Removal of brown trout (Salmo trutta L): Changes in population dynamics in a weir basin in Western Norway

The trout population in a 10.6 ha weir basin in western Norway has been studied over thirteen years. Baseline investigations were carried out during 1974–78, three to five years after construction of the weir. Studies in 1983 showed dominance of the 1977 and 1978 year‐classes. This dominance lasted until 1985. Removal of 75 per cent of the population for stocking in reservoirs reduced the fish biomass from 1.43 to 0.28 kg 100 m−2. This increased the growth rate and condition factor of the remaining fish in subsequent years. The trout enter the basin mainly at age 2+ and 3+. No increase in the number of immigrants was observed during the first two years after removal of fish. In the third year, however, a five‐fold increase was measured. Most of these fish belonged to the 1985 year‐class, which had little contact with the strong classes of 1977 and 1978. We conclude that density‐dependent factors control immigration to the basin, whereas a combination of density‐dependent and independent factors act on the recruitment in the riffle areas. We suggest that the highest recruitment of fish is obtained by reducing the density‐dependent effect through removing a very high proportion of the young fish, 2+ and 3+, every year.