This study evaluated the endpoint biochemical responses and associated physiological status of rainbow trout (Oncorhynchus mykiss) of different body sizes during salinity acclimation under varying temperature and salinity conditions. An L₉ (3⁴) orthogonal design was applied using three nominal body-size levels (100, 250, and 500 g), three temperatures (10, 15, and 20 °C), and three salinities (10, 20, and 30‰) over 60 days. Hepatic superoxide dismutase (SOD), alkaline phosphatase (ALP), and acid phosphatase (ACP) activities and plasma malondialdehyde (MDA), urea nitrogen (BUN), ammonia, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) were measured. For inferential analysis, the three fish sampled from each tank were averaged to obtain one tank-level value, yielding 18 tank-level observations. Tank-level main-effects analysis showed that body size was significantly associated with ALP, ACP, MDA, BUN, ammonia, ALT, and AST. Temperature was significantly associated with SOD, ACP, MDA, BUN, and AST, whereas salinity was significantly associated with MDA, ALT, and AST. MDA and AST were the only endpoints associated with all three factors. The biochemical responses were pathway-specific rather than uniformly directional across treatment combinations. Because interactions could not be independently estimated in the L₉ design, the identified main effects should be interpreted as orthogonal screening associations. Overall, body-size class was associated with the broadest range of endpoints and should be incorporated into future experimental assessments of rainbow trout within the tested salinity range of 10–30‰.

Using a three-factor orthogonal design, we investigated the transcriptomic and histological responses of triploid rainbow trout (Oncorhynchus mykiss) gills across three salinities (10, 20, and 30 ppt) over 60 days, with temperature and body size as background conditions. Results showed that 20 ppt was associated with extensive transcriptional reprogramming, with 3333 differentially expressed transcripts (DETs), whereas 30 ppt induced only 120 DETs; WGCNA further identified salinity-correlated modules involved in ion transport and ribosome biogenesis. Histological alterations remained within physiological ranges with no salinity-dependent severity gradient, as confirmed by semi-quantitative scoring. These findings demonstrate that triploid rainbow trout gills employ distinct molecular strategies at medium versus high salinity. Within the 10–30 ppt range examined, 20 ppt was the salinity at which transcriptional reprogramming was most active, suggesting that this intermediate salinity warrants further investigation as a potential acclimation stage.

The gut microbiome of rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar) is increasingly recognized as a functional interface linking dietary inputs, epithelial barrier integrity, mucosal immunity, environmental stress, disease susceptibility, and antimicrobial-resistance risk in intensive aquaculture. Based on available salmonid studies and relevant evidence from broader fish and aquaculture systems, this review synthesizes current knowledge on salmonid gut microbial composition, nutritional modulation, microbiome–mucosal immune interactions, aquaculture stressors, antibiotic exposure, antibiotic resistance genes (ARGs), mobile genetic elements (MGEs), metagenomics, multi-omics, and emerging microbiome-informed decision-support tools. Current evidence does not support a universally stable single-core microbiota in these species. Instead, community structure is shaped by developmental stage, freshwater–seawater transition, intestinal segment, digesta versus mucosa sampling, diet, temperature, stress, health status, and methodological workflow. Feed substitution and functional additives can remodel the gut microbiota, but these shifts should be interpreted alongside histology, barrier function, metabolic profiles, immune indicators, and disease-resistance phenotypes. Antibiotic exposure may reduce acute bacterial disease pressure while disturbing community structure and potentially enriching ARGs or ARG–MGE associations. Risk assessment should therefore move beyond ARG abundance toward host–ARG–MGE linkage using shotgun metagenomics, metagenome-assembled genomes, long-read sequencing, Hi-C, and externally validated multi-omics models. Machine learning and artificial intelligence approaches may support feature screening, risk stratification, and decision support, but their application in salmonid gut-health management remains at an early stage and requires external validation across sites, production stages, diets, and seasons.

