Influence of near bottom fish distribution on the efficacy of a combined hydroacoustic video survey

Abstract

Combining hydroacoustics and underwater video is an effective tool for generating fish population estimates. However, hydroacoustics cannot be used to differentiate fish from the seafloor within an area known as the acoustic dead zone. A common way to address this is to exclude data near the bottom. The effect of this exclusion zone on population estimates of nearshore semi-pelagic rockfish is unknown. This study explores the effect of a near bottom (0–1 m) exclusion zone by comparing ROV video data to data from a combined hydroacoustic and video method. Higher densities of semi-pelagic species (Black and Blue/Deacon Rockfish) were observed in the combined acoustic and video method, suggesting that most of the population resides above the exclusion zone. Demersal rockfish observed by the ROV did not contaminate acoustic data of semi-pelagic species, since they remained within the exclusion zone. Results demonstrate that extrapolation of school data into the exclusion zone provided a realistic correction to the acoustic data for Black Rockfish. Our work demonstrates that excluding the data within 1 m of the bottom does not negatively affect the ability of the combined video hydroacoustic method to sample semi-pelagic rockfish.