515-528. (Corresponding author, Dr. J. Gordon Bell, E-mail: g.j.bell@stir.ac.uk)

Post-smolts of Atlantic salmon were fed with diets in which the added fish oil was substituted by 0%, 10%, 25%, 50% or 100% rapeseed oil (RO). After 16 weeks of feeding the fish were sampled and the remaining fish were fed a commercial grower diet containing fish oil (FO) for 12 weeks. There were no effects of the inclusion of RO on growth or feed conversion. Increasing the inclusion of RO increased the levels of 18:2w6 and 18:3w3 and lowered the levels of 20:5w3 and 22:6w3. Transfer to the FO washout diet, restored the levels of 20:5w3 after 4 weeks and 22:6w3 after 12 weeks. But, even after 12 weeks, the levels of 18:2w6 were significantly higher in fish previously fed 50% and 100% RO, although the absolute values were reduced by 48% and 65%, respectively. The study concluded that rapeseed oil is a potential substitute for fish oil in Atlantic salmon feeds, however inclusion levels above 50% can significantly reduce HUFA levels in the fish. The HUFA levels can be restored by feeding a diet containing fish oil for a period of 12 weeks.

4. Effect of canola oils as alternative lipid resources in diets for juvenile red seabream (Pagrus auratus). B.D. Glencross, W.E. Hawkins & J.G. Curnow. 2003. Aquaculture Nutrition 9:305-315. (Corresponding author, Dr. Brett Glencross, E-mail: bglencross@fish.wa.au)

This study evaluated crude and refined canola oils and refined soybean oil as potential fish oil substitutes in red seabream. Each of the oils was used to replace 25%, 50%, 75% and 100% of added fish oil in the diet. The diet had 60% fishmeal, 10% total lipids and 4% added oil. A negative control diet containing no added oil and a positive control diet containing 8% added fish oil were also included. A significant reduction in growth was observed when the fish oil was replaced with 100% crude canola oil. Reduction in growth was observed with the negative control diet as well. The positive control diet gave the best growth. No other significant growth effects were observed. The growth data showed that the red seabream utilized the plant oils.

There was a considerable effect of the dietary treatments on fatty acid composition of the fish. Significant increases in 18:2w6 (linoleic acid) and 18:3w3 (linolenic acid) were observed with the inclusion of plant oils. There were some, but not dramatic decreases, in the levels of long chain HUFA when plant oils were included. A sensory evaluation study compared the cooked fillets of fish fed the 100% canola oil, 100% soybean oil and 100% fish oil treatments. It rated the canola oil fed fish the highest in overall acceptability.

5. 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. B.D. Glencross, W.E. Hawkins & J.G. Curnow. 2003. Aquaculture Nutrition 9:409-418. (Corresponding author e-mail: Dr. Brett Glencross, bglencross@fish.wa.au)

Three months ahead of the study, three groups of red seabream were reared on a fish oil based diet or diets where the fish oil (FO) had been replaced with canola (CO) or soybean oil (SO). After three months, some of the fish that had been fed with CO or SO were put on FO while other fish were continued with the dietary treatments they had been receiving for three months. All fish were fed for another 32 days. There were no significant differences between the dietary treatments except for a slight growth improvement in fish on SO switched to FO. The fatty acid composition of the fish switched from CO or SO to FO changed significantly. Levels of 18:2w6 (linoleic acid) and 18:3w3 (linolenic acid) decreased while the levels of long chain PUFA, 20:5w3 (EPA) and 22:6w3 increased when the fish were returned to FO. The study demonstrated that it is possible to reverse the fatty acid changes in red seabream that has been fed plant oil based diets, by feeding them with a fish oil based diet.

6. Dietary lipid sources for seabream and seabass: growth performance, tissue composition and flesh quality. M. Izquierdo, A. Obach, et al. 2003. Aquaculture Nutrition 9:397-407 (Corresponding author, Dr. Marisol Izquierdo, E-mail: marisoli@iccm.rcanaria.es).

The study assessed the effect of partial replacement of fish oil by vegetable oil on growth, dietary fatty acid utilization and flesh quality of gilthead seabream and seabass. Five diets with identical levels of energy and protein were formulated. The diets had 25% lipid content. The control diet had only fish oil as the added lipid source. In other diets, fish oil was added at a level high enough to keep the w3 HUFA levels above 3% so that the essential fatty acid requirements of the species were met. Fish oil was replaced by soybean oil (SO), rapeseed oil (RO) and linseed oil (LO) or a mixture of them. Feed intake, growth and feed utilization of fish in the dietary treatments were comparable to commercial grow-out during the experimental period. The results showed that it is possible to replace up to 60% of the fish oil by SO, LO and RO or a mix of the oils in diets for seabream and seabass, without compromising fish growth as long as the essential fatty acid requirements are met. Fatty acid profile of liver and muscle reflected the fatty acid profile of the diet. However, cooked fillet of fish fed vegetable oils was very well accepted by trained judges.