Dietary micronutrient supplementation in low fishmeal based diets for optimum growth and immune status of European sea bass (Dicentrarchus labrax) juveniles

The present study aimed to evaluate the optimal dietary micronutrient supplementation of plant-based diets for juvenile European sea bass. A 90-day feeding trial was performed, where fish were fed a high fishmeal (FM) based diet (62% DM; positive control) or low FM diets (12.5% FM and krill meal), supplemented with increasing levels of a micronutrient mixture (Nutrient Package, NP) consisting of vitamins, minerals and other micronutrients. The supplementation levels ranged from 0% (no supplementation, negative control) to 400%, where 100% supplementation level corresponded to the requirement levels recommended for other fish species. Fish growth and feed performance (SGR, DGI, FCR, PER) were significantly impaired at levels NP0 and NP25 compared to the FM control. Body Fe and Zn concentrations were positively affected by the graded levels of the NP, while Mn and Se concentrations remained stable. Growth parameters were particularly correlated to body Fe and Zn, while immune parameters were mainly correlated to body Zn. Significant immunosuppression was evidenced (concerning the respiratory burst activity) when low FM diets were not supplemented with NP. High inclusion levels of the NP showed a negative impact on lysozyme activity. Histological examinations did not reveal any significant effects on gills, gut and liver morphology. At identical NP supplementation level (50%), fish fed low and high FM diets showed similar growth and innate immunity suggesting that long-term feeding with plant-based diets do not significantly affect the fish when adequate minerals and vitamins supplementation is provided. Quadratic regression analysis showed that the inclusion of the NP between 260 and 300% improves growth and immune parameters of fish fed low-FM diets as well as Zn body incorporation. Overall, the present study suggested the positive impact of NP supplementation of PM-based diets on both fish growth and health but future NP could include less Mn, more Fe, closer to levels obtained in fish fed FM (245 mg·kg⁻¹), and Zn at 122 mg·kg⁻¹.