Increasing dietary plant proteins affects growth performance and ammonia excretion but not digestibility and gut histology in turbot (Psetta maxima) juveniles

A trial on turbot (Psetta maxima) juveniles was undertaken to evaluate the effect of replacing dietary fishmeal (FM) with a complementary mixture of plant proteins (PP) consisting of soybean meal (SBM), wheat gluten meal (WGM) and corn gluten meal (CGM). Four practical diets were formulated to progressively replace 25% (PP25), 39% (PP39), 52% (PP52) and 66% (PP66) of FM protein. Forty animals per tank (initial weight 24.2 ± 4.4 g) were randomly distributed into twelve 450-liter square tanks connected to a closed recirculation system (temperature 18±1°C). The diets were tested in triplicate for 77 days. Final weight ranged from 126.6 g (PP25) to 99.5 g (PP66). Voluntary feed intake of group PP66 (55.5 g fish-1) was significantly lower than in PP25. Specific growth rate in fish fed diet PP25 (2.14% day-1) was significantly higher than those found both in PP52 (2.01% day-1) and PP66 (1.82% day-1) groups. Feed conversion ratio (FCR) in groups PP25 and PP39 (0.66-0.67) were lower in comparison with those of the other groups (FCR 0.70-0.74). None of the diets affected whole-body composition and hepatosomatic index. Condition factor significantly decreased with increasing dietary PP (2.06, 2.02, 1.97 and 1.91) whereas a significant increase in viscerosomatic index was observed in fish fed PP52 and PP66 (6.70-6.95) in comparison with the other two groups (6.31-6.21). Reduced protein retention was found in groups fed diet PP52 and PP66, with protein efficiency ratio (PER) ranging from 2.71 to 2.63 and gross protein efficiency (GPE) from 40.3 to 40.0, in comparison with the other two groups (PER 2.83 and GPE 45.5-45.8), even though apparent digestibility coefficients were not reduced. Ammonia excretion, (g total ammonia nitrogen 100 g protein intake-1), was significantly higher for group PP25 (3.41) than group PP52 (2.61). Gut histology examined in four different sites of intestine (pyloric caeca, proximal, intermediate and distal intestine) revealed no noticeable differences among fish of the various diet groups. In conclusion, substitution of a mixture of SBM, WGM and CGM for up to 52% of FM protein did not reduce feed intake, and at 39% substitution, turbot maintained optimal growth rate and nutrient utilization. Worsened FCR of fish fed diets containing higher plant protein levels was not associated with a reduced digestibility of ingredients or alterations of gut histology.