The impact of introduced pike in a small stream

The abundance and size-structure of a formerly allopatric brown trout (Salmo trutta L.) population in an inlet stream to a small lake were monitored for 9 years (1987-1995) after the first observation of introduced pike (Esox lucius L.), and revisited 18 years later (2013). All age groups of brown trout were reduced after the pike introduction, especially older fish of age≥2+, but less so for age-0+ and 1+ fish. We suggest that the decline of older brown trout is mainly due to the high predation pressure from pike when migrating into the adjacent lake to feed. Young stream-dwelling pike of age-0+ and 1+ which ranged between 6 and 27 cm in length may also exert a predation pressure on juvenile brown trout that remains in the stream. Keywords Stream * Brown trout * Pike predation * Density * Population structure