Assessment across life stages reveals superior habitat suitability in reintroduced historical habitats for an endangered salmon species

Abstract Accurate assessment of habitat suitability is crucial for the conservation of endangered species. However, conventional measures often overlook the complexity of ecological requirements across life stages and how different causes of local extinctions (e.g. habitat degradation versus overharvesting) can lead to misinterpretation of optimal habitat conditions. We investigated how life‐stage specific ecological requirements and historical causes of population decline affect habitat suitability assessment in the critically endangered Formosan landlocked salmon ( Oncorhynchus masou formosanus ). We compared Chichiawan Creek (a remnant population) with Hehuan Creek (a reintroduced population) using field surveys and in situ experiments. We quantified redd density, nest site selection and hatching rates. We also estimated multidimensional niche hypervolumes based on five key aquatic environmental parameters (water temperature, conductivity, pH, dissolved oxygen and stream velocity) for the nesting and hatching stages, and assessed the influence of these factors on reproduction and early development. Habitat suitability, defined as the relationship between environmental variables and species' fitness, was higher in Hehuan Creek than Chichiawan Creek for both the nesting and hatching stages. The hatching hypervolume was substantially smaller (49.5%) than the nesting hypervolume, indicating more stringent environmental requirements for successful egg development compared to adult nesting behaviour, and representing a critical ecological bottleneck for conservation planning. Redd density increased with stream velocity but decreased with water conductivity and temperature. Female salmon selected nest sites with lower stream velocities, more neutral pH values, lower conductivity and higher dissolved oxygen levels. Hatching success was most strongly associated with lower water conductivity, temperature and moderate stream velocity, while indicators of eutrophication levels (dissolved oxygen and pH) had less influence on egg development. Synthesis and applications . Our results challenge the assumption that remnant habitats represent optimal conditions for endangered species, especially when historical local extinctions were caused by overharvesting rather than habitat degradation. This study provides a framework for comprehensive conservation strategies that integrate historical distribution data with quantitative assessments of stage‐specific ecological requirements for salmonids. This approach will enable conservation practitioners to better identify suitable areas for protection and reintroduction, improving endangered species management under rapid environmental change.