Potential migration routes for the alien species, pink salmon (Oncorhynchus gorbuscha) : investigating access in west and south coast rivers of Sweden

Abstract: Invasive alien species are one of the main drivers of ecosystem change globally, presenting challenges to biodiversity that take many forms Pink salmon, Oncorhynchus gorbuscha, has recently been observed on the Swedish west coast where its presence is the result of secondary spreading from populations introduced to the White and Barents Seas in northwestern Russia. In Sweden, O. gorbuscha is recognised as having high invasion potential but is not presently classed as invasive. Potential problems arising from an established pink salmon distribution on the Swedish west and south coasts are speculated to relate to instances where habitat use by the alien species overlaps with that of Sweden’s native salmonids. Aiming to facilitate further research into pink salmon distribution, this study investigates 29 rivers on Sweden’s south and west coasts with regards to accessibility for migrating pink salmon looking to spawn. Mapping of potential migration routes for pink salmon was performed using GIS to explore, extract, analyse and process existing data on migratory barriers, fishways, Atlantic salmon distribution and pink salmon observations. The resulting maps were analysed alongside data on physical characteristics of each river drainage basin and existing literature on pink salmon to comment on potential impacts of the alien species on native ecosystems. The study concludes that the most severe potential impacts are posed by pink salmon migrating further upstream than expected. This could give rise to competition between pink salmon and indigenous salmonids for optimal spawning grounds as well as exacerbate nutrient loading in rivers. Furthermore, factors contributing to vulnerability of each river drainage basin to pink salmon establishment were recognised as highly complex. A first step to assessing vulnerability was identified as prioritising either the native salmonid populations or the RDB’s ecosystem. The prospect of intentionally introduced pink salmon establishing in Sweden highlights other challenges forced upon natural systems by humans. It is however, quite paradoxically, man-made barriers in the form of hydropower dams that currently constitute the largest preventative measure to pink salmon spreading in most river drainage basins.