Total fishmeal replacement by defatted Tenebrio molitor larvae meal induces alterations in intermediary metabolism of European sea bass (Dicentrarchus labrax)

Tryptophan-supplemented diet modulates the metabolic response of European seabass (Dicentrarchus labrax) juveniles reared under space-confined conditions and submitted to acute inflammation

The energetic costs of being in stressful conditions require the involvement of hormones associated with metabolic support, which may also influence immune function. The present work aimed to explore the links between tryptophan nutrition and metabolic responses in European seabass (Dicentrarchus labrax) held under space-confined conditions, and subsequently submitted to an immune challenge. To study that, two dietary treatments were evaluated, i.e. control diet (CTRL) and CTRL-based diet supplemented with tryptophan (0.3%; TRP) to fish under space-confinement conditions (10 kg/m3) or not (5 kg/m3). Dietary treatments were offered for 15 days after which fish were intraperitoneally injected (i.p.) with Photobacterium damselae piscicida. Liver was sampled before the immune challenge (at end of the nutritional trial, 0 h) and at 4, 24, 48 and 72 h post-injection. Fish-fed TRP exhibited distinct metabolic profiles compared to those fed CTRL diets, particularly in energy metabolism and stress response. CTRL-fed fish in space-confined conditions showed a gradual reduction of lipid oxidative enzyme activity post-injection, pointing to acute stress-induced lipid catabolism feedback. In contrast, stressed fish fed TRP under same rearing conditions presented reduced glucose levels and cortisol production but unchanged 3-hydroxiacil-CoA dehydrogenase (EC 1.1.1.35, HOAD) activity patterns, suggesting an inhibitory and modulatory role of tryptophan in stress response. Discriminant analysis revealed that fish fed TRP under space-confined conditions resembled fish CTRL-fed under non-stressful conditions. These findings suggest that tryptophan dietary supplementation for stressed fish modulates their metabolic responses and potentially mitigates the negative effects of rearing stressful conditions after subsequent acute stress induced by an immune challenge.

Effects of Dietary Plant Protein Replacement with Insect and Poultry By-Product Meals on the Liver Health and Serum Metabolites of Sea Bream (Sparus aurata) and Sea Bass (Dicentrarchus labrax)

The liver health of Gilthead sea bream and European sea bass, fed with fish meal-free diets, including various proportions of plant proteins, as well as insect and poultry by-product meals, was investigated through biochemical and histological analyses using a new liver index (LI) formula. Four isoproteic (45% Dry Matter, DM) and isolipidic (20% DM) diets were compared, including a plant-based control diet (CV) and three other test diets, in which 40% of a plant protein-rich ingredient mixture was replaced with meals from Hermetia illucens (H40) or poultry by-product (P40) alone, or in combination (H10P30). The trials lasted 12 and 18 weeks for sea bream and sea bass, respectively. The results obtained thus far highlighted species-specific differences in the physiological response to dietary changes. In sea bream, the biochemical and histological responses suggest favorable physiological and liver health statuses, with higher serum cholesterol (CHO) and triglyceride (TAG) levels, as well as moderate hepatocyte lipid accumulation, with the H10P30 diet compared to the CV (p < 0.05). In sea bass, all diets resulted in elevated serum TAG levels and lipid accumulation in the liver, particularly in fish fed the P40 one (p < 0.05), which resulted in the highest LI, coupled with a higher frequency of severe lipid accumulation, hypertrophy, cord loss, peripheral nuclei displacement, and pyknosis. In conclusion, sea bream adapted well to the test diets, whereas sea bass exhibited altered hepatic lipid metabolism leading to incipient liver steatosis, likely due to the high lipid contents of the diets, including the insect and poultry meals. The LI formula developed in this study proved to be a reliable tool for assessing the effects of dietary changes on the liver health of sea bream and sea bass, consistent with biochemical and histological findings.

Fishmeal, plant protein, and fish oil substitution with single-cell ingredients in organic feeds for European sea bass (Dicentrarchus labrax)

Single-cell ingredients (SCI) are considered promising nutrient sources which are produced using environmentally friendly biotechnological processes. The aim of the current study was to evaluate the replacement of fishmeal, plant protein sources, and fish oil with SCI in organic feeds for European sea bass (Dicentrarchus labrax). Bacterial protein, yeast protein, and microalgae were used to replace fishmeal trimmings, soya bean meal, and fish oil from trimmings. Triplicate groups (30 fish per replicate) of European sea bass (14.4 ± 2.4 g) were fed the experimental diets for 71 days. The results showed that the incorporation of SCI at all levels of inclusion significantly enhanced nutrient digestibility. Additionally, growth performance parameters were not affected by SCI inclusion, exhibiting similar or improved values. Moreover, a tendency for improved anterior and posterior gut structure was observed and a significant increase of lysozyme activity at the two highest inclusion levels of SCI was determined. Overall, the results showed that the inclusion of SCI at 15% (bacterial: yeast: algae-9.4: 4.7: 1) is possible without compromising any of the parameters evaluated. According to these findings, a higher substitution of fishmeal trimmings, plant protein sources, and fish oil from trimmings with SCI in organic diets for European sea bass (D. labrax) can be further evaluated in future studies.