Introduction
Aquaculture sustainability is increasingly challenged by climate change, with projections indicating more frequent marine heatwaves and prolonged periods of thermal stress (Haberle et al., 2024). While elevated temperatures can enhance growth within species-specific thermal optima (e.g., up to 28°C for gilthead sea bream), exceeding these thresholds disrupts metabolism, impairs immune function, and destabilizes gut homeostasis, resulting in tissue damage and shifts in microbiota composition. Functional dietary additives have shown promise in mitigating these effects; however, the potential of alternative protein sources and novel feed formulations remains unclear (Fernandes et al., 2024). This study evaluated the performance of three successive gilthead sea bream stocks over a four-year temperature profile that mirrors natural climate-driven thermal fluctuations, assessing the long-term effects of alternative protein sources and novel fish feed formulations on growth performance, histopathology, blood metabolites, multi-tissue (liver, white skeletal muscle, anterior intestine, head kidney gene expression) and resilience to environmental stressors.
Materials and Methods
For the study, gilthead sea bream (Sparus aurata ) juveniles from Mediterranean origin (Avramar , Burriana, Spain) were raised at the Institute of Aquaculture Torre de la Sal (Spain) under natural photoperiod and temperature conditions. The study spanned three consecutive production cycles (Trial 1: April 2020–December 2021, Trial 2: April 2021–December 2022, Trial 3: May 2022–December 2023). In Trials 1 and 2, attention was focused on growth performance with fish fed standard commercial diets (Biomar , Spain) . In trial 3, three experimental diets were tested. The control diet (CTRL) mimicked current commercial formulations with FM, FO and plant ingredients as the main dietary protein and lipid sources. The other two experimental diets were partially or totally devoid of FM with plant ingredients and poultry meal as the main FM replacer in the PAP diet , while the main FM replacer for the named ALT diet were plant proteins together with insect proteins and microbial biomass. Otherwise, salmon-oil by-product and DHA algae oil were used as alternative oils in PAP and ALT diets, respectively. Across the trial, blood and tissue samples were collected in summer, autumn, and winter f or biochemical, histological and gene expression analyses, using customized tissue-specific arrays with 106 selected markers of growth, lipid metabolism, antioxidant defence, and immunological status. In parallel, the effect of the nutritional background on stress susceptibility and resilience was assessed in a crowding stress test (changing the density from 16 to 48 kg/m3 ) by the individual tracking of physical activity and respiratory frequency, using AEFishBIT biosensors externally implanted to the operculum (Calduch-Giner et al., 2022).
Results and Discussion
The temperature always represents a factor that strongly affects metabolic activity and growth performance of ectotherm animals. Accordingly, the rise of incidence of intense marine heatwaves and heat stress events are worsening risks for aquaculture, especially in the Mediterranean basin. However, the present results exhibited a contrary and promising trend for gilthead sea bream, because the achieved growth align with the historical water temperature records in the Mediterranean Sea along the summers of 2022 and 2023, reaching the commercial size in a 1-2 months shorter period (Fig. 1A). This important finding highlights that a correct feeding schedule is able to promote an enhanced g rowth with the water temperate rise regardless of the use of different fish feeds. Certainly, the achieved growth with the three tested diets was almost undistinguishable , and no signs of histopathological damage were found in both liver and intestine in any experimental dietary group . However, going deeper with the analyses, the transcriptomic profiles revealed different temporal metabolic strategies that clustered together different fed fish for a given particular tissue. Thus, liver and muscle gene expression patterns cluster ed together CTRL and ALT groups at the end of trial (Fig. 1B) , mostly due to the confluent expression pattern of markers of lipid metabolism and mitochondrial respiratory chain . In contrast, the differential expression pattern of immune-related genes of t he intestine clustered together CTRL and PAP fed fish (Fig. 1C). These different gene expression features also shaped a different stress response with a more reactive behaviour (increased activity) immediately after stress confinement in ALT fed fish, which rendered an enhanced activity/respiratory quotient with 2-3-fold lower plasma cortisol levels under standard basal conditions (Fig 1D). According ly, it was hypothesized that the ALT feed would avoid a chronic stress condition (improved welfare and wellbeing) , which supports at the same time a more reactive response when fish are stress challenged .
Concluding Remarks
This work demonstrates how a correct management of animals during the production cycle , combined with adequate feed formulations, can contribute not only to mitigate the negative impact of climate change, but also to take advantage of a scenario of a global warming. This feature was achieved with different feed formulations, although gene expression analysis and behavioural tests allows different physiological trade-offs that need to be considered in depth to ensure a customized and more efficient aquaculture.
Funding
This study forms part of the ThinkInAzul program and was supported by MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Generalitat Valenciana (THINKINAZUL/2021/024, IP1: J. Pérez-Sánchez)
References
Calduch et al., (2022 ). Frontiers in marine Science 9:854888 ; Haberle, I. et al. (2024). Aquaculture, 578, 740052 ; Fernandes, A. M. Et al. (2024). Animals, 14(15), 2166.