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

Abstract

Norway is the world’s largest producer of farmed Atlantic salmon and is home 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 >25 000 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.

Swimming with the fishes: validating drift diving to identify farmed Atlantic salmon escapees in the wild

“…Drift diving has only recently (since 2014) been included as a standard method used in the Norwegian monitoring program of farmed escaped salmon (Glover et al 2019), supplementing the traditional method based on validation of fish gathered from angling (Diserud et al 2019, Glover et al 2019). Compared to angling-based methods of assessing the number and proportions of farmed escaped salmon (e.g.…”

“…Retention of farmed fish within net pen facilities has greatly improved with time (Jensen et al 2010), yet escape events still occur at levels that pose a significant threat to wild populations of salmon (see Wringe et al 2018 for a recent example). To circumvent the ecological ramifications of escaped farmed salmon, various stakeholders have increased awareness and efforts to monitor the presence of escapees in wild populations (Glover et al 2019) and to mitigate the effects that can occur between wild and farmed salmon.…”

“…Introgression of domesticated escapees is considered to be one of the most significant threats to native S. salar populations in countries where salmon aquaculture is practiced (Taranger et al 2015, Forseth et al 2017. This phenomenon has been documented in the wild for decades (Diserud et al 2019, Glover et al 2019, and extensive introgression and hybridization has been observed in areas where aquaculture and wild S. salar overlap (Clifford et al 1998, Glover et al 2013, Karlsson et al 2016, Wringe et al 2018. While consequences arising from escaped salmon are likely to be population-dependent (Fraser et al 2010, Glover et al 2013, Heino et al 2015, Wringe et al 2018, the offspring of domesticated salmon, and their hybrids with wild salmon, display reduced survival in the wild (McGinnity et al 1997, Fleming et al 2000, Skaala et al 2012.…”

“…Drift diving is one of the main sampling methods used to quantify the number and proportion of escaped farmed S. salar throughout rivers in Norway (data from 133 rivers included in the national monitoring program in 2017; Glover et al 2019). In order to identify farmed salmon escapees through drift diving, it is necessary to be able to distinguish them from wild conspecifics by identifying variations in several phenotypic traits (e.g.…”

Model-based evaluation of the genetic impacts of farm-escaped Atlantic salmon on wild populations

“…hybridization) between wild and escaped Atlantic salmon Salmo salar from aquaculture operations have been documented across the natural range of the species where the 2 co-occur (Glover et al 2017, Keyser et al 2018. Escaped farmed Atlantic salmon regularly occur in both Europe and Atlantic Canada (Keyser et al 2018, Diserud et al 2019, Glover et al 2019 and have been commonly found in rivers at distances of up to 200 km from the nearest aquaculture site, although distant occurrences at sea have also been reported (Hansen et al 1993, Hansen & Jacobsen 2003, Jensen et al 2013. As a consequence, hybridization between wild and domestic salmon can be both spatially extensive and represent a significant proportion of a population's annual production (Glover et al 2013, Karlsson et al 2016, Sylvester et al 2018.…”