Enhanced acoustic backscatter due to high abundance of sand dollars, Dendraster excentricus

Detailed acoustic surveys of benthic sediments were conducted in July 1995 and September 1998 in the vicinity. of Humboldt Bay. California. During the se surveys, a hand of enhanced acoustic backscatter was observed offshore f rom the bay entrance, approximately parallel to the isobaths, in water dept hs ranging from 16-24 m. In order to assess the cause of the increase in ba ckscatter levels, a,ore comprehensive study was conducted in August and Sep tember 1999 using 100 kHz side-scan sonar, bottom grab sampling and underwa ter video recording. New observations indicated that a dense population of sand dollars (Dendraster excentricus) coincided with the enhanced backscatt er band. Compared to the two previous acoustic studies, the central section of the band expanded westward by 180 m and the southern section of the ban d shifted eastward by 160m, possibly resulting from a change in the biologi cal or physical factors which influence the location and breadth of sand do llars.