A field examination of acoustical scattering from marine organisms at 70 kHz

Acoustical scattering from aquatic organisms is an extremely complicated problem and can only be dealt with by empirical or semi-empirical models. In order to use sonars as a remote-sensing tool to study biological sound scatterers in the ocean, one must rely on such models. Our research at the Gulf Stream Boundary near Cape Hatteras, North Carolina, involves studying heterogeneous distributions of fish, crustaceans (e.g., shrimp), cephalopods (e.g., squid), and soft-bodied animals (e.g., salps) and their relationship to the front. To quantify the scattering from the animals, we use existing scattering models. However, the models were based on data from animals in artificial environments such as a laboratory tank or an underwater cage near a ship. Since scattering strengths depend on the animal´s behavior, depth, and whether it is alive or dead (and possibly preserved), data from the artificial environments may not accurately represent the scattering from the animals in their natural environments. Furthermore, the scattering models are mainly based on species not found in the area studied; therefore, we have considered them as general models for selected taxonomic groups. Working under this assumption, the present study examines the robustness of the general models to a field application. We used size frequency distribution of the various animal types from trawl data as input into the scattering models to produce a predicted value of the acoustic volume scattering strength. We also examined the field sonar data for potential problems in comparing trawl and sonar data (e.g., large animals which have high target strengths can avoid trawls). Our comparison between the predicted and field measured values demonstrates the potential usefulness of the general models in areas with different taxonomic assemblages. We find most data to lie within 5 dB of the predictions with some data systematically outside the 5-dB bars, which indicates animals at some stations were avoiding the trawl.