Replacement of fishmeal by increasing levels of plant protein blends in diets for Senegalese sole (Solea senegalensis) juveniles

A growth trial was conducted to evaluate the growth potential and nutrient utilization of Senegalese sole fed diets containing increasing replacement levels of dietary fishmeal (FM) by mixtures of plant protein (PP) sources. Six extruded isonitrogenous and isolipidic diets (55% crude protein and 8% crude fat on a dry matter basis) were fed to juvenile sole (mean initial body weight: 8g±0.06) during 92days. A reference diet containing FM as the main protein source (Control) was compared with five PP based-diets with increasing PP levels: 25% (PP25), 35% (PP35), 45% (PP45), 60% (PP60) and 75% (PP75). PP45, PP60 and PP75 diets were supplemented with crystalline amino acids to simulate the level present in the Control diet. The environmental impact of the tested diets was assessed through the determination of N and P budgets and calculating approximate Fish-in:Fish-out (Fi:Fo) ratios. Fish fed PP25 and PP75 reached a final body weight and daily growth index (DGI) similar to the Control group (P>0.05). Diet PP45 displayed the lowest DGI (0.8 vs 1.3%), lowest protein efficiency ratio (1.03 vs 1.8) and highest FCR (1.7 vs 1.01). The dietary treatments with highest percentage of FM replacement (PP60 and PP75) displayed similar FCR values as the Control (1.2–1.0). Apparent digestibility coefficients (ADCs) of protein varied between 77 and 85% and were not significantly affected by the PP inclusion level. Whole body protein and energy content did not differ significantly among treatments, but protein gain was significantly higher in the Control group compared to PP35, PP45 and PP60 groups. The results indicate that Senegalese sole can effectively cope with plant protein-based diets, but growth rate and nutrient gain mainly depend on the selection of adequate plant protein blends, rather than on the plant protein incorporation level. Sole fed the highest PP level (PP75) showed good growth performance, efficient dietary nutrient utilization and a lower Fi:Fo ratio compared to the Control. The present results clearly show that increasing FM replacement level can have a positive environmental impact as reduces P fecal waste and the fishmeal used per kg of sole produced.