Acoustic characterization of gelatinous plankton aggregations: four case studies from the Argentine continental shelf

Abstract During routine acoustic surveys for the assessment of fish abundance in the Argentine Sea, large-scale, plankton-like scattering layers covering thousands of square nautical miles are commonly observed. Net sampling revealed that many of these scattering layers comprised gelatinous zooplankters aggregated in dense concentrations in the main. Even though echoes from gelatinous zooplankton are expected to be weak, because of the low reflectivity of their bodies, dense aggregations are capable of producing sound-scattering levels high enough to mask even the overlapping echoes from swimbladdered fish. The objective of this study is to relate the aggregations of four gelatinous species identified by means of nets to the presence of sound-scattering layers. Selected sections of echo recordings from aggregations of Lychnorhiza lucerna (Scyphozoa), Iasis zonaria (Salpidae), Mnemiopsis leidyi (Ctenophora), and Aequorea sp. (Hydrozoa) were obtained at 38 or 120 kHz, at different locations along the Argentine shelf. Some features of the spatial distributions of the aggregations are described. The feasibility of remote detection for different gelatinous groups is of great importance considering the impact that blooms of these organisms could have on some particularly sensitive ecosystems (e.g. fish spawning and nursery grounds). The characterization of specific aggregation and behavioural patterns will allow the mapping ofjellyfish distributions by the analysis of previous cruise databases. This methodology will provide a baseline for the study of spatial and temporal changes and trends in their abundance.