Introduction
Marine aquaculture is increasingly challenged by rising water temperatures, particularly in the Mediterranean, where temperatures close to 30 °C may induce chronic thermal stress in European sea bass (Dicentrarchus labrax). Temperature-dependent growth responses in fish have been widely described (Fonds et al., 1992). In parallel, functional ingredients such as inulin may contribute to intestinal homeostasis, microbiota stability and innate immune modulation under intensive farming and heat stress conditions (Srirengaraj et al., 2023; Zhou et al., 2022). The present study evaluated the effects of three diets differing in protein and lipid levels, dietary energy content, and inulin inclusion on growth performance, immunology, histopathology, welfare, and fillet proximal composition of D. labrax reared under chronic thermal stress.
Materials and Methods
A 12-week nutritional trial was conducted in a recirculating aquaculture system using 450 D. labrax specimens with an initial body weight of 113.24 ± 15.82 g. Fish were randomly distributed into 9 tanks (50 fish tank-1) in triplicate groups. Water temperature was progressively increased from 22 to 30 °C during the first week and then maintained at 30.0 ± 1.0 °C. Three diets were tested: i) diet CD, commercial diet (46.5 % protein, 20 % lipid); ii) diet HE, high-energy diet (44 % protein, 24 % lipid); and ii) diet FD, functional diet (46.5 % protein, 18 % lipid) supplemented with 0.5 % inulin. Productive performance was assessed through final body weight, specific growth rate, feed intake, feed conversion ratio and survival. Immunological analyses included reactive oxygen species in plasma, lysozyme activity in plasma and mucus, protease/anti-protease activity and total proteins in mucus, and phagocytic activity in anterior kidney. Histopathology was evaluated in liver and intestine. Welfare assessment included plasma glucose, triglycerides, cholesterol and cortisol, together with hepatic triglycerides, cholesterol, superoxide dismutase, catalase, glutathione peroxidase and malondialdehyde. Final fillet proximal composition was also determined.
Results
Initial body weight was similar among treatments. At the end of the trial, fish fed diet FD showed the highest final body weight (209.23 ± 6.23 g fish-1) and specific growth rate (0.75 ± 0.04 % day-1), significantly higher than fish fed diet HE (191.84 ± 1.93 g fish-1; 0.64 ± 0.01 % day-1; P < 0.05), while diet CD presented intermediate values (204.07 ± 6.46 g fish-1; 0.72 ± 0.04 % day-1). Feed conversion ratio did not differ among treatments (1.82–1.83), and survival was 100 % in all groups. Most immune indicators followed similar trends among diets and did not show significant differences at the final sampling, except for phagocytic activity, which was higher in fish fed diet HE. Welfare assessment revealed significantly higher plasma glucose and triglyceride levels in fish fed diet HE, whereas fish receiving diets FD and CD displayed a more favourable metabolic profile under prolonged exposure to 30 °C. Histopathological evaluation revealed normal lipid vacuolization in liver and preserved intestinal integrity, with no relevant lesions associated with any diet. Final fillet proximal composition was not significantly affected by dietary treatment.
Discussion
The results indicate that increasing dietary lipid to 24 % was not advantageous under chronic heat stress, as the high-energy diet was associated with poorer growth performance and a less favourable metabolic response. In contrast, the functional diet supplemented with inulin provided the best overall response, combining the highest growth with lower plasma glucose and triglycerides levels and maintaining histological safety and final fillet quality. The tendency towards greater antioxidant enzyme activation in fish fed diet HE also suggests higher oxidative pressure in this group. Overall, moderate lipid levels combined with functional ingredients, such as inulin, appear to improve physiological adaptation of D. labrax specimens to prolonged exposure to high water temperature.
Acknowledgment
This project is part of the 2025 Production and Marketing Plan of APROMAR (PO No. 30), in collaboration with the General Secretariat for Fisheries of the Ministry of Agriculture, Fisheries and Food, and co-financed by the European Maritime, Fisheries and Aquaculture Fund (EMFAF).
References
Fonds, M., Cronie, R., Vethaak, A. D., & Van der Puyl, P. (1992). Metabolism, food consumption and growth of plaice (Pleuronectes platessa) and flounder (Platichthys flesus) in relation to fish size and temperature. Netherlands Journal of Sea Research, 29(1–3), 127–143. https://doi.org/10.1016/0077-7579(92)90014-6
Srirengaraj, V., Razafindralambo, H. L., Rabetafika, H. N., Nguyen, H.-T., & Sun, Y.-Z. (2023). Synbiotic agents and their active components for sustainable aquaculture: Concepts, action mechanisms, and applications. Biology, 12(12), Article 1498. https://doi.org/10.3390/biology12121498
Zhou, C., Yang, S., Ka, W., Gao, P., Li, Y., Long, R., & Wang, J. (2022). Association of gut microbiota with metabolism in rainbow trout under acute heat stress. Frontiers in Microbiology, 13, Article 846336. https://doi.org/10.3389/fmicb.2022.846336