Validation of acoustic echo counting for studies of zooplankton behavior

Acoustic methods can be employed to census zooplankton in large volumes of water rapidly, but ecological interpretations of the results are often complicated by ambiguities concerning the identity of acoustic backscatters. Here, a multi-step approach is developed to evaluate the potential of acoustic echo counting for behavioral studies of zooplankton using OASIS (Optical–Acoustic Submersible Imaging System), an instrument designed for concurrent optical and acoustic imaging of zooplankton. A combination of field and laboratory analyses (1) demonstrate general agreement between echo counts and depth-stratified net sampling, (2) show that fish and zooplankton can be distinguished on the basis of target strength (TS), (3) indicate that zooplankton taxa (euphausiids and gammarid amphipods) could not be differentiated on the basis of TS, and (4) demonstrate a positive relationship between body size and TS in euphausiid crustaceans. Simulation of the system's performance suggests that at high densities of reflective targets, echo counting increasingly underestimates the true number of targets present. These results confirm that echo counting is a valuable method for behavioral studies and underscore the importance of independent verification of the identities of acoustic backscatters.