Introduction

During the previous couple of decades, the world aquaculture industry has considerably developed and remained one of the fastest-growing food-producing sectors. The global increase in aquaculture production has resulted in increased worldwide demand for fish oil to meet ever developing aquafeed industry. It has been shown that terrestrial lipid sources including vegetable oils are promising alternatives for dietary fish oil due to their increased production, price stability, and availability. Incorporating vegetable oil instead of fish oil in aquafeed formulation could improve the sustainability of aquaculture development and also increase the profitability of aquaculture industry (Xie et al., 2018; FAO, 2020). The present research was conducted to elucidate the effects of different dietary MUFA contents and LA:ALA ratios, by dietary incorporation of different propositions of vegetable oils including sunflower, linseed, coconut and olive oils instead of fish oil on growth indices and muscle fatty acids profile of Oncorhynchus mykiss fed on a plant-based diet without any marine ingredients.

Methodology

This study was conducted to evaluate the effect of different dietary levels of MUFA and different ratios of linoleic acid (LA) to linolenic acid (ALA) on growth performance and muscle fatty acid profile of rainbow trout fingerlings. 650 juveniles of triploid rainbow trout with an average initial body weight of 14.5 ± 0.17 g were randomly allocated into seven triplicated experimental groups in 300-liter polyethylene tanks. Therefore, seven iso-nitrogenous and iso-lipid experimental diets including diet 1 (55.33, 1.00; dietary MUFA content and LA:ALA ratio), diet 2 (25.30, 1.00), diet 3 (55.13, 2.05), 4 (24.92, 2.03), diet 5 (54.94, 8.06) and diet 6 (24.91, 8.06) and a control diet containing fish oil, diet 7 (33.60, 5.97) were formulated. The basal diet did not contain any fish meal. The experimental fish were fed the diets for 4 months during the first stage of the trial. Afterwards, during the second stage of the trial, all experimental groups were fed diet 7, the basal diet which contained fish oil as the only dietary lipid source for one month. At the end of each stage, the fish were weighed to the nearest 0.1 g to calculate growth indices. Muscle samples were also taken after humanly sacrificing fish for fatty acid profile studies.

Results

At the end of the first four-month feeding period, there was no significant difference among various experimental groups regarding weight gain (WG), relative WG, daily weight gain (DWG), specific growth rate (SGR), and survival rate (p>0.05). Similarly, feeding various experimental groups with diet containing only fish oil as the only dietary lipid source for one month also did not result in any significant differences among the groups (p>0.05). At the end of the first stage, there were no significant differences regarding muscle SFA contents of various experimental groups (p>0.05). Muscle MUFA contents reflected dietary MUFA content so that the highest MUFA proposition was observed in treatment 1 (p<0.05). However, n-6 PUFA and n-6 HUFA contents of fish in treatments 5 and 6 fed diets contained the highest dietary LA:ALA ratio (i.e., 8.0) was the highest (p<0.05). Muscle n-3 HUFA contents of the fish fed diets with lower dietary LA:ALA diet (≤2.0) were considerably higher than treatments 5 and 6 (p<0.05). However, those fish fed with diet 7 showed the highest muscle n-6 HUFA content (p<0.05). At the end of the first experimental period, muscular contents of EPA, DHA and PUFA of all experimental groups were significantly decreased (p<0.05) comparing to those fish fed diet containing fish oil (treatment 7). At the end of the both periods, muscle n-6 PUFA/n-3PUFA was significantly increased in treatment 5 (fish fed diet containing 55% MUFA and LA:ALA ratio of 8) compared to the control group (treatment 7, p<0.05). At the end of the second feeding period, there were no significant differences regarding muscle SFA and PUFA contents among various experimental groups (p>0.05). Similar to the previous stage, muscle MUFA contents of those fish that had previously been fed diets with higher MUFA contents was higher but the difference was milder compared to the first feeding period. Muscle n-3 PUFA and n-3 HUFA contents of fish did not show any differences among various experimental groups (p>0.05). However, muscle C20:4n-6, n-6 PUFA and n-6 HUFA contents still showed significant differences among groups to the extent that treatments 5 and 6 showed the highest propositions (p<0.05). Feeding all experimental groups with the finisher diet containing fish oil could to some extent restore the muscular contents of the aforementioned fatty acids, although muscle DHA content of treatment 5 was significantly lower compared to treatment 7 (p<0.05).

Discussion and conclusions

Our results showed that different vegetable oils blends could replace fish oil in plate-sized rainbow trout feed. Complete replacement of dietary fish oil by such terrestrial oils blends with different MUFA contents and LA:ALA ratios could affect muscle fatty acid profiles of fish which were characterized by high MUFA, ArA, and LA and lower EPA and DHA contents when compared to those fish fed diet containing fish oil. Although, rainbow trout was considerably able to de novo synthesis of EPA and DHA, increasing dietary LA:ALA ratio, i.e., >2.0, resulted in decreased muscle EPA and DHA content revealing that increased dietary LA:ALA ratio might affect liver desaturation and elongation enzymes activity which finally affect muscle fatty acid deposition requiring further studies regarding the enzymes activity and expression. Such decreased EPA and DHA contents warrant further consideration in terms of the nutritional quality of fish fillet for human consumption when replacing dietary fish oils with vegetable oils. In addition, feeding the fish with a finisher diet contained fish oil for a short period could restore the muscular fatty acids profile to improve the nutritional quality of fish. The efficacy of such a feeding strategy was considerably higher in those fish previously been fed diet 2 contained 25% MUFA with LA:ALA ratio of 1.0. However, the complete conclusion warrants more detailed studies in in terms of nutrient digestion and absorption, immune competence and finally reproductive performance of fish.

Conflict of Interest

The authors declare that they have no conflict of interest.

Acknowledgment

We are grateful to the technicians and staff of the Fisheries Department of Natural Resources Faculty and Artemia and Aquaculture Research Institute, Urmia University for their sincere assistance during the experiment.