Characteristics of the deep scattering layer in the Gulf of Mexico as they relate to sperm whale diving and foraging behavior

Abstract

In June 2005 we used a 38-kHz ADCP and two Simrad fishery echosounders to collect backscatter data and passive acoustic array recordings of sperm whale sounds, as part of the Sperm Whale Seismic Study (SWSS). Dive profiles and presumed foraging depths of the whales were derived using a 3-D tracking technique [see abstract by Thode and Norris (this conference)]. Relationships are examined between dive profiles and acoustic backscatter intensity measurements from the main deep scattering layer (DSL) at a daytime depth of 400–600 m. Passive detections of creaks, believed to be associated with prey acquisition, are merged with dive profiles to provide insight between mid-water backscatter returns and foraging effort. Results to date indicate whales diving in three depth ranges: deeper than 800 m, between 500 and 800 m, and less than 500 m. Dive patterns indicate some whales dive through the main DSL into deeper secondary layers seen by the 38-kHz ADCP. There seems to be no difference in dive patterns between night and day. Calibration analysis of ADCP backscatter versus 70-kHz Simrad backscatter shows a positive functional relationship, with 4 units of relative acoustic backscatter intensity equal to 1 dB of Sv.