Effect of Dietary Incorporation of Yellow Mealworm as a Partial Fishmeal Replacer on Growth, Metabolism, and Intestinal Histomorphology in Juvenile Meagre (Argyrosomus regius).

Efforts have been made to find alternatives to fish meal (FM), as the sustainability of aquaculture depends on it. Insect meal (IM) is a potential candidate to partially replace FM, being more sustainable and economically viable. In this experimental trial, three diets were tested with different yellow mealworm incorporation: a control diet with no IM, a diet with an inclusion of 10% IM (Ins10), and a diet with an incorporation of 20% IM (Ins20). The diets were tested on 10.5 g meagre for 47 days. The results showed that an IM inclusion higher than 10% affected both growth (2.6 vs. 2.2) and FCR (1.5 vs. 1.9) of meagre juveniles. However, this reduction in growth did not result from lower protein retention or changes in muscle fibre area or density. Little differences were observed in the activity of pancreatic and intestinal enzymes except for aminopeptidase total activity which was higher in the control and Ins10 compared to Ins20 (3847 vs. 3540 mU/mg protein), suggesting no limitations in protein synthesis. Also, the alkaline phosphatase intestinal maturation index was higher in the control group compared to the IM groups (437 vs. 296). On the contrary, several differences were also found in the proteolytic activity in the hepatic and muscle tissues of meagre juveniles fed the Ins10 diet. The inclusion of IM had no impact on intestine histomorphology but changes were detected in the enterocytes of fish from control and Ins10 which showed hypervacuolization and nucleus misplacement compared to the Ins20 treatment. Nevertheless, a higher percentage of Vibrionaceae was recorded for meagre fed on the Ins20 diet. Since no signs of inflammation were observed in the distal intestine, this suggests IM incorporation could have had an important impact on intestinal health due to its antimicrobial properties. This is supported by an increase in the haematocrit in the treatments where IM was added (20 to 25%). In conclusion, incorporations of IM at percentages up to 10% do not seem to have a negative impact on meagre performance at this age but can enhance the fish immune system and protection against intestinal inflammation.

Plant-based diets fed to juvenile meagre Argyrosomus regius with low methionine and taurine supplementation led to an overall reduction in fish performance and to an increase in muscle fibre recruitment.

Methionine and taurine are amino acids (AA) that are usually deficient when fish meal is replaced by plant proteins. In this study, three diets were tested in juvenile meagre (initial weight: 13.4 g) for 8 weeks. The D1 diet had 0.2% methionine and 1% taurine supplementation; the D2 and D3 diets had 0.6% methionine and 1% and 2% taurine supplementation, respectively. The results showed that meagre fed the D1 diet had lower specific growth rate (2.2 to 2.5), lower feed efficiency (0.9 to 1.2) and higher food conversion rate (FCR, 1.1 to 0.8) as well as a lower activity of the alanine aminotransferase (ALAT) enzyme. Furthermore, a higher recruitment of muscle fibres (46% compared to 36%) as well as a higher fibre density was observed (1019 compared to 870 fibres mm-2 ). This study shows that meagre requires a sufficient quantity of methionine in plant-based diets to avoid a reduction in fish performance. Furthermore, taurine supplementation in the D1 diet was not able to mitigate the effects of methionine deficiency. A higher taurine supplementation did not improve meagre performance.

Effect of Tryptophan Dietary Content on Meagre, Argyrosomus regius, Juveniles Stress and Behavioral Response.

There are a high number of stressors present in aquaculture that can affect fish welfare and quality. One way of mitigating stress response is by increasing dietary tryptophan. In this study, three diets containing 0.5% (Tript1), 0.6% (Tript2), and 0.8% (Tript3) of tryptophan were tested in 32 g juvenile meagre for 56 days. At the end of the trial, survival, growth, and proximate composition were similar between treatments. Significant differences were found in the plasma parameters before and after a stress test consisting of 30 s of air exposure. Blood glucose levels were higher in the post-stress for all treatments (e.g., 63.9 and 76.7 mg/dL for Tript1 before and after the stress test), and the hemoglobin values were lower in the post-stress of Tript1 (1.9 g/dL compared to 3.0 and 2.4 g/dL for Tript2 and Tript3, respectively). In terms of behavior, three tests were carried out (novel tank diving and shoaling assays, and lateralization test), but no significant differences were found, except for the number of freezing episodes during the anxiety test (1.4 for Tript3 compared to 3.5 and 4.2 for the other treatments). This study suggests that supplementation with dietary tryptophan, particularly in higher dosage (0.8%), can reduce anxiety-like behavior in meagre exposure to acute stress (novel tank). Although the remaining results showed mild effects, they provide some clues as to the potential of this amino acid as a stress mitigator in aquaculture.