Hydroacoustic assessment of fish abundance in the presence of gas bubbles

Methane ebullition from bottom sediments is abundant in many productive water bodies. The occurrence of the methane bubbles in the water column may cause erroneous quantification of fish densities in aquatic ecosystems when using hydroacoustic methods, since the target strength (TS) of bubbles strongly overlaps with that of fish. In this study, an approach that allows separation of TS distributions of fish and bubbles is proposed. The method was applied to data collected in a shallow reservoir with a 120-kHz echo sounder. Separation between fish and bubbles was based on the difference in their trajectories on echograms. To distinguish the target trajectories, the water column was sampled at low boat speed at restricted locations. Sampling was also carried out at high boat speed to assess the abundance and TS distribution of all targets in the entire reservoir. The information from the two different sampling modes was integrated in a simple mathematical algorithm, which allowed calculating bubble and fish densities in the reservoir. The total abundance of gas bubbles was higher than the density of fish. The elaborated method can be useful for quantification of dominant objects when spatial distributions are heterogeneous.