Introduction
The rapid expansion of the aquaculture industry, combined with the ongoing effects of climate change, has increased the exposure of farmed fish to environmental stressors, among which temperature fluctuations represent one of the most critical challenges. Elevated water temperatures can disrupt metabolic balance, impair immune function, and promote the overproduction of reactive oxygen species (ROS), leading to oxidative stress and cellular damage.
In this context, dietary strategies based on natural compounds have gained increasing attention. Polyphenols, a diverse class of plant-derived bioactive molecules, are known for their strong antioxidant and anti-inflammatory properties and their ability to modulate immune responses. Grape by-products from the wine industry represent an important and sustainable source of such polyphenols, offering a valuable opportunity for the valorisation of agro-industrial waste through their conversion into functional feed ingredients for aquaculture (Quagliardi et al. 2024).
The inclusion of polyphenol-rich additives in aquafeeds has been associated with improved growth performance, enhanced antioxidant capacity, and increased resistance to stress in several fish species (Mansoori et al. 2024; Ahmadifar et al. 2012; Torricelli et al. 2024). However, the efficacy of different polyphenol sources and their optimal dietary concentrations, particularly under thermal stress conditions, remains to be fully elucidated.
Therefore, the present study aimed to evaluate the effects of different concentrations of two grape-derived polyphenol-based additives on growth performance, oxidative status, and selected biochemical parameters in rainbow trout under thermal stress conditions.
Materials and Methods
Five experimental diets were formulated: four supplemented with two polyphenol-based additives at two inclusion levels, and a control diet without additives.
A total of 450 rainbow trout (Oncorhynchus mykiss) (initial weight: 30 g) were randomly distributed into experimental groups and fed for 60 days. Fish were hand-fed to apparent satiation twice daily, and water temperature (13.0 ± 1 °C) and oxygen levels were continuously monitored.
At the end of the trial, growth performance (FBW, SGR, FI, FCR, survival) and morphometric and nutrient efficiency indices (CF, VSI, HSI, MFI, PER, GPR, LER, GLE) were evaluated. Samples were collected to assess carcass composition, liver histology, gene expression of inflammatory markers, plasma biochemistry, liver metabolomics and oxidative stress biomarkers (total antioxidant capacity (TAC), glutathione (GSH), antioxidant enzyme activity (CAT), lipid peroxidation (TBARS), protein carbonylation (PC), and mitochondrial ROS production (mtROS)).
Fish were then subjected to a thermal stress challenge, with temperature increased to 20 °C (2°C day-1) and maintained for 15 days. At the end of the stress period, survival and growth parameters were recorded, and sampling was repeated to assess physiological and biochemical responses.
Results and Discussion
Fish fed diets supplemented with polyphenol-based additives are expected to show growth performance, FI, and FCR comparable to or slightly improved compared to the control group, particularly at moderate inclusion levels. Recent studies have reported enhanced growth and feed utilization in rainbow trout fed polyphenol-rich diets, likely due to improved metabolic efficiency and antioxidant status (Mansoori et al. 2024).
Oxidative stress biomarkers are expected to be significantly affected by dietary treatments. Fish receiving polyphenol-supplemented diets are likely to exhibit higher TAC, increased GSH, and enhanced CAT activity, along with reduced lipid peroxidation, protein carbonylation, and mitochondrial ROS production. These responses are consistent with findings highlighting the antioxidant role of polyphenols in fish (Ahmadifar et al. 2012).
Gene expression analyses are expected to show modulation of immune and inflammatory markers, with a potential reduction of pro-inflammatory cytokines and improved immune status, as recently observed in fish fed plant-derived bioactive compounds (Torricelli et al. 2024).
Histological observations are likely to reveal generally preserved intestinal and hepatic structures, with possible improvements at moderate inclusion levels and mild alterations at higher doses, reflecting metabolic adaptation.
Following the thermal stress challenge, fish are expected to show increased oxidative stress and physiological disturbance due to elevated temperature, as widely reported for rainbow trout (Hosseinpour et al. 2024; Yousefi et al. 2024). However, fish fed polyphenol-enriched diets are likely to exhibit improved resilience, with higher survival rates and better maintenance of antioxidant defences.
Overall, polyphenol supplementation is expected to enhance antioxidant capacity and improve the resilience of rainbow trout under thermal stress conditions.
Conclusion
Overall, the trial is expected to show that moderate inclusion levels of polyphenol-based additives can enhance antioxidant status and support immune responses in rainbow trout without negatively affecting growth performance. These effects are likely to be more evident under thermal stress, improving fish resilience and reducing oxidative damage.
Given the impact of climate change on aquaculture, particularly rising water temperatures, polyphenols represent a promising strategy to improve fish robustness and support more sustainable production systems.
References
Ahmadifar, E. et al. (2021) "Benefits of dietary polyphenols and polyphenol-rich additives to aquatic animal health: an overview." Reviews in Fisheries Science & Aquaculture 29.4: 478-511.
Hosseinpour, F. et al. (2024) "Acclimation to higher temperature and antioxidant supplemented diets improved rainbow trout (Oncorhynchus mykiss) resilience to heatwaves." Scientific Reports 14.1: 11375.
Mansoori, A. et al. (2024) "Polyphenol-rich extracts enhance growth, immune function, and antioxidant defense in juvenile rainbow trout (Oncorhynchus mykiss)." Frontiers in Nutrition 11: 1487209.
Mousavi, Shalaeh, et al. (2020) "Administration of grape (Vitis vinifera) seed extract to rainbow trout (Oncorhynchus mykiss) modulates growth performance, some biochemical parameters, and antioxidant-relevant gene expression." Fish physiology and biochemistry 46.3: 777-786.
Quagliardi, M. et al. (2024) "Use of grape by-products in aquaculture: New frontiers for a circular economy application." Heliyon 10.5.
Torricelli, M. et al. (2024) "Gene expression study in gilthead seabream (Sparus aurata): effects of dietary supplementation with olive oil polyphenols on immunity, metabolic, and oxidative stress pathways." International Journal of Molecular Sciences 25.22: 12185.
Yousefi, M. et al. (2024) "Dietary thymol supplementation promotes antioxidant responses and thermal stress resistance in rainbow trout, Oncorhynchus mykiss." Animals 14.20: 2988.