The acoustic dead zone: theoretical vs. empirical estimates, and its effect on density measurements of semi-demersal fish

• Published in: ICES Journal of Marine Science
• Main Authors: Mello, L. G. S., Rose, G. A.
• Format: Text
• Language: English
• Published: Oxford University Press 2009
• Subjects: SEAFACTS; Newfoundland; atlantic cod; Gadus morhua
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/fsp099v1; https://doi.org/10.1093/icesjms/fsp099

<qd> Mello, L. G. S., and Rose, G. A. 2009. The acoustic dead zone: theoretical vs. empirical estimates, and its effect on density measurements of semi-demersal fish. – ICES Journal of Marine Science, 66: 000–000. </qd>The height of the acoustic dead zone, the region near the seabed where fish cannot be resolved acoustically, was calculated both theoretically (DZ t ) and empirically (DZ e ). The DZ e was based on measurements of depth and trawl geometry from sensors (SCANMAR) mounted on a bottom trawl deployed during a survey off Newfoundland and Labrador in winter 2007. Acoustic data were acquired while trawling, using a 38-kHz echosounder (Simrad EK500) with a hull-mounted transducer. The DZ e was calculated as the difference between the trawl-footrope depth and the corresponding acoustically sensed, seabed depth. EK500 and SCANMAR estimates of seabed depth were significantly different. The fish caught were mostly Atlantic cod ( Gadus morhua ). The estimates of DZ e ranged between 2.0 and 3.5 m and were greater than DZ t by 0.1–0.9 m in more than half the cases. Three values of acoustically derived cod densities were estimated for each tow, without dead-zone correction and with corrections for DZ t and DZ e . When compared with DZ t corrections, DZ e resulted in negative (6–12%) and positive (9–35%) corrections to cod density. A general linear model revealed that the seabed depth gradient, standard deviation of estimated fish density in the dead zone, and wind direction and force explained 85% of the difference between DZ t and DZ e estimates. These factors affected the detection of the seabed and biased acoustically derived indices of demersal-fish abundance.