Improved fish detection in data from split-beam sonar

Hydroacoustic split-beam techniques have been applied to enumerate salmon migrating in the river Tana (northern Norway) during the summers 1998 and 1999. Analysing data by single echo detection and tracking was difficult. Missing echoes in tracks from fish, combined with noise in the output from the single echo detector was seen as reasons for this. An improved counting method is presented. Contours from moving targets are detected by image analysis. Then, detected single echoes within these contours are combined into tracks. This procedure reduces problems related to noise, and to tracking fish with few accepted single echoes.