Growth is a polygenic and environmentally controlled trait with the most influential genes being those of growth hormone (GH) and insulin-like growth factor-I (IGF-I), as they consist the core of the hypothalamic-pituitary-somatotropic (HPS) axis. As a result, the influence of environmental and dietary conditions on GH and IGF-I gene expression has a great potential in planning to optimize fish health and production, as growth rate of fish is a key factor in aquaculture. In this review, we provide a molecular and physiological overview of GH and IGF-I gene expression as well as all the participating factors regulating GH/IGF-I axis in growth. We further discuss the genetic, dietary and environmental factors affecting growth rate. More importantly, how external factors in combination with diet such as temperature, photoperiod, salinity, pollutants and stocking density affect GH and IGF-I expression and growth rate. In addition, there is evidence that photoperiod effect varies among species by altering the seasonal cycle of GH and IGF-I gene expression and biological cycle of fish. Pollution has also a variable effect on GH, IGF-I and fish growth rate, depending on the pollutant, species and sex. Finally, increased stocking density influences the HPS axis and growth, dependent on species, temperature and the stage of growth. Notably, interactions among these factors were observed by the reported importance, primarily feeding-related stress one of the most important factors, while environmental would add or subtract on each other's optimal or suboptimal conditions in a hierarchy of significance, as further discussed.
