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Do triploid Salmo trutta stocked into a chalk stream in the spring prey on wild Salmo salar smolts?


Correspondence: W. D. Riley, The Centre for Environment, Fisheries & Aquaculture Science (Cefas), Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK (e-mail:


The predation impact of recently stocked triploid brown trout, Salmo trutta L., on migrating wild Atlantic salmon, Salmo salar L., smolts was investigated in two field-based experiments. The first experiment employed a unique experimental facility to monitor 57 wild S. salar smolts through an enclosure containing a known density of stocked S. trutta to determine a predation rate. None of these smolts were preyed upon by the stocked S. trutta. The second experiment investigated the diet of free-ranging stocked triploid S. trutta in a chalk stream during the spring. Although at least 6% of the free-ranging stocked triploid S. trutta became piscivorous on Phoxinus phoxinus (L.), the results suggest that large, recently stocked, triploid S. trutta with a high condition factor do not represent a predation threat to wild S. salar smolts. However, it is recommended that a precautionary approach is maintained and the findings are not generalised until further investigation permits explicit management advice to be developed, and that the stocking of large triploid S. trutta is avoided before May/ June (regional variations to apply) each calendar year, where this can reasonably be achieved.