Abstract
<title>Abstract</title> <p> Rainbow trout ( <italic>Oncorhynchus mykiss</italic> ) is a globally significant salmonid and primary aquaculture model, yet molecular mechanisms governing growth across its full developmental spectrum remain poorly characterized. This study investigated spatio-temporal expression of five somatotropic axis genes <italic>gh</italic> , <italic>igf-1</italic> , <italic>mstn</italic> , <italic>ghrl</italic> , and <italic>lep</italic> across six ontogenetic stages from fertilized egg to maturity in brain, muscle, liver, and gut. <italic>GH</italic> was brain-dominant throughout ontogeny, peaking embryonically due to maternally deposited transcripts, yet showed a paradoxical negative correlation with body size at maturity, consistent with IGF-1-mediated negative feedback and metabolic reallocation toward reproduction. <italic>IGF-1</italic> and <italic>lep</italic> peaked synchronously in liver and muscle during fingerling stages, with hepatic <italic>igf-1</italic> fold-changes reaching 4.5, identifying these as primary windows of somatic growth investment. <italic>MSTN</italic> displayed a parabolic muscle expression profile, co-regulating hyperplasia and hypertrophy alongside <italic>igf-1</italic> as a homeostatic developmental brake. <italic>GHRL</italic> surged in gut at the advanced fingerling stage, marking gastrointestinal endocrine establishment and transition to exogenous foraging. PCA resolved a dominant coordinated somatic growth axis ( <italic>igf-1</italic> , <italic>ghrl</italic> , <italic>lep</italic> , <italic>mstn</italic> ) explaining 67% of transcriptional variance, with <italic>gh</italic> loading independently, confirming functional decoupling of upstream from downstream effectors. These findings establish a transcriptional baseline offering actionable targets for precision feeding regimes and marker-assisted selection in aquaculture. </p>