Investigation on replacement of fish oil by vegetable oils (Rice bran, soybean and linseed) in diet of rainbow trout (Oncorhynchus mykiss)

The aim of the present study was to determine the impact of replacing the fish oil by
alternative lipid sources in diets for rainbow trout (Oncorhynchus mykiss) on growth, fatty
acid profiles of muscle. Five isonitrogenous and isoenergetic semipurified diets were
formulated containing 100% rice bran oil (RBO), 100% linseed oil (LO), 100% Soybean oil
(SO) and a 1:1:1:1 rice bran, linseed, soybeen and fish oil mixture (MX) were compared to a
pure fish oil (FO) diet. The diets were fed to apparent satiation twice a day to triplicate groups
of 30 rainbow trout with an initial weight of 90g for 15 weeks at 12°C.At the end of the
experiment, fillet samples were collected from fish for the measurement of fatty acid profile
and fillet composition. Fish fed the FO diet had the highest weight gain over the experimental
period (280.28 g), However no differences in growth rate and CF were observed across diet
groups. Feed conversion ratios (FCR) ranged from 1.12 to 1.24. Significant difference in
FCR was found in the group fed an FO diet. The fillet lipid concentrations and fatty acid
composition of the fish were significantly affected by the experimental diets. Fish fed the FO
diet contained significantly lower lipid levels (20.18%) than those fed the 4 other diets. The
highest level of n-3 PUFA concentrations were recorded in fish fed the LO diet (21.8) with
significant difference than other diets and the lowest were in those fed the RBO diet (7.84).
Fish fed the SO diet contained significantly higher level of n-6 PUFA concentrations (31.72)
than other diets and the lowest were in those fed the FO diet (19.15). Fish on the FO diet had
a significantly greater percentage of DHA (10.10%) in muscle tissue compared with fish on
all other diets and the lowest was in those fed the RBO diet (2.81%). However, the highest
level of eicosapentaenoic acid (EPA) concentrations was recorded in fish fed the SO diet
(1.84%) and the lowest in those fed the diet (0.67%). This study showed considerable
reductions (about 3-fold) in the n-3/n-6 ratio when FO in the fish diet was replaced by rice
bran and soybean oils, which may be extremely negative from a nutritional point of view. In
other hand linseed oil had the highest n-3/n-6 ratio (0.91) that was comparable with fish oil n-
3/n-6 ratio (0.89). The results in this study imply that an appropriate mix of vegetable oils and
FO can replace the sole use of FO in fish feeds.