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Fishery Bulletin 96(2), 1 998 
squid were most likely the major component of the 
observed DSL’s, although a variety of crustaceans 
were undoubtedly present in the survey areas. 
Nonmigrating, epipelagic fish and squid are likely 
represented by near-surface backscattering during 
the day. 
The small fish and squid that we argue were de- 
tected by our acoustic systems are dolphin prey (Fitch 
and Brownell, 1968; Miyazaki et al., 1973; Perrin et 
al., 1973; Robison and Craddock, 1983; Robertson and 
Chivers, 1997). The ranges of volume scattering 
strength we observed — more than three orders of 
magnitude among bins (Fig. 2), up to lOx variation 
between night and day and 500x variation among 
daily means — must be important factors in dolphin 
feeding strategies and habitat choices. Thermocline 
depth, which influences prey distribution, is an im- 
portant component in dolphin habitat variability 
(Reilly and Fiedler, 1994). 
We observed a positive correlation between the 
abundance of dolphins (observed during daylight) and 
the abundance of prey at night, when spotted and 
spinner dolphins are known to feed on mesopelagic 
prey near the surface (Perrin et al., 1973; Robertson 
and Chivers, 1997). Over 90% of the dolphins identi- 
fied in both surveys were common, spotted, spinner, 
and striped dolphins. In the eastern tropical Pacific 
(PODS92), a region characterized by a strong and 
shallow thermocline (Wyrtki, 1967), nighttime prey 
abundance was more closely related to thermocline 
depth. Off Baja California (PODS93), the thermocline 
had less influence on distribution of prey. Dolphin 
community structure was also different: common 
dolphins represented 61% of the identified dolphins 
in this region, compared with only 37% in the 
PODS92 survey area. 
Our results indicate that dolphin distribution can 
be related to prey abundance measured acoustically 
and that the physical environment influences the 
abundance of dolphin prey. Marchal et al. (1993) ob- 
served a similar DSL in the eastern tropical Atlantic 
that was related both to thermal structure and to 
tuna abundance. Simple and readily available sonar 
systems can yield useful results when survey time 
or resources are limited. Monitoring long-term 
changes in zooplankton or micronekton biomass, as 
Roemmich and McGowan ( 1995) did with net samples 
off southern California, could be facilitated by the 
application of such acoustic methods. However, in 
present and future studies of cetacean feeding and 
cetacean responses to habitat variability, we will use 
multispectral acoustic systems and validation by net 
samples to quantify availability and distribution of 
specific prey types. 
Acknowledgments 
We thank Valerie Philbrick for collecting these data; 
Denny Sutton and Jim Anthony of NOAA’s Pacific 
Marine Center for support in maintaining the acous- 
tic systems; Roger Hewitt and Dave Demer for assis- 
tance in the calibration of the EK-400 echo sounder; 
Ken Richter, Dave Demer, and Paul Smith for com- 
ments on an earlier draft of this manuscript; and 
Steve Reilly for continued support of environmental 
studies during marine mammal surveys. 
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