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A hull -mounted 38 -kHz phased -array acoustic Doppler current profiler (ADCP ) was used to acoustically survey the continental margin of the northern Gulf of Mexico (GOM ) during 6 cruises in 2002 -2003 . This is the first backscatter survey with a 38 -kHz ADCP in the Gulf of Mexico . ADCPs have been used as a proxy to measure the volume backscatter return from plankton in the water column , however previous studies were restricted to the upper 200 to 300 meters due to the relatively high frequency of operation (150 -300 kHz ) of the transducers . In addition to measuring deep water current velocities , the 38 -kHz phased -array ADCP can measure Relative Acoustic Backscatter Intensity (RABI ) as deep as 1000 meters . The daytime depth of the main deep scattering layer at 400 to 500 meters was resolved , and locally high backscatter intensity can be seen down to 800 meters . The objectives were to determine how to analyze RABI from the instrument to resolve scattering layers , and then to seek secondary deep scattering layers of potential prey species below the main deep scattering layer , from 600 to 800 meters in the feeding range for Gulf of Mexico sperm whales .
Based on RABI from the 38 -kHz ADCP , secondary DSLs in sperm whale diving range were more commonly recorded over the continental shelf than in the deep basin region of the Gulf of Mexico . The daytime depths of migrating plankton showed variation depending on physical circulation features (cyclone , anticyclone , proximity to Mississippi river , and Loop Current ) present . Vertical migrations compared between concurrently running 38 and 153 -kHz ADCPs showed an overlap of acoustic scatterers recorded by the two instruments , however the 153 -kHz instrument has much finer vertical resolution . Vertical migration rates were calculated and simultaneous net tow samples from one of the cruises was used to compare abundance estimates by the two methods . |
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