| Summary: | This work aimed to appraise growth performance, digestive capacity and immune and stress responsiveness in European seabass (Dicentrarchus labrax) when fed diets supplemented with seaweeds. A control diet (without supplementation) was tested against 6 practical diets supplemented either with Gracilaria spp., Ulva spp., or Fucus spp., at 2.5 or 7.5% levels, plus an additional diet with a blend of the three seaweeds, each supplemented at 2.5% (Mix). Seabass juveniles (24.0 ± 6.3 g) were fed the experimental diets for 84 days. Dietary seaweed supplementation had no effect on growth rate (DGI: 1.0–1.1), voluntary feed intake (11.3–12.6 g kg −1 ABW−1 day−1), feed conversion ratio (FCR: 1.2–1.4) and protein efficiency ratio (PER: 1.5–1.8). Lipase activity was significantly higher in fish fed Mix diet when compared to control (P < 0.05). Glutathione peroxidase (GPx) was significantly affected by seaweeds presence regardless its level of supplementation (P < 0.05). The innate immune system was significantly altered by dietary seaweed supplementation (P < 0.05). Fish fed the 7.5% seaweed supplemented diets had a significant decrease in the ACH50 level, when compared to fish fed 2.5% seaweed supplemented diets (P < 0.05). Moreover, a combined effect of seaweed and supplementation level significantly affected lysozyme (LYS) activity (P < 0.05). Fish fed diet with Ulva at 2.5% had an increase in LYS when compared to fish fed control and Ulva at 7.5% diets. Overall, our results indicate that the use of dietary seaweed supplementation improves immune and antioxidant responses in European seabass without compromising growth performance. Keywords: Dicentrarchus labrax, Gracilaria spp., Ulva spp., Fucus spp., Digestive enzymes, Growth, Innate immunity, Oxidative stress, Seaweeds
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