Fiedler et a I.: Dolphin prey abundance determined from acoustic backscatter data 
243 
offshore part of the study area and increased 
towards the southeast corner near Cabo 
Corrientes (20°N, 105°W). 
Time-depth sections (Fig. 2) show a close 
relation between the deep scattering layer 
and the thermocline at night. The DSL as- 
cended through the thermocline in the evening, 
then remained just above the thermocline un- 
til it descended in the early morning. The DSL 
followed changes in the depth of the ther- 
mocline during some nights (cf. 22-23 August, 
Fig. 2). Where the thermocline is deep, S v is 
maximum at 10-20 m above the thermocline 
and then declines closer to the surface. Where 
the thermocline is shallow, S v is maximum at, 
or very close to, the surface. The nighttime 
depth of the DSL was significantly correlated 
with thermocline depth, except for the 38-kHz 
DSL in 1993 (Table 2). Mean S v above the ther- 
mocline was slightly, but not significantly, 
greater above more shallow thermoclines 1992. 
Dolphin abundance, as indexed by sight- 
ing rates of schools or individuals per 100 km, 
was significantly correlated with nighttime 
mean S v above the thermocline in 1992 and 
with 0-400 m mean S v in 1993 (Table 3, the 
150-kHz results are shown here owing to 
larger sample sizes but the 38-kHz^ results 
were similar). In 1992, both mean S v above 
the thermocline and dolphin abundance were 
high near the countercurrent thermocline ridge 
along 9°N (Fig. 6). In 1993, mean S v above the 
thermocline and dolphin abundance were rela- 
tively high near the mouth of the Gulf of Cali- 
fornia, at intermediate thermocline depths. 
Discussion 
Deep scattering layers have been observed 
throughout the Pacific with low-frequency 
depth sounders (<30 kHz). Tont (1976) re- 
ported mean daytime depths of 319 m in the 
eastern tropical Pacific and 282 m in the Cali- 
fornia Current, similar to the depths observed 
here (Fig. 2). Small mesopelagic fish with gas- 
filled swimbladders are now recognized to be 
the most important low-frequency back- 
scatterers in the open ocean (Chapman, 1947; 
Marshall, 1951; Brooks, 1977), although early 
reports attributed DSL’s to cephalopods 
(Lyman, 1948) or euphausiids (Hersey and 
Moore, 1948). 
What do our acoustic observations represent? Vol- 
ume scattering strength (dB) represents a sum of in- 
20 - 
-120 -115 -110 -105 
Figure 5 
Maps of daily (0800-1600) and nightly (2000-0400) 38-kHz mean 
volume scattering strength (S v , dB) in the surface layer (0-100 m) 
during the PODS93 survey, 28 July-6 November 1993. 
dividual target strengths (TS), except at very high 
target densities where shadowing and multiple scat- 
-115 
110 
