Effect of Experimentally Increased Intraspecific Competition on Sedentary Adult Brown Trout (Salmo trutta) Movement and Stream Habitat Choice

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Abstract
Movements and habitat choice of 19 brown trout (Salmo trutta) resident in a stream was monitored during one season, while in the next season the population was increased by introducing 130 wild brown trout captured further downstream. No differences in the movement patterns of resident trout at the two population densities, or of residents compared with nonresidents, were found. Median movement distance was 0 m. Sixty-six percent of the movements were less than ±50 m, 11% were between 50 and 100 m, and 23% were greater than ±100 m. Introduced nonresident trout occupied habitats different from those occupied by residents. Nonresidents were more often observed in shallower areas with less cover and finer substrate, i.e. less preferred habitats. It is hypothesized that this was caused by dominance of resident trout due to their larger size and prior residence. Both resident and introduced trout exhibited preferences for deep stream areas, e.g. water > 50 cm deep, mean water velocities in the range 10–70 cm/s, coarse substrate > 128 mm, and a > 50% overhead cover compared with available habitat. At both low and high population densities, the trout inhabiting pools were significantly larger than trout inhabiting other habitat types. No homing tendency was observed for the displaced trout.