294 
Fishery Bulletin 106(3) 
sumption that these 166 EMs are derived from the first 
echo only. Because both of these assumptions are typi- 
cally presumed to be correct for this type of acoustic 
analysis, these findings may be of use for interpret- 
ing seafloor substrate classifications for determining 
EFH. 
Materials and methods 
Data collection and conversion 
Data were collected in the *.raw format from a 38-kHz 
Simrad single-beam echosounder on the FV Gladiator 
during the 2003 NMFS bottom trawl survey in the Gulf 
of Alaska (Table 1). The transducer gain was 24.5 dB, 
transmit power was 1500 W, beam angle was 9°, pulse 
length was 4.096 ms, and the sampling interval was 
1.024 ms. These Simrad files were calibrated in Echo- 
View® (vers. 3.30.60.05, SonarData Pty. Ltd, Hobart, 
Tasmania, Australia), and short (~1.5 km, -1440 pings) 
seafloor sections corresponding to 15-minute dura- 
tion bottom tows conducted at 1.54 m/s (3 knots) were 
exported into binary files by using the Echolmpact 
export module for import into QTC IMPACT™. This 
Echolmpact export module was specifically designed 
by the two companies to convey acoustic data in an 
appropriate format from EchoView to QTC IMPACT. We 
also examined EMs recorded directly by QTC VIEW™ 
(QTC, Sidney, British Columbia, Canada), without any 
prior EchoView® processing, at preset gains (ping inten- 
sities or amplitudes) by four external research cruises: 
the Alaska Department of Fish and Game (ADFG RV 
Resolution 2003 cruise), the Canadian Department 
of Fisheries and Oceans (Canadian coast guard ship 
RV John P. Tully 2002 cruise, and the RV Pallasi 
2004 cruise), and the New Zealand National Institute 
of Water and Atmosphere (RV Rangithi 1999 cruise) 
(Table 1). 
Gain settings 
In automated seafloor echo-processing systems, there 
may be a mismatch between the seafloor echo strength 
(gain) and the ability of the processing system to iden- 
tify the abrupt rise or spike that represents the begin- 
ning of the seafloor reflection. It is necessary to adjust 
the gain setting such that the inflection point can be 
distinguished from the earlier portion of the echo, which 
is the water column above the seafloor. Therefore sev- 
eral postprocessing gain settings in QTC IMPACT were 
applied to subsets of the NMFS 2003 FV Gladiator data 
sets in order to maximize the number of pings strong 
enough for automatic bottom detection (or bottom pick- 
ing) and to minimize the number of pings that would be 
too strong for the dynamic range of 96 dB of sound that 
QTC software can process. Otherwise, louder portions 
of pings would have had to have been automatically 
decreased to 0 dB, a process known as clipping, and 
quieter portions of pings would have had to be automati- 
