Abstract
The aim of this study was to investigate the effects of three commercial diets, standard, immunostimulant-supplemented (β-glucan, vitamins C and E) and plant-based, on the degree of oxidative stress in tissues of rainbow trout (Oncorhynchus mykiss). Selected immune and haematological indices were measured and challenge with Aeromonas salmonicida was conducted. The plant-based diet systemically modulated all oxidative stress biomarkers (thiobarbituric acid reactive substances; reduced and oxidised glutathione and their ratio) in the intestine. The ratio was elevated in all organs (liver, kidney, muscle and intestine) and an enhancement of respiratory burst and complement activity was observed even in the control fish. With the standard diet, an elevation of thiobarbituric acid reactive substances in the intestine and a decrease in some immune indices appeared after challenge. Less distinctive changes and the lowest mortality rate (the highest being with the plant-based feed) were obtained with immunostimulants. Increased total immunoglobulin, relative lymphocytosis and a decrease in the phagocyte count were observed. This study contributes especially by a thorough examination of oxidative stress indices in different tissues. The exact composition of these commercial diets is a trade secret; however, knowledge of their effects is extremely important for fish farmers using them; therefore, this study has a great practical impact.
Highlights
In recent years, the growth of aquaculture has led to the frequent substitution of fish meal with plant-based feeds that are more economically sustainable
The highest levels of GSH in rainbow trout were found in the liver and kidney, followed by the intestine, with the lowest concentrations found in muscle
Comparable levels and similar distribution of GSH were observed in gilthead seabream (Sparus aurata) fed a vegetable oil and carbohydrate rich diet (Castro et al 2016)
Summary
The growth of aquaculture has led to the frequent substitution of fish meal with plant-based feeds that are more economically sustainable. Le Boucher et al (2011) were able to show a significant reduction in the growth rate of rainbow trout fed a plant-based feed, without a negative impact on the survival rate of the fish. The addition of β-glucans (polysaccharides that occur naturally in the cell walls of bacteria and yeasts) as a supplement to fish diet enhances non-specific immune reactions such as phagocytosis (Dalmo and Bøgwald 2008), resulting in an improved reaction to stress (Jeney et al 1997; Dawood et al 2017). Non-specific immunity tends to be the main mechanism involved in the infection process (Magnadóttir 2006; Esteban et al 2015), but significant natural variability needs to be taken into account (Anderson 1992)
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