Integrative analysis of physiology, transcriptomics, and metabolomics suggests body-weight-dependent alterations in triploid rainbow trout (Oncorhynchus mykiss) under natural summer thermal condition.

Body size is an important property of fish, and the body weight is one of the major elements influencing fish metabolism and potentially the response to environmental stress such as high temperature. To investigate the size-dependent adaptation to summer heat stress, we performed an integrative analysis of serum biochemical parameters, transcriptomes, and metabolomes in triploid rainbow trout (Oncorhynchus mykiss) reared under naturally elevated summer temperatures (20 ± 1 °C). Individuals were categorized by body weight into small- (SRT, 1.02 ± 0.19 kg), middle- (MRT, 2.19 ± 0.27 kg), and large-sized (LRT, 3.38 ± 0.38 kg) groups. Our findings suggest that body weight was associated with distinct physiological and molecular profiles under a common thermal challenge. At the biochemical level, SRT fish exhibited a significantly higher superoxide dismutase (SOD) activity and lower protein carbonyl levels than LRT fish (P < 0.05), indicating that smaller individuals maintain superior antioxidant capacity and experience less oxidative damage, and they were associated with cellular maintenance over rapid growth under thermal stress. Transcriptomic analysis identified vdac3, plin1, lpl, and adipoqa as key genes differentially expressed across weight groups. Notably, plin1 and lpl both involved in lipid storage and mobilization, they were significantly upregulated in LRT fish, pointing to enhanced fat deposition capacity in larger individuals. Whereas vdac3 showed a decreasing expression trend with increasing body weight, hinting at reduced mitochondrial stress resilience in larger fish. Metabolomic profiling further revealed that the weight-dependent metabolic shifts were reflected in distinct pathway enrichment patterns, each carrying specific biological implications. Specifically, the enrichment of amino acid biosynthesis pathways (LRT vs. MRT) indicates enhanced protein turnover and anabolic activity in larger fish, supporting their continued somatic growth. The serotonergic synapse pathway (LRT vs. SRT) suggests that neuroendocrine regulation differs with body size, potentially reflecting altered stress-coping mechanisms in large versus small individuals. Meanwhile, fatty acid metabolism and degradation (MRT vs. SRT) point to differential energy utilization strategies, with medium-sized fish exhibiting distinct patterns of lipid catabolism compared to their smaller counterparts (P < 0.05). Integrative correlation analysis reinforced these interpretations, revealing significant associations between key lipid-metabolism genes (plin1, lpl) and metabolites such as arachidonic acid and 3-methyl-2-oxobutanoic acid-strong evidence that fat metabolism is a central, weight-dependent process under thermal stress. In summary, our multi-omics study, conducted under a common natural thermal challenge, suggests that smaller triploid rainbow trout were associated with greater antioxidant and immune defenses, whereas larger individuals were characterized by enhanced lipid metabolic and fat deposition capacities, revealing a weight-dependent divergence in adaptation strategy to elevated temperature.