Sea Technology

JUL 2017

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38 st / July 2017 www.sea-technology.com from July 29 to August 12, 2016. The mission transited the Gulfstream Pipeline in 27 to 65 m of water. The glider path zigzagged over the pipeline and surrounding rocky rubble that attracts reef-associated sponges and soft corals, as well as fishery species like snappers and groupers. The AZFP was set to transmit pulses at 1 Hz, or about one transmission per 25-cm linear distance. The 7° transducer beam has a swath of 12 percent or approximately 12 m at 100-m range. If a fish were ensonified, it would have been likely that several sequential transmissions would reflect from the fish and pro- vide multiple measures of backscatter and estimate of tar- get strength. To optimize vertical resolution to detect fish in close proximity to the seafloor, the shortest calibrated pulse length of 150 microseconds was used. Data were logged to a maximum range of 100 m. Data Analysis, Visualization AZFP data were read into Echoview acoustic analysis software, which provides a collection of semiautomated im- age and signal-processing algorithms for single-beam echo- sounder data. First, glider position, depth and orientation data were read and synchronized to the raw transmission and backscatter data. Because the glider only provides GPS position information on each surfacing in normal configura- tion (about every 2 to 3 hours), Echoview assumes linear tra- jectory and constant speed between surfacings and applies interpolated GPS coordinates to each pulse transmission. Glider depth and orientation were recorded at 1 Hz and were paired directly with each pulse transmission and used to correct for range and depth of ensonified seafloor and ob- jects in the water column. Even though the AZFP logs data only on glider descent, there were frequent occasions when the AZFP logged data at the surface prior to engaging the de- scent. The pitch of the glider was used to filter data, accept- ing only pulse transmissions when the glider was between 15° and 45° of horizontal (typical glider descent is at 22.5° from horizontal). Maximum backscatter for each pulse was used to mark the range and depth of the seafloor and ex- clude the seafloor backscatter from measures of backscatter in the water column originating from biological organisms. Other sources of noise and interference were also noted in the echogram, including interference from the glider system and pulse transmissions from the glider's 170-kHz altime- ter. These sources of interference were minimal, and back- ground noise removal algorithms in Echoview were used to further clean the data. The final product was an echogram that projects backscatter in the water column from within 1 m of the glider to the seafloor along the entire mission path. Relative acoustic biomass, measured as area backscatter coefficient corrected for range and beam width, was esti- mated along the glider path at 50-m intervals and plotted on a nautical chart to show hotspots of biological biomass along the glider path. Acoustic backscatter was processed in two ways. First, low-density backscatter likely from in- dividual fish was analyzed by detecting single targets and estimating their target strength, an acoustic measure gen- erally proportional to fish size. Individual targets were as- signed a GPS coordinate and position in the water column and stored in a database. Schools of fish were identified, delineated with a polygon and assigned the centroid as a GPS coordinate. The area in the polygon was integrated for total acoustic backscatter, an indicator of fish density in the school. Lower backscattering layers likely representing plankton were also noted in the echograms. Biological fea- tures identified through this analysis were then related to oceanographic features such as water density stratification and peaks in chlorophyll. Conclusion The AZFP integrated glider represents a novel ocean eco- system survey tool available to fisheries research. This proj- ect has demonstrated the utility of a low-power, small form echosounder in detecting indicators of biomass for plankton and fishes in the pelagic and near-benthic environments. While it cannot replace the full capabilities of fisheries re- search vessels, the unmanned system could extend marine ecosystem acoustic surveys beyond the range and endur- ance of vessels. Ocean gliders are less limited by sea condi- tions and could extend surveys of fishery resources outside typical survey seasons (e.g., winter in the Atlantic Coast). Indicators or biomass hotspots mapped by the glider AZFP could guide additional sampling by research vessels carry- ing additional tools, such as nets or video systems to assess species abundance. With additional research in multiple- frequency acoustics for species classification, and evaluat- ing onboard data processing, new biological metrics could be collected on board ocean gliders, allowing for near-real- time simultaneous and synoptic measures of oceanographic features and the biological resources they support. Acknowledgments The AZFP integration into the TWR glider project was funded by a grant from the NMFS's Advanced Sampling Technology Working Group to the authors and National Marine Fisheries Southeast Fisheries Science Center (Todd Kellison). Operational field deployments in the eastern Gulf of Mexico are ongoing and have been funded by the Florida RESTORE Act Centers of Excellence Program administered by the Florida Institute of Oceanography (FIO). Design and engineering of the glider echosounder was led by ASL En- vironmental Sciences (Jan Buermans, Matt Stone and Rene Chave) and Teledyne Webb Research (Chris DeCollibus, Michael Brissette, and Clayton Jones). Data reading, analy- sis and visualization assistance was provided by Echoview (Toby Jarvis and Briony Hutton). The authors also acknowl- edge the crew of the FIO RV Bellows for support during glider calibration and field trials. ST J. Christopher Taylor is an ecologist in the Marine Spa- tial Ecology Division of NOAA's National Center for Coastal Ocean Science (NCCOS) in Beaufort, North Carolina. He has 19 years of experience developing coastal management decision support tools from fisheries acoustics surveys in ocean ecosystems us- ing research vessels, unoccupied and autonomous systems. Chad Lembke is a mechanical engineer with the Cen- ter for Ocean Technology at the University of South Florida's College of Marine Science in St. Petersburg, Florida. He has designed, developed and operated custom and off-the-shelf technology to assist faculty, researchers and private enterprises for two decades.

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