Accession Number:

ADA393673

Title:

Effects of Macrofauna on Acoustic Backscatter from the Seabed: Field Manipulations in West Sound, Orcas Island, WA, USA

Descriptive Note:

Journal article,

Corporate Author:

NAVAL RESEARCH LAB STENNIS SPACE CENTER MS MARINE GEOSCIENCES DIV

Report Date:

2001-06-15

Pagination or Media Count:

26.0

Abstract:

Previous observations with a bottom-mounted, radially scanning sonar BAMS at 40 kHz suggested that macrofaunal activities influence low-angle, acoustic backscatter from seafloor sediments. In order to test that possibility experimentally, we measured and modeled time series of backscatter strength at both 40 and 300 kllz prior to manipulation and then introduced several macrofaunal species at Imown abundances to randomly selected locations within the ensonified area. We worked in West Sound, Orcas Island, WA, at a water depth of 20.4m and for the more frequently recorded 40-kHz series extracted effects by the time-series method known as intervention analysis, wherein the intervention was the experimental alteration. We observed increased backscafler from patches of the small protobranch bivalve Acila castrensis, and of the cockle Clinocardium nuttali, from bait used as chum for fishes and crabs, and from tethered crabs Cancer magister other treatments showed no significant change. All of the effective treatments involved increased backscatter at 300 kHz from animals that have obvious hard parts or air bladders. Power calculations for intervention analysis and geoacoustic modeling suggest that failure of other treatments to show no significant effects on backscatter strenght stems from the small size of the organisms and structures used relative to the 40-kllz wavelenght 3.7cmand to low sound-speed contrasts between surficial sediments at this site and overlying water at both frequencies, producing low backscatter levels from both volume heterogeneity and surface microtopography. This experiment demonstrates, however, that low-angle acoustic backscatter can be used to observe at least some populations of benthic animals over a large area ca. 8000m2 and that intervention analysis can be a useflil tool where logistics permit repeated observation but few or no spatial replicates - frequently the case in ecological manipulations.

Subject Categories:

  • Biology
  • Hydrology, Limnology and Potamology
  • Acoustics

Distribution Statement:

APPROVED FOR PUBLIC RELEASE