Validation of current acoustic dead‐zone estimation methods in lakes with strongly sloped bottoms

Abstract

Inland water bodies contain extremely steep‐sloped bottoms compared with those typically occurring during marine vertical surveys. These steep bottom slopes can cause high acoustic dead zones, biasing our estimates of living organisms. The studies so far have used the assumption that the first contact between the acoustic wavefront and bottom will be at the point where the radial ray from the transducer is normal to the bottom (90°), which we refer to as the normal ray assumption (NRA). Nevertheless, as acoustic energy dramatically decreases laterally due to the beam's pattern, this assumption may not be fulfilled further from the acoustic axis. It is reasonable to believe that the methods assuming the NRA can fail at quite steep slopes. We installed a calibration benthic gillnet of known height at sites with different bottom slopes, ranging from 12° to 50°. The gillnet's float‐line served as a good visible marker above the monitored lake bottom and was successfully used for measuring the acoustic dead‐zone height empirically. By comparing the observed and modeled dead zones based on the NRA, we can show that the current methods for their estimation are invalid at quite sloping angles. We conclude that the current dead‐zone estimation methods are not always applicable for surveying inland water bodies with extremely steep bottom slopes. Installing a simple calibration net as an off‐bottom marker can provide help for in‐situ dead‐zone measurements.