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 thei r activity and behaved more aggressively, whereas 10-cm L-S (age-0) trout t ended 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 v elocities in winter than in summer, and increased their use of instream cov er in winter. When both size classes were present, only small trout decreas ed their use of low water velocities and cover. The results indicate that i ntercohort competition may cause spatial segregation among size groups of b rown trout, especially in winter when trout attempt to minimize their maint enance costs. (C) 1999 The Fisheries Society of the British Isles.