237 
Abstract .—Acoustic backscatter 
data from a 38-kHz echo sounder and 
a 150-kHz acoustic Doppler current 
profiler (ADCP) were collected during 
Southwest Fisheries Science Center 
marine mammal surveys in the east- 
ern Pacific aboard the NOAA ship 
David Starr Jordan in 1992 and 1993. 
These data were processed to give pro- 
files of volume scattering strength. A 
deep scattering layer dominated the 
time-depth patterns of backscatter. 
This layer migrated from a depth of 
300-400 m during the day to 0-100 m 
at night, when it was located just 
above the thermocline. The source of 
backscatter was primarily small fish 
and squid, which are dolphin prey. 
Dolphin sighting rates were correlated 
positively with nighttime volume scat- 
tering strength above the thermocline. 
Spatial and temporal variability of 
prey biomass in the surface layer, as 
indexed by volume scattering strength, 
may have important consequences for 
dolphin feeding, as well as for distri- 
butional responses to habitat variabil- 
ity by dolphins. 
Manuscript accepted 19 June 1997. 
Fishery Bulletin 96: 237-247 (1998). 
Dolphin prey abundance determined 
from acoustic backscatter data 
in eastern Pacific surveys 
Paul C. Fiedler 
Jay Barlow 
Tim Gerrodette 
Southwest Fisheries Science Center 
National Marine Fisheries Service, NOAA 
PO. Box 271, La Jolla, California 92038 
E-mail address (for R Fiedler): PFiedler@noaa.gov 
Acoustic data, consisting of the re- 
turns of pulses of sound from tar- 
gets in the water column, are rou- 
tinely used to assess fish distribu- 
tion and abundance (Forbes and 
Nakken, 1972; Johannesson and 
Mitson, 1983; MacLennan and 
Simmonds, 1992). Such data are 
now also being used to quantify the 
biomass and distribution of zoo- 
plankton and micronekton (Greene 
and Wiebe, 1990; Hewitt and Demer, 
1993). Innovative techniques using 
multifrequency amplitude and 
phase information can give esti- 
mates of size, movement, and acous- 
tic properties of individual plank- 
tonic organisms (Farmer and Hus- 
ton, 1988; Holliday et al., 1989). 
Another approach in acoustic stud- 
ies of zooplankton and micronekton 
has been to employ instrumentation 
that is used routinely, notably the 
acoustic Doppler current profiler 
(ADCP, Roe and Griffiths, 1993). 
Several studies have used uncali- 
brated ADCP echo intensity data, 
with various levels of processing, to 
estimate relative patterns of plank- 
ton biomass distribution in space 
and time (Flagg and Smith, 1989; 
Plueddemann and Pinkel, 1989; 
Smith et ah, 1989; Heywood et al., 
1991). 
We have been collecting echo in- 
tensity data from a 38-kHz echo 
sounder since 1986 and from a 150- 
kHz ADCP since 1992 on Southwest 
Fisheries Science Center marine 
mammal surveys. This report de- 
scribes data collection and process- 
ing and presents results from two 
cruises during which nearly com- 
plete data sets were obtained (popu- 
lation of Delphinus stocks 1992 
[PODS92] and 1993 [PODS93]) 
(Fig. 1). Other data collected on 
these surveys provide a context for 
these results in terms of predators 
and habitat. Line-transect data 
were collected for dolphin abun- 
dance estimates (Mangels and 
Gerrodette, 1994a, 1994b). We use 
sighting rate here as an index of 
dolphin abundance. Physical and 
biological oceanographic data were 
also collected to characterize habitat 
variability (Philbrick et al., 1993). 
Materials and methods 
A 38-kHz Simrad EK-400 scientific 
sounder was used on NOAA ship 
David Starr Jordan beginning in 
1986. Rather than a commercial 
echo integrator, we used an acous- 
tic data acquisition (ADA) system, 
consisting of an analog-to-digital 
converter and a personal computer. 
A 150-kHz ADCP was installed on 
the Jordan in 1991. We collected 
nearly complete sets of 38- and 150- 
kHz acoustic data in 1992 and 1993. 
A sonar equation relating volume 
backscattering strength per m 3 ( S v , 
