The Feeding Ecology of Invasive Pink Salmon Juveniles in Northern Norwegian Rivers and Their Role as Prey to Native Salmonids

ABSTRACT Increasing numbers of invasive Pacific pink salmon ( Oncorhynchus gorbuscha ) are spawning in northern Norwegian rivers, leading to large numbers of juveniles migrating to coastal waters. This raises concerns about food competition with native salmonids in rivers, although details about the timing and intensity of pink salmon feeding and the characteristics of their freshwater diet remain unclear. Some evidence indicates that pink salmon can also serve as a food source for native salmonids, as do the carcasses of spawned adults, eggs and juveniles. This study investigated largely unexplored aspects of the emergence timing and feeding behaviour of juvenile pink salmon. The prevalence, timing and impact of fish size on freshwater feeding was examined across 10 rivers in northern Norway, using stomach content and stable isotope ( δ 13 C and δ 15 N) analyses to assess diet composition. Similar methods were applied to assess whether native juvenile salmonids feed on pink salmon tissue, such as juveniles, during their migration. Predictions of the emergence times of alevins show that later spawning, thought to cause increased potential negative interactions with native fish, is an unlikely strategy for pink salmon in northern Norway. In three of the rivers, some of the largest free‐swimming pink salmon juveniles sampled in this study (30–35 mm) began external feeding on freshwater invertebrates later in the season (June and July), with larval and pupae chironomids as well as blackfly and mayfly nymphs dominating their diets. Feeding was not observed in all rivers, highlighting the need for further study on the prevalence and intensity of freshwater feeding. Stomach content and stable isotope analyses revealed that some larger native Atlantic salmon and brown trout juveniles consumed pink salmon juveniles, complicating assessments of their overall impact on native salmonids.