Pink salmon in Norway: the reluctant invader

In 2017, Norway experienced an invasion of the Pacific salmonid pink salmon ( Oncorhynchus gorbuscha ) in numbers never before seen in rivers all along the coast. Significant numbers were also caught in other parts of northwestern Europe. Pink salmon has been observed in variable numbers in Norwegian waters in the summer and autumn of most years since 1960, after the first successful Russian introduction of pink salmon fry in rivers draining to the White Sea in northwest Russia in 1959. With the exception of 1960, pink salmon have been most abundant in odd years, based on the odd-year broodline of the 2-year life salmonid. Even-year fish has generally been less abundant, but in recent years, significant numbers of this broodline have also been caught. In this paper we review the available information on pink salmon in Norwegian rivers and discuss (1) to what extent the presence of this species in Norway has been driven by Russian introductions and natural reproduction in Russian, and lately in Norwegian, rivers, and (2) the likelihood of reproducing populations of pink salmon being established in more Norwegian rivers. Considering the continued propagule pressure in terms of adult pink salmon entering and spawning in Norwegian rivers, it is puzzling that self-propagating populations apparently only have been established in some rivers in the northernmost part of the country. The potential impact of pink salmon on native salmonids and river ecosystems is discussed briefly. Extensive research is required to understand the mechanisms that determine the fate of pink salmon as an alien species, and specifically the possible impact of pink salmon on native salmonids and the environment in the recipient rivers and in the ocean.