Domesticated escapees on the run: the second-generation monitoring programme reports the numbers and proportions of farmed Atlantic salmon in >200 Norwegian rivers annually

Norway is the world's largest producer of farmed Atlantic salmon and is home to similar to 400 rivers containing wild salmon populations. Farmed escapees, a reoccurring challenge of all cage-based marine aquaculture, pose a threat to the genetic integrity, productivity, and evolutionary trajectories of wild populations. Escapees have been monitored in Norwegian rivers since 1989, and, a second-generation programme was established in 2014. The new programme includes data from summer angling, autumn angling, broodstock sampling, and snorkelling surveys in >200 rivers, and >25000 scale samples are analysed annually. In 2014-2017, escapees were observed in two-thirds of rivers surveyed each year, and between 15 and 30 of the rivers had >10% recorded escapees annually. In the period 1989-2017, a reduction in the proportion of escapees in rivers was observed, despite a >6-fold increase in aquaculture production. This reflected improved escape prevention, and possibly changes in production methods that influence post-escape behaviour. On average, populations estimated to experience the greatest genetic introgression from farmed salmon up to 2014 also had the largest proportions of escapees in 2014-2017. Thus, populations already most affected are those at greatest risk of further impacts. These data feed into the annual risk-assessment of Norwegian aquaculture and form the basis for directing mitigation efforts.