The Efficiency of Modified Canola Oil as a Replacement of Menhaden Fish Oil in Practical Diet for Pacific White Shrimp, Litopenaeus vannamei

Abstract

Lipids are an essential component of the diet serving as source of energy and components of cell structures. Marine species require a certain level of specific fatty acids in their diet as they have limited ability to synthesize and/or transform precursors into essential fatty acids (EFA). As fish oil is rich in EFA it has served as the primary lipid source in aquaculture diets. Due to fish oils advantages of omega-3 (n-3) and omega-6 (n-6) long-chain highly unsaturated fatty acids (LC-HUFAs), EPA (Eicosapentaenoic acid, C20:5n-3), DHA (Docosahexaenoic acid, C22:6n-3) and ARA (Arachidonic acid, C20:4n-6), aquaculture currently uses 90% of this resource. In order to expand the aquaculture industry, fish oil supply should be replaced by a source of lipids which are suitable to meet the shrimp requirements for fatty acids, and more available in terms of production. Canola oil is not only cheaper than fish oil, but also more available and sustainable in terms of production for a long-term period. As with other terrestrial oil sources, the limited availability of LC-HUFA in canola oil means it cannot meet the EFA requirements of the shrimp. However, with the development of technology to genetically modify lipid metabolism of canola, the fatty acid profile is modified to produce higher level of n3 fatty acids, in particular DHA and EPA, and can possibly serve as a true alternative to fish oil. The development of modified canola oil is a relatively new technology which must be validated and is thus the subject of this work. The objective of the first study was to estimate the efficacy of genetically modified canola oil as a replacement for fish oil in Pacific white shrimp feeds. In this experiment, a series of fishmeal free diets were formulated to contain 36% protein and 8% lipid. The lipid from menhaden fish oil (MFO, 5.45 g/100g diet) was replaced by the modified canola oil (MCO-1) at 25%, 50%, 75%, and 100% in the first series of diets. A diet of MCO-2 replaced 100% MFO was also formulated. In the second series, MFO was replaced by 50%, 75%, and 100% using standard canola oil (SCO). At the conclusion of a 6-week growth trial, there was a significant depression in final weights of shrimp maintained on diets containing 100% MCO-1, MCO-2, or SCO as compared to shrimp reared on the basal diet with MFO. The reduced performance at high levels of replacement is likely caused by low levels of DHA not meeting the threshold of the requirement for this EFA. Another possible explanation was due to rancidity of the oil which could be an issue as peroxide value of this diet was considerably higher than that of the menhaden fish oil-based diet. Hence, palatability or toxicity of the oxidized products could have contributed to the poor response at 100% inclusion. The second study was similar to the first but with a new source of the modified canola oil, which was stabilized to reduce oxidation. As before, MFO was replaced with graded level of MCO-3 up to 100% replacement. The first five of the eight diets were designed with fishmeal, soybean meal and corn protein concentrate as the primary protein sources while the other three were fishmeal free based. In theory, fishmeal does not only contain protein, but also provide lipid and fatty acids which contribute to EFA levels of the diets. In all diets, menhaden fish oil served as the primary lipid source and was sequentially replaced with a modified canola oil at 25%, 50%, 75%, and 100%. Then, the three fishmeal-free based diets were evaluated using 75% MCO-3 as well as 75% MCO-3 in combination with hydrolyzed salmon (amino salmon, AS) by product meal which served as an attractant to replace MFO. Results of the 6-week growth trial indicate that 100% of MFO could be successfully replaced by the MCO-3 without causing deficiencies in shrimp performance, survival, and FCR. Whether or not palatability of the diet was affected by the MCO-3 in the diet was not clearly detected; however, mean final weights of the shrimp offered the diet containing AS did not show a significant improvement. Finding an alternative source of lipid to replace fish oil in commercial shrimp feeds is required if the industry is to continue to expand. According to the overall results of this study, the modified canola oil has a higher level of n3 fatty acids and an increased level of EFA which are required by many species. Results of this work confirm that the MCO can be used as the primary lipid source in production diets for shrimp. As there were slight reductions in performance, further research into the efficacy of the MCO is warranted. It is also recommended that work should be conducted in more practical conditions to demonstrate the efficacy under less controlled conditions.