Acoustic Characterization of the Mesopelagic Boundary Sound Scattering Layer

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The acoustic properties of mid-water biologically induced sound scattering layers in coastal environment is poorly understood, especially around sub-tropical and tropical islands in the Pacific, where the bottom slope can very steep. The sound scattering layer can represent a large acoustic target and can easily be mistakenly as a false bottom and can interfere with sonar systems. The sound scattering layer around the Hawaiian Islands is known as the mesopelagic boundary community (MBC) of micronekton comprising of small fishes, shrimps, and squids. This layer, like most deep-scattering layers, undergoes vertical migration. However, the Hawaiian boundary's migration also has a large horizontal component that is predictable along some coastlines. The project team will characterize the acoustic environment of the mesopelagic boundary community by performing 1) multi-frequency acoustic backscatter measurements at multiple grazing angles at different portions of the layer to account for differences in density and layer thickness, 2) acoustic, video and oceanographic measurements to estimate density and species and the relationship of oceanographic conditions at the same locations of the backscatter measurements, 3) acoustic and chlorophyll a measurements to estimate zooplankton and phytoplankton abundance and density at the locations of the backscatter measurements, 4) island scale spatial and temporal distribution of the mesopelagic boundary sound scattering layer at various locations around an island, with Oahu serving as a model for other islands in temperate and tropical Pacific. This research will increase our understanding of the environment and the characteristics of the mesopelagic boundary community. The mesopelagic boundary community is an important part of the coastal ecosystem associated with Pacific and other islands that have steep bottom slopes and increase understanding of its ecology is important for preserving, managing and operating in this environment. This community of micronekton also serves as an important food resource for many commercially important species including billfish, tunas, and bottomfish and also spinner dolphins.