Review on monitoring adult salmonid ( Oncorhynchus and Salmo spp.) escapement using fixed-location split-beam hydroacoustics

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Since 1992, split-beam hydroacoustic techniques have been used to monitor adult salmonid escapement ( Oncorhynchus and Salmo spp.) in 14 rivers in North America and Europe. Monitoring in rivers is one of the more challenging applications for fisheries acoustics. Rivers typically have a high reverberation level, uneven bottom bathymetry, and nonlaminar hydraulics, requiring sophisticated equipment and careful deployment, calibration, and testing. The major issues that were addressed in order to obtain estimates of adult salmon escapement included hydroacoustic equipment and techniques, site selection, transducer deployment, and fish behavior. Fixed-location hydroacoustic techniques were employed, utilizing narrow-beam transducers aimed horizontally, monitoring migrating fish in side-aspect. Fish were tracked in three dimensions as they passed through the acoustic beam. A bottom substrate of low acoustic reflectivity enabled the acoustic beam to be aimed close to the bottom. Sites were selected where fish were actively migrating, not holding or milling. In most cases, migrating salmonids were strongly shore- and bottom-oriented, where water velocities were slowest. Diel distributions of fish passage were weighted toward nighttime. Other results included fish size and velocity. Potential improvements in riverine monitoring capabilities include quadrature demodulation and FM Slide/Chirp signals.