Early marine growth and spawning body size vary by year and region in invasive pink salmon (Oncorhynchus gorbuscha)

Abstract Non-native pink salmon ( Oncorhynchus gorbuscha ) have increased rapidly in Norwegian rivers since 2017, yet their marine ecology remains poorly understood. To assess variation in marine growth, we analysed scales from 476 individuals returning to northern (Barents Sea region) and central (Atlantic region) Norway in 2019 and 2021. Clear differences in growth and body size emerged between regions and years, with fish returning in 2019 experiencing the poorest growth and reaching the smallest adult size. Differences in growth histories indicate that pink salmon captured in the two regions may have experienced distinct marine feeding resource conditions, possibly because they followed different migratory pathways, beginning in the first weeks at sea. Approximately one-third of individuals experienced distinct periods of depressed growth during their first spring and summer at sea, and these events were more common among fish returning to central Norway. Mid- to late-season growth during the first year at sea strongly influenced adult size, highlighting the importance of favourable conditions during this period. At the same time, it was shown that growth after the winter could more than compensate for poor growth up until then. The pronounced variability in growth performance suggests that environmental conditions during the marine phase strongly shape adult size and may influence the future invasion potential of pink salmon in the North Atlantic. As this invasive species continues to expand, continued monitoring of growth and improved understanding of their marine ecology and environmental drivers of growth will be essential for anticipating invasion dynamics and impacts on native ecosystems.