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White sea bream (Diplodus sargus) and brown meagre (Sciaena umbra) were caught from the wild in Aegean Sea (Greece), and studied for their proximate, fatty acid and amino acid composition to evaluate their nutritional value for human consumption and their potential as candidate fish species for Mediterranean aquaculture diversification. Both species exhibited very low muscle fat, ranging at 1%. White sea bream was found to have higher muscle eicosapentaenoic (EPA) and docohexaenoic (DHA) contents and total n-3 fatty acids than brown meagre. A superiority of white sea bream in aspects of thrombogenicity was indicated, showing higher atherogenic (0.679) and thrombogenic indices (0.377) comparing to the respective values for brown meagre (0.610 and 0.579). The two-studied species exhibited high-quality protein as food source, with white sea bream showing a higher essential to non-essential amino acid (EAA/NEAA) rate, 0.764 vs 0.704, respectively. The individual to total essential amino acids rates of lysine and leucine were the highest ones for both studied species, while no significant differences were observed among them.

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.

Plant-based proteins are increasingly being used in aquafeeds, but one of the limitations is taurine deficiency that can be especially detrimental for carnivorous fish. In this study, taurine supplementation in high plant protein diets (low fish meal, 15%) was investigated on the growth performance and fillet organoleptic characteristics of European seabass ( Dicentrarchus labrax ), juveniles (78 ± 0.4 g). Five diets were formulated to contain 0 (C−), 5 (T5), 10 (T10) or 20 (T20) added taurine (g/kg), while a control diet (C+) included two-fold higher amount of fishmeal (30%) with no taurine supplementation. Seabass fed the T20 or C+ diet showed similar growth, which was significantly higher compared to those in the C-treatment. Histological examination of the muscle, liver and intestine were similar among treatments. No effect on spoilage bacterial growth or production of total volatile basic nitrogen was observed. Taurine supplementation significantly reduced fillet drip loss, while the fillets of European seabass fed the T10 and T20 diets showed significant higher and lower hardness and adhesiveness values, respectively. Moreover, fillet chewiness was also found to be significantly higher in fish fed the T20 diet compared to C+ and C− diets. Overall, it seems that high dietary taurine supplementation acted as a growth promoter and concurrently improved significantly the postharvest quality characteristics of seabass, which may be attributed to its role in maintaining cell membrane integrity and permeability.

As the demand for fish food increases globally, there is a need for the aquaculture industry to expand its farmed fish species base in order to increase productivity and sustainability. To unlock a species potential for aquaculture, the nutrient requirement of the fish in terms of protein, amino acids, lipids, minerals and vitamins must be well understood to ensure efficient nutrient utilization and rapid growth. The new development in aquafeed production has witnessed commercial fish feed industries incorporating large quantities of plant-based ingredient in fish feed formulation (Lee et al., 2020). However, the main constraint when alternative proteins of plant origin are used in the fish diets is the limiting content in some essential amino acids (EAA), mainly lysine and methionine. Lysine is