In recent decades, populations of the non-native pink salmon Oncorhynchus gorbuscha have successfully expanded their range in the White Sea basin. Understanding the physiological characteristics of pink salmon in these novel aquatic ecosystems is crucial for assessing potential risks to native ichthyofauna, including other salmonid species. Timing of pink salmon entry from the sea into the rivers often varies during spawning migration. We selected two sampling time points of the species entry into the Keret River after crossing the critical salinity barrier from the sea to the river: in mid-July (the peak) and one and a half months later, at the end of August (termination/end). To evaluate steroidogenesis and energy exchange in fish, we measured the individual concentrations of sex steroids and thyroid hormones in the blood serum. Additionally, levels of glucose and cortisol were quantified to assess fish osmotic stress after crossing the critical salinity barrier (from the sea to the river). We observed that larger males and females predominantly migrated to the river at the end of the spawning season. During migration termination, both sexes exhibited a four- to five-fold decrease in estradiol-17β concentrations compared to fish that had passed upstream earlier, indicating the completeness of gonad development. It appears that gonad development in later spawners nearly completes in brackish water. The observed reduction in triiodothyronine levels by three to four times from July to August suggests differing energy mobilization and utilization processes between the two groups of spawners. Earlier and later spawners also differed in their stress responses, as the former exhibited lower levels of cortisol and glucose than the latter. Such biochemical heterogeneity likely characterizes the successful adaptation of non-native fish to new habitats, which is a critical factor that needs to be considered when assessing their impact on aquatic ecosystems.