Improving hydroacoustic sampling in large rivers: Evaluating factors influencing target detectability

Abstract

Mobile hydroacoustic sampling is increasingly being used to monitor fish populations in large rivers; however, several factors may influence the ability to sample fishes with this gear in these environments. This study sought to address the effects of vessel speed, wind speed, the coefficient of variation (CV) of transducer pitch angle, and beam compensation (BC) on hydroacoustic detectability to advance and inform mobile, horizontally oriented hydroacoustic sampling in large rivers. A series of four replicate surveys was conducted past 23 artificial targets at three vessel speeds in a large river. Wind speed data during surveys were obtained, and post-processing of hydroacoustic data was conducted separately under maximum BC values of 6 and 12 dB. Results revealed significant negative effects of wind speed and transducer pitch CV on buoy detectability, as well as negative interactions between vessel speed and wind speed, with detectability decreasing as wind speed increased, particularly at slower vessel speeds. We suggest this may be due to increased vessel and transducer motion during periods of increased wind speed. Detectability also significantly increased under an expanded BC due to a greater proportion of the water volume being analyzed; however, further work is necessary before implementing greater BC settings in large rivers. These results ultimately display the interactive and negative effects of wind speed and vessel speed on horizontal hydroacoustic detectability in large rivers, revealing important considerations for hydroacoustic monitoring and sampling efforts in these systems.