Rainbow trout (Oncorhynchus mykiss) is an important cold-water aquaculture species and a tractable salmonid model for functional genomics. Rapid advances in chromosome-level genome assemblies, genetic variation resources, regulatory annotations, tissue and cell models, controlled challenge systems, and genome-editing technologies have established an increasingly integrated framework for linking genomic variation with measurable phenotypes. Evidence from studies of mucosal immunity, disease-resistance genetics, and environmental stress responses indicates that these resources can improve candidate-gene prioritization and mechanistic interpretation across molecular, cellular, tissue, and whole-fish levels. However, differential gene expression, quantitative trait locus and genome-wide association signals, genomic predictions, and cell-type localization remain largely associative and rarely provide direct evidence of causality. Interpretation is further complicated by salmonid-specific whole-genome duplication, because retained paralogues may exhibit tissue-specific divergence, functional redundancy, and compensatory responses. Further progress will require the integration of pangenomics, regulatory annotation, single-cell and spatial analyses, and paralogue-aware functional perturbation in both cell-based and whole-fish systems. Such integration will strengthen causal inference, clarify genotype–phenotype relationships, and support disease-resistant breeding, healthy aquaculture, and environmental risk assessment.

Rainbow trout (Oncorhynchus mykiss) is a globally important cold-water aquaculture species, yet the molecular events underlying the transition from embryo to free-living larva remain poorly resolved. In this study, eukaryotic reference-based transcriptome sequencing was used to compare genome-wide gene expression profiles between eggs (eyed-egg stage) and newly hatched larvae of O. mykiss. A total of 618 differentially expressed genes (DEGs) were identified, of which 297 were upregulated and 321 downregulated. Gene Ontology (GO) enrichment analysis showed that DEGs were significantly enriched in immune system process, response to stimulus, and biological regulation. Within the immune system process category, 4 DEGs were upregulated and 11 were downregulated in larvae, indicating a more complex remodelling of immune-related gene expression than simple activation. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis identified only Glycolysis/Gluconeogenesis (map00010) as significantly enriched (padj < 0.05). Nitrogen metabolism (map00910) and tryptophan metabolism (map00380) showed enrichment trends but were not statistically significant after multiple-testing correction. Because these pathways contained only a small number of DEGs and displayed mixed regulation, no overall pathway-level up- or down-regulation was inferred. These results suggest a possible metabolic remodelling during the transition from yolk-dependent anabolism to catabolic utilization in larvae. Transcript assembly further identified 134,770 potential novel isoforms and 2902 intergenic transcripts classified as putative novel transcripts, enriching the genome annotation of O. mykiss. Trend clustering indicated coordinated regulation of immune and stress-response genes together with downregulation of amino acid and energy metabolism genes, revealing programmed temporal regulation during this developmental transition. These findings provide foundational data for understanding early development in rainbow trout and candidate molecular targets for health-oriented aquaculture management.

Salinity, temperature and body weight dominate seawater acclimation in rainbow trout (Oncorhynchus mykiss), yet few studies simultaneously explore their main effects and potential correlative interactive patterns in hepatic responses. A 60-day L₉ (3³) orthogonal trial was performed on triploid rainbow trout with three gradients of body weight, temperature and salinity. Hepatic transcriptomics revealed that salinity drove global transcriptional remodeling and high salinity induced far fewer DEGs than medium salinity. WGCNA screened a salinity-positive blue module (r = 0.408, p = 0.0346), while alternative splicing confirmed extensive salinity-dependent post-transcriptional regulation. Semi-quantitative histology showed that 20 °C was associated with more pronounced salinity-caused hepatocellular vacuolation and karyopyknosis in the orthogonal test. Survival statistics indicated that salinity was the only factor with significant main effects (p < 0.05), and the 500 g-10 °C-10 ppt group obtained the highest survival. This multi-omics and histological dataset reveals a suggestive regulatory hierarchy-like pattern: salinity acts as the primary driver, temperature serves as a synergistic amplifier, and body weight plays a minor modulatory role. These findings provide a theoretical basis for developing size-specific salinity acclimation protocols in commercial triploid rainbow trout farming.