Abstract
The production of salmonids in sea-cages has been developed for monoculture of the target species. However, we show here for the first time, that wild fish may enter sea-cages used for farming of Atlantic salmon (Salmo salar L.) in Norway, out-grow the mesh size, and thereafter become permanently trapped. Within seven different sea-cages located in western Norway, eight different species of wild fish were identified; European eel (Anguilla anguilla), sea trout (Salmo trutta L.), cod (Gadus morhua), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens), pollack (Pollachius pollachius), hake (Merluccius merluccius) and whiting (Merlangius merlangus). In the two most extreme cases, a 5 × 5 × 7 m cage with 311 farmed salmon (903 g) also contained 542 whiting (79 g), 77 haddock (43 g), and 5 cod (26 g), and a 12 × 12 × 15 m cage with 1695 farmed salmon (559 g) also contained 1196 haddock (35 g), 1115 whiting (31 g), 46 cod (23 g), 23 saithe (48 g), 15 pollock (22 g), 5 sea trout (54 g), and 2 hake (29 g). The present study thus demonstrates that aquaculture cages designed for monoculture may attract and effectively ‘trap’ wild fish. We did not investigate the frequency of this occurrence, and the ecological significance of these observations remains unclear. However, with the ever-increasing number of sea-cages used for global aquaculture, this is clearly a topic for further research.
Highlights
World aquaculture production is increasing, and within the marine environment, sea-cages that are open to the surrounding water masses represent the standard approach to production
In Norway, which is the world's largest producer of Atlantic salmon (Salmo salar L.), 3460 sea-cages were distributed among 535 localities along the coastline as of January 2016
This is the first report of wild fish trapped inside aquaculture sea-cages in Norway, the worlds largest producer of farmed Atlantic salmon
Summary
World aquaculture production is increasing, and within the marine environment, sea-cages that are open to the surrounding water masses represent the standard approach to production. A number of challenges linked with environmental sustainability of cage-based salmonid aquaculture have been reported, and of these, farmed escaped salmon that may display genetic interactions with wild conspecifics (Glover et al, 2017), and increased mortality of wild smolts via infection with the. These cages were investigated with respect to the species observed, their abundance and size
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