Effects of high‐level fishmeal replacement by plant proteins supplemented with different levels of lysine on growth performance and incidence of systemic noninfectious granulomatosis in meagre (Argyrosomus regius)

The potential use of plant protein (PP) blends (soybean, wheat, rapeseed, corn gluten and wheat gluten) in the diet of juvenile meagre (Argyrosomus regius) was investigated at increasing levels to replace fishmeal (FM) (33% and 56%) in six isonitrogenous (480 g/kg) and isoenergetic (22 MJ/kg) diets, which were supplemented with crystalline lysine. Meagre juveniles (36 ± 0.6 g initial weight) were reared in triplicate for 60 days at 19.4 ± 2.4°C to evaluate their growth performance, feed utilization parameters, body proximate composition and the prevalence of systemic noninfectious granulomatosis. Results indicated that there was no significant difference (GLM ANOVA, p > 0.05) in growth performance and feed utilization parameters in meagre fed the diet containing 300 g/kg FM (33% FM replacement) compared to the control group (450 g/kg FM inclusion), although a trend showing inferior body gain and feed conversion ratio was observed. However, higher levels of FM replacement (56%) by PP blends (200 g/kg FM inclusion) significantly impaired growth performance, feed conversion and protein efficiency rates (p < 0.05), which may be linked to a decrease in feed intake and/or reduced levels of bioactive compounds or other micronutrients present in FM. On the other hand, increasing dietary lysine levels from 25 to 29 g/kg in the diets containing the same PP content and 200 g/kg inclusion of FM significantly improved growth performance in juvenile meagre. The replacement of FM did not affect lipidosomatic and hepatosomatic indexes in any of the experimental groups evaluated (p > 0.05). The aetiology of granulomatosis found in different tissues was not due to the presence of bacteria, as no bacterial structures were detected in histological slides when samples were stained with the Gram, Ziehl‐Neelsen and Fite‐Faraco staining. The presence of chronic systemic noninfectious granulomatosis was observed in meagre from all the experimental groups regardless the level of FM replacement by PP blends, indicating that the onset and progression of granulomatosis occurred insidiously at earlier life stages of meagre and persisted at variable levels thereafter. The liver and kidney were found to be the most severely affected tissues.