Abstract
Twenty Hoplias aimara were tagged intraperitoneally with radio‐transmitters in the Sinnamary River, French Guyana. In November 1993, 13 tagged fish were released in an area which would be flooded by the Petit‐Saut reservoir impoundment in mid‐1994. Seven other tagged fish were released in January 1994, 20 km upstream of the upper limit of the reservoir. Hoplias aimara showed site fidelity: 75% of fish returned to their capture site before the test area was inundated. The remaining fish stayed close to the release area. Monitoring during three 24‐h cycles before reservoir filling showed that H. aimara has a limited home range, which is less marked in unconstrained reaches during the wet season (April to August) than in constrained reaches during the dry season (September to November). In natural conditions, H. aimara has a wider home range in unconstrained river reaches than in constrained river reaches. After closure of the dam, in September and October 1994 (dry season, low water) more than half of the total H. aimara tagged in the flooded zone migrated upstream following the rise of water.