Abstract

Knowledge of the distribution and biomass of fishes within an estuarine system is needed to make effective management use of the concept of Essential Fish Habitat (EFH). Four basic habitat types are recognized in a typical estuary: marsh, marsh edge, open bay, and channel. Fish distributions and species diversities within these habitat types can vary with season, tide, lunar cycle, weather, and bottom type. Resolution of the influence of these and other variables on EFH is confounded by limitations of traditional sampling gears. Recently, hydroacoustics has been shown to be a useful sampling tool that avoids some of the selectivity issues encountered with traditional gear types. A survey was conducted in coastal Louisiana to evaluate the quantitative capabilities of hydroacoustics in a shallow water setting. Survey objectives were to 1) determine the nekton biomass associated with two different habitats (bay and channel), 2) compare hydroacoustic data to concurrently collected wing-net data, and 3) determine effect and magnitude of ambient acoustic backscatter on nekton biomass estimates.\Based on acoustic sampling, there was no diurnaI diference in channel biomass; however, based on channel wing-net samples there was a three-fold greater biomass at night as compared to day. Based on wing-net data, bay anchovy, Anchoa mitchilli, dominated the catch (90-97%) in all sites. Diurnal differences between gear types was attributed to avoidance of the wing-net during the day. Length frequency data compared to acoustic data showed alto 3 dB shift from Love's equation. Acoustically derived densities reflected densities from wing-net data. The high sediment load and plankton abundances common to coastal waters do not bias acoustic estimates. The data thus far suggest a positive outlook for the use of hydroacoustics as a tool for estimating biomass and abundance of nekton in ultrashallow systems such as estuaries, coral reefs, and mangroves.

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