Intercohort competition causes spatial segregation in brown trout in artificial streams

When together in a flume, 13-cm L-S (age-1) trout appeared to increase their activity and behaved more aggressively, whereas 10-cm L-S (age-0) trout tended to move less and were rarely aggressive in the presence of the larger size class. Both size classes were less mobile and preferred lower water velocities in winter than in summer, and increased their use of instream cover in winter. When both size classes were present, only small trout decreased their use of low water velocities and cover. The results indicate that intercohort competition may cause spatial segregation among size groups of brown trout, especially in winter when trout attempt to minimize their maintenance costs. (C) 1999 The Fisheries Society of the British Isles.