A simple method to correct the results of acoustic surveys for fish hidden in the dead zone

In lentic freshwater systems, vertical acoustics may underestimate fish abundance in the acoustic dead zone where fish detection capability is limited. To estimate this bias, the height of fish above the lead-line of a benthic multi-mesh gillnet (1.5m high) was used to quantify both the vertical distribution of fish near the bottom and the proportion residing within the acoustic dead zone. The study was carried out at the percid-dominated Biesbosch Reservoirs in the Netherlands. Acoustic dead zones were estimated at 7cm above flat bottoms, and 1234cm above 8 degrees sloped bottoms at depths of 527m, respectively. Depending on the habitat, 36 to 75% of the gillnet catch by number was present in the acoustic dead zone, representing 551% of the biomass. Near-bottom depths were highly preferred by ruffe Gymnocephalus cernua, often used by perch Perca fluviatilis and pikeperch Sander lucioperca, plus seemingly devoid of smelt Osmerus eperlanus. The total amount of fish hidden in the acoustic dead zone was estimated to be 1339% of the whole water column. The proportion of biomass obscured in the dead zone was lower (112%). The conclusion is that undetected fish in the acoustic dead zone can seriously bias density assessment, which can be corrected by concurrent sampling with benthic gillnets.