Statistical combination of multifrequency sounder-detected bottom lines reduces bottom integrations

Abstract

Accurate quantification of biological abundance using acoustics requires complete removal of the sea floor’s contribution from the echo.  Acoustic-trawl surveys often utilize a single sounder-detected bottom (SDB) line for bottom determination, but combining bottom detections from multiple frequencies could provide a more accurate representation.  Using bottom detection lines from five different frequencies, 222 nautical miles of trackline from four distinct acoustic-trawl survey areas were investigated to determine if the mean or median statistical combination of multiple frequency SDB lines resulted in fewer bottom integrations compared to a single frequency SDB.  The difference in bottom detection depth between the statistical combination lines and the 38 kHz SDB was also investigated.  The mean statistical combination line reduced bottom integrations by an average of 45% and the median produced a 43% reduction in integrations over all areas.  Additionally, bottom depth averaged 5.9 and 6.9 cm shallower for the mean and median, respectively, compared to the 38 kHz SDB.  Overall the mean tended to be marginally better at reducing the bad bottom integrations while at the same time slightly less divergent from the 38 kHz SDB compared to the median.   Based on these results, beginning in 2009, the Alaska Fisheries Science Center’s Midwater Assessment and Conservation Engineering Program began using the mean statistical combination line to represent the true bottom, replacing the 38 kHz SDB line historically used.

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