The limited availability of fish oil (FO), as the main source of lipid in aqua-feeds, calls for the identification of alternative lipid source for the development of sustainable fish farming. In this context, the utilization of vegetal oil (VO) sources, due to their relatively low cost and stable production, has dramatically increased as an important alternative in recent years. From a nutrition point of view, VOs are rich in C18 polyunsaturated fatty acids (PUFA) such as linoleic acid (LA; 18:2n-6) and α-linolenic acid (LNA; 18:3n-3), but the main drawback of using VOs is their FA profile (a different n3:n6 ratio) in comparison to FO. However, digestive and absorptive processes such as enzymatic activities can be affected. Grape seed oil (GO) includes the unsaturated fatty acids, namely oleic acid (18:1n-9) and LA with a higher ratio of unsaturated to saturated fatty acids than animal fats and it does not contain cholesterol. Commercial diets for rainbow trout (Oncorhynchus mykiss) mostly contain FO as the main lipid source. Therefore, successful replacement of FO with GO would be an alternative for reducing the dependence on this ingredient and its associated costs. This work was aimed to investigate the effects of dietary increasing levels of GO replacing FO on the activity of digestive enzymes in rainbow trout.
The rainbow trout (40.16 ± 0.04g mean individual weight) was fed 2% of their body weight per day for 60 days at 15.0±1.5 °C with a mean oxygen concentration of 9.6± 0.1 mg/L. Five experimental diets were formulated with pure FO (D1) and increasing amounts (D2: 25 %, D3: 50%, D4: 75% and D5: 100%) of GO as dietary lipid (Table 1). For determination of digestive enzymes activity, fish pyloric caeca and intestine were separated from viscera and then separately homogenized and all enzyme activities were measured at 37 °C by spectrophotometer.
The results indicated with increasing level of GO in the diets was promoted a decrease in α-amylase activity whereas trypsin and lipase activities were increased from GO25 diet to GO50 diet and then decreased (Table 2). Based on the enzymatic findings, it can be concluded that the GO50 diet was performed best among the tested diets in the experiment.