From colonization to spatial structuring: tracing invasion dynamics and natal origins using isoscapes in a non-native salmon

Abstract

Biological invasions are key drivers of global ecological change, often involving complex interactions between dispersal, establishment, and local adaptation. Chinook salmon, native to the North Pacific, is an invasive exotic species in Patagonia that causes significant ecological impacts. To contribute to the understanding of invasion mechanisms, we used 87Sr/86Sr isotope ratios to assess natal origin areas and describe migration routes in the Santa Cruz River Basin (Argentina). Water 87Sr/86Sr was measured at 22 monitoring stations during both peak-flow and base-flow seasons to evaluate spatial and seasonal variation. The base-flow dataset revealed four distinct isotopic clusters, showing 100% classification accuracy. Spawning-condition fish were captured in two tributary rivers that serve as spawning areas in the upper basin—the Guanaco and De las Vueltas rivers—and their natal origin was assigned by contrasting otolith core 87Sr/86Sr ratio with the water-based isotopic baseline using maximum-likelihood estimation. All fish captured at the Guanaco River spawning site were assigned to the local cluster, while fish from the De las Vueltas originated from three distinct clusters, suggesting greater natal dispersal. Otolith profiles revealed two divergent migratory patterns between spawning areas, with individuals varying in their residence time between nursery areas and the migratory corridor. These findings demonstrate that 87Sr/86Sr ratios are effective for reconstructing natal origin and migratory history in recently colonized systems. More broadly, they offer insight into key invasion processes—such as natal fidelity, early dispersal, and life-history plasticity—that drive the establishment and spread of non-native species.