Assessing the extraordinary measures used to counteract invasion of pink salmon into the North Atlantic

Pink salmon is an invasive fish species to the North Atlantic Ocean and Barents Sea regions, spreading from intentional releases in north-west Russia.We assess the removal methods used in 94 Norwegian rivers in 2023.In total, 250 000 adult pink salmon were caught when returning from the ocean feeding migration and removed before they were able to spawn.Successful removals show that it is possible to construct bank-to-bank traps to prevent pink salmon from entering even relatively large rivers -despite thousands and even tens of thousands of pink salmon approaching single rivers within a few weeks.Pink salmon by far outnumbered native salmonids, with 11 times more than Atlantic salmon, 35 times more than sea trout, and 13 times more than Arctic char entering the rivers based on trap catches.Traps were used in 50 rivers: resistance board weirs in six rivers, picket weirs in 16 rivers, and wire mesh traps in 28 rivers.Resistance board weirs and picket weirs were easier to keep clean and operate and ensured better fish welfare than wire mesh traps.Timing of river entry of pink salmon and native salmonids overlapped.Native salmonids were sorted out and released alive upstream of the traps.Immediate mortality of native salmonids caught in all three type traps was low (0.1-0.5% of captured fish).Northern Norway is the gateway for spread of pink salmon into the North Atlantic, and future distribution of pink salmon depends on the success of measures in this area.Experiences gained can be used to further develop a comprehensive strategy for removal of pink salmon to improve construction and operation of effective traps with complete coverage in rivers both in Norway and internationally.Improved trap design is also valuable when using large weirs in rivers for other monitoring and management purposes.