Physiologically mediated responses in gilthead sea bream (Sparus aurata) fed sustainable diets: seasonal growth under warming conditions.

BACKGROUND: The Mediterranean Sea is warming approximately 20% faster than the global average, underscoring the urgent need for adaptive aquaculture strategies, including the evaluation of novel feed formulations under environmentally relevant conditions. To address this objective, the present study assessed the performance of gilthead sea bream during the unusually warm summers recently recorded in the Western Mediterranean, where temperatures have increased by approximately 0.1 °C per year over the past decade. The rising temperatures between 2020 and 2025 were associated with progressively shorter production cycles, particularly in the 2022 cohort, where growth rates increased by approximately 10-15%.

METHODS: In this cohort, we conducted a ten-month trial (May 2022-February 2023) to evaluate juvenile performance using a plant-based diet representative of current commercial formulations, along with two experimental diets in which fishmeal and fish oil were partially replaced by either processed animal proteins and salmon oil by-products (PAP diet) or a combination of insect proteins, microbial biomass, and algal oil (ALT diet).

RESULTS: Growth performance remained largely comparable across dietary treatments, and histological analyses of liver and anterior/posterior intestine at three sampling points (July, November, February) revealed no tissue alterations, indicating preserved organ integrity throughout the trial. Despite similar growth outcomes, fish relied on distinct physiological strategies to sustain performance. The ALT diet was associated with reduced basal stress-axis activity, more proactive behavioral responses following confinement, and enhanced intestinal transcriptional activity, suggesting improved physiological responses to aquaculture stressors. Conversely, the PAP diet was associated with a down-regulation of liver and white muscle transcriptomes, which may indicate a potential trade-off that could affect cold-adaptation capacity.

CONCLUSION: Overall, these findings suggest that alternative and low-trophic feed ingredients following circular economy principles can sustain growth and tissue health even under atypically warm conditions, while also modulating stress physiology in ways that may enhance robustness in a rapidly warming Mediterranean. By identifying diet-specific physiological signatures, this study provides actionable insights for the design of aquafeeds better adapted to warming conditions, supporting the transition toward more sustainable and adaptive aquaculture systems, particularly in the Mediterranean region, where accelerating warming demands rapid innovation to maintain productivity, animal welfare, and long-term sector sustainability.