Abstract
To gain more knowledge about escaped farmed salmon Salmo salar in the wild, we investigated the proportion of escapees, body length at escape, proportion escaped as smolts/post-smolts (≤300 mm) and number of winters in the wild (winter zones in the growth pattern in the scale samples) in catches on the Norwegian coast and in an adjacent fjord from 2013 to 2017. The mean proportion of escapees was higher on the coast (26%) than in the fjord (4%), and escapees caught on the coast had a slightly larger mean body length at escape (607 mm) than in the fjord (557 mm). However, the mean proportion escaped as smolts/post-smolts did not differ significantly between the coast (8%) and the fjord (11%). There was also no significant difference in the mean proportion of farmed salmon with 1 or more winter zones after escape (50% on the coast and 56% in the fjord). The proportions of escapees with 1, 2, 3 or 4 winter zones after escape were 28, 20, 2 and 0.4% in catches on the coast and 30, 21 and 4% in catches in the fjord, respectively. This study found that the proportion of escapees was considerably higher in coastal waters than in the fjord. Escapees consisted of farmed salmon from several escape events over several years, and approximately 50% of the escapees had one or more winter zones after escape. Thus, escaped farmed salmon may pose a threat to wild salmon populations for several years after the escape event.
Highlights
Norwegian commercial production of farmed Atlantic salmon Salmo salar L. started in the 1970s, and in 2017 the biomass produced exceeded the biomass of Norway’s catch of wild salmon by a factor of more than 1500
Proportion of escapees was still higher on the coast than in the fjord when only including fish caught in the time period when both locations were sampled (GLMM: intercept estimate (± SE) = −1.09 ± 0.15, contrast fjord = −2.17 ± 0.09, z = −24.97, p < 0.0001)
In this study we investigated the proportion of escapees and the escape history of farmed salmon caught on the Norwegian coast and in a large adjacent salmon fjord containing a major salmon river (Namsen River)
Summary
Norwegian commercial production of farmed Atlantic salmon Salmo salar L. started in the 1970s, and in 2017 the biomass produced exceeded the biomass of Norway’s catch of wild salmon by a factor of more than 1500. Production of farmed salmon in Norway was 1220000 t, while the reported mass of the catch of wild salmon was 782 t (Anon 2018b). Presence of escaped farmed salmon in Norwegian rivers was first documented in the 1980s (Gausen & Moen 1991, Lund et al 1991). Presence of escaped farmed salmon in rivers has been monitored in Norway since 1989 (Fiske et al 2006, Diserud et al 2019, Glover et al 2019), and monitoring in rivers was further increased when a national monitoring program was established in 2014 (Glover et al 2019).
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