The tropical Sine-Saloum estuary Senegal is an ecologically important area for fish population dynamics. Daily cycles of fish movements in a tidal channel were observed using both split-beam and multibeam sonar. A trap-net was set and lifted regularly and simultaneously with the acquisition of acoustic data. The relationship between the two sets of data was satisfactory when acoustic records for single targets were separated from those containing fish schools. The heterogeneity of fish spatial and temporal distributions is examined, as well as the rapidity of changes of fish population density in an environment subject to tides. The consequence of this on sampling strategies is discussed. q 1998 Elsevier Science B.V.