Point-source violations: split-beam tracking of fish at close range

Split-beam positional estimates of fish detected in a river at close range often do not correspond to the actual position of the target. These inaccur acies create problems in determining whether a fish is moving upstream or d ownstream. We hypothesize that these positional estimates are degraded by t wo factors: size of target relative to beam diameter, and the complex scatt ering of the fish. These parameters create a near-held effect, within which the phase measurements of the returning echoes are corrupted. Examples of fish tracks from near and far range fish detected by a split-beam echo soun der an provided to illustrate these inaccuracies. Experimental data from te thered spheres and complex targets show Increasing distortion with target c omplexity and proximity to the transducer. (C) 2000 Ifremer/CNRS/INRA/IRD/C emagref/Editions scientifiques et medicales Elsevier SAS.