Growth, fatty acid profile, antioxidant enzymes activities and lipid peroxidation level was investigated in rainbow trout (Oncorhynchus mykiss) fed two iso-nitrogenous (48% protein) and iso-lipidic (20% lipid) feeds: FF; fish meal/oil-based, FP; plant meal/oil-based diet with four feeding strategies: 1) only FF, 2) morning FP, afternoon FF (FP/FF), 3) one day FF, the other day FP (FF-FP), and 4) only FP to the apparent satiation for 150 days. The feeding trial was carried out in semi-recirculating water system equipped with 12 tanks (100 L volume), mechanical and bio-filtration, and constant oxygenation, in triplicates.
At the end of the feeding trial, growth performance and feed efficiency were significantly influenced by the dietary treatments (P<0.05). The highest growth was observed in the FP/FF group (final weight; 279.7 g), while the lowest growth (final weight; 209.7 g) was observed in the FP group (Fig. 1). Fatty acid composition of the whole-body fish reflected the fatty acid composition of the diets, with significant differences between dietary treatments (P<0.05). While the highest total saturated fatty acids (SFA), monounsaturated fatty acids (MUFA), n3, and n3/n6 ratio were detected in the FF group (32.1, 33.3, 22.3 and 1.8, respectively), the lowest values were recorded in the FP group, where the highest polyunsaturated fatty acids (PUFA) were detected (48.7%). Long chain n-3 PUFAs such as eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) decreased with the reception of dietary plant oil. Hepatic lipid peroxidation level (malondialdehyde level; MDA), activities of antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT), and total glutathione level were significantly influenced by the dietary treatments (P<0.05). The lowest and highest MDA levels were observed in FP (0.48 nmol/mg tissue) and FF (1.21 nmol/mg tissue) groups, respectively.
Our results showed that alternative feeding with FF and FP diets promoted growth in rainbow trout, with the better performance when the interval between 2 diets were shorter. EPA and DHA levels in fish fed only plant oil-based diet decreased dramatically; however, considerable amount of EPA and DHA were secured when 2 diets were used in alternation. Our overall results suggested that using plant meal/oil-based and fishmeal/oil-based diets alternatively could be a good strategy in rainbow trout culture to improve growth and feed efficiency, and oxidative status without dramatic decrease in EPA and DHA.