Zooplankton Aggregation Near Sills
Defence Research and Development Canada Atlantic Dartmouth, Nova Scotia
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This report outlines acoustic methods and results from a 3-year collaborative study of zooplankton aggregations in a coastal fjord. Mid-water aggregations of zooplankton were surveyed using a calibrated, three-frequency 38, 120, and 200 kHz vessel-based echo-sounder system, a multi-net towed zooplankton net BIONESS, and a high-resolution in situ camera system ZOOVIS. Dense daytime layers of crustacean zooplankton near 70 to 90 m depth were found in the lower reaches of the inlet, especially concentrated by tidal flows around a sill which rises above the layer. Quantitative Euphausiid and Amphipod backscatter measurements, combined with in situ species, size, and abundance estimates, were found to agree closely with size- and orientation-averaged fluid-cylinder scattering models. Similar in situ scattering measurements of Siphonophores were found to have a much stronger low-frequency 38 kHz scattering strength, in agreement with a simple bubble scattering model. Distinctive near-surface flow lines near the sill were found to coincide with strongly sheared pycnoclines and high levels of turbulent dissipation. The combination of a lack of zooplankton present in these layers and a reasonable agreement with turbulence scattering models suggests these acoustically-visible layers are due to micro-structure scattering. A new high-resolution multi-beam sonar was used to map the ecologically important sill at Hoeya Head with 3 m resolution, and to sample in two-dimensions mid-water aggregations of fish, zooplankton, and turbulence.