Effects of supplementing a plant protein-rich diet with insect, crayfish or microalgae meals on gilthead sea bream (Sparus aurata) and European seabass (Dicentrarchus labrax) growth, physiological status and gut health

This study was aimed at evaluating growth response, gut health and physiological status of seabream (GSB) and European seabass (ES) juveniles fed diets totally deprived of fish meal and supplemented with alternative ingredients. Fish were fed four isoproteic (45%) and isolipidic (20%) diets in which meals from black soldier fly (Hermetia illucens, H) prepupae, whole red swamp crayfish (Procambarus clarkii, RC) or a microalgae blend (Tisochrysis lutea and Tetraselmis suecica, MA) were used as supplements to replace 10% protein of a fish meal-free, plant protein-based diet high in soybean meal (CV). Each diet was fed to triplicate fish groups kept in a RAS (Temperature 23.5 °C, Salinity 29 ppt) over 12 (GSB) and 18 (ES) weeks. In both fish species, MA diet resulted in the worst specific growth rate (SGR), feed conversion ratio (FCR) and protein efficiency ratio (PER) compared to CV. Diet H significantly improved the SGR only in GSB without affecting FCR. The dry matter apparent digestibility coefficient (ADC) was highest in the H diet (approximately 76%) and lowest in MA (around 73%), while an intermediate value was recorded for the RC diet, in both fish species. Slightly inflamed intestine and altered mucosa morphology were observed in both fish species fed on CV diet, highlighting a higher susceptibility of GSB to this diet. On the contrary, diets H, RC and MA led to a modulation of inflammatory gene expression in the distal intestine from the two fish species. However, while H and RC diets lead to a more evident intestine status improvement in GSB, in ES this result was evident even in response to MA diet administration. The MA diet induced a lowering of nutritional status compared to CV in both GSB and ES. Overall the results have shown that 10% protein from H and RC in a plant protein-based diet, tends to improve fish growth and gut health in both fish species. In ES also the diet including microalgae turned in a beneficial effect on fish gut health, compared to CV one, although it needs further refinement to reduce the adverse impact on growth and physiological status. Our findings suggest the potential use of insect and crayfish meals as functional ingredients in fish meal-free diets for GSB and ES, while further studies are needed to refine the use of microalgae as functional feed supplements in diets for sub-adult gilthead seabream and European seabass.