Effect of fish meal replacement by plant protein sources on non-specific defence mechanisms and oxidative stress in gilthead sea bream (Sparus aurata)

Partial or total replacement of fish meal by a mixture of plant protein (PP) sources (corn gluten, wheat gluten, extruded peas, rapeseed meal and sweet white lupin) balanced with indispensable amino acids was examined in juvenile gilthead sea bream over the course of a 6-month growth trial. A diet with fish meal (FM) as the sole protein source was compared to diets with 50%, 75% and 100% of replacement (PP50, PP75, and PP100). The possible influence of diets on growth performance, plasma metabolites, gut integrity, liver structure, anti-oxidant and immune status was evaluated. Final body weight was progressively decreased with PP inclusion, but in PP50 and PP75-fed fish, feed efficiency (FE) was significantly improved and specific growth rates remained unchanged or slightly reduced in comparison to fish fed the FM diet. In fish fed PP100 diet, FE remained unchanged and feed intake and growth decreased dramatically. In this group of fish, liver fat deposition was also largely increased, enterocytes showed an increased number of lipidic vacuoles and/or deposition of protein droplets, and the submucosa of intestine was dilated/hypertrophied and infiltrated with eosinophilic granular cells. Plasma glucose levels did not differ among the four groups, but a significant and progressive decrease of plasma cholesterol and plasma protein levels was found with FM replacement. The glutathione redox status in blood and liver increased with the increase of PP content and this increment was statistically significant in the liver of the three PP-fed groups. Glutathione reductase and γ-glutamyl transferase were also enhanced by PP inclusion. Plasma lysozyme levels were not altered by the dietary treatment, but respiratory burst of head kidney leucocytes and plasma myeloperoxidase values were significantly increased in PP75 and PP100 fish, respectively. Complement (ACH₅₀) was significantly increased in PP50 fed fish and decreased in PP75 and PP100 groups. As a general conclusion, substitution of FM by a mixture of PP sources exerted an anti-oxidative effect, compromised growth performance only at the 100% level, and decreased one of the immune defence mechanisms at above 75% level.