Development of a method for estimating the probability of detecting fish through a hydroacoustic beam

Active hydroacoustic sampling techniques are widely used to monitor fish densities and behavior in aquatic environments. Since the 1970´s, this technology has been used to assess a wide range of fisheries-related questions, including upstream and downstream migrations, stock assessments, behavioral evaluations, and others. When properly applied, hydroacoustics is a powerful and effective fisheries assessment tool, providing high sampling coverage that is unobtrusive and nonselective. Like any sampling technique, an understanding of the factors potentially influencing the ability to detect organisms and quantify the sampled volumes with a hydroacoustic system enhances the ability to accurately interpret the results and derive quantitative estimates. Several factors can affect the probability of detecting fish using hydroacoustics. These include: 1) the sampling environment; 2) hydroacoustic system data collection methodology and parameters; and 3) characteristics and distribution of the fish being monitored. Interactions between these factors can influence both the ability to detect fish of a given minimum size and the effective sampling volume of the hydroacoustic system. This paper describes methods for estimating the probability of detecting a fish within a hydroacoustic beam under different conditions. Examples demonstrating the effect of each of these parameters on the ability to detect fish using hydroacoustics are presented.