Incidence and timing of wild and escaped farmed Atlantic salmon ( Salmo salar) in Norwegian rivers inferred from video surveillance monitoring

• Published in: Ecology of Freshwater Fish
• Main Authors: Svenning, Martin‐A., Lamberg, Anders, Dempson, Brian, Strand, Rita, Hanssen, Øyvind Kanstad, Fauchald, Per
• Other Authors: Norwegian Seafood Research Fund
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2016
• Subjects: Atlantic salmon; Salmo salar
• Online Access: http://dx.doi.org/10.1111/eff.12280; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Feff.12280; https://onlinelibrary.wiley.com/doi/pdf/10.1111/eff.12280

Abstract Run timing of escaped farmed Atlantic salmon Salmo salar vs. wild fish was compared by the use of video camera surveillance in 15 rivers over several years, covering 1600 km of the Norwegian coastline (from 58°N to 69°N). Annual runs of wild salmon varied among rivers from <200 fish to more than 10 000. During the surveillance period that for most rivers extended from late May to early October, larger‐sized salmon (fish ≥ 65 cm) generally entered the rivers earlier than small fish. The percentage of salmon identified as escaped farmed fish ranged from 0.1% to 17% across rivers with an average of 4.3%. Estimates of escapees are, however, assumed to represent minimum values because an unknown number of farmed fish passing the video cameras may have been misclassified as wild fish. By the use of a linear mixed model and generalised additive mixed models, it was found that the relationship between run timing and fish length differed significantly between farmed and wild salmon. While small‐sized farmed and wild fish (<65 cm) entered the river at about the same time, wild large salmon returned on average 1–2 weeks earlier than similarly sized escapees. The proportion of large‐sized farmed escapees also increased until late August and decreased thereafter. In contrast, there was a relatively constant and lower proportion of small‐sized escapees throughout the season. Within the surveillance period, there was no evidence of any exceptionally late runs of fish classified as escaped farmed salmon.