Activities Sonar Transducers, which send a sonar beam across the river, are installed along each bank of the river. When a fish swimming up-river travels through the beam, it is recorded on a computer. Weirs, or large panels, are installed along shore to force the swimming fish farther out into the river where they can be counted in the sonar beam. This method is accurate but very expensive. Sonar also fails to identify what species of fish are passing the study site. Additional sampling with nets must be used to verify the species being recorded in the sonar count. Return to Activity Text and graphics by USFWS staff Last modified 18 January, 2006