Evaluation of a Doppler sonar system for fisheries applications

We report on tests of a general-purpose, coherent, Doppler sonar system undertaken to explore its capabilities when applied to detecting discrete fish targets. This 250-kHz, 30 kHz bandwidth instrument provides for phase coding of the transmit pulses and coherent sampling of successive acoustic returns. Towtank tests were used to determine the basic operating capabilities of the system. Under these ideal conditions a single-ping velocity accuracy of between 3 and 8 cm s−1 can be achieved approaching the theoretical limit for this instrument. Field trials were undertaken on the Fraser River in British Columbia near the Pacific Salmon Commission's field site, allowing comparisons with conventional fisheries sonar systems. While the long-term goal is to apply the Doppler sonar system in a larger-scale marine environment, the passage of migrating salmon provides an ideal test opportunity with fish of predictable and well defined swimming behaviour. Individual fish-swimming speed can be measured with an accuracy of between 5 and 10 cm s−1. By comparison, water velocity measurements made with the same instrument can only achieve a theoretical accuracy of 60 cm s−1.