Fiedler et al.: Dolphin prey abundance determined from acoustic backscatter data 
241 
PODS93 
PODS92 
38 kHz S v (dB) 
Figure 3 
Relationships between 38-kHz and 150-kHz volume scattering strength (S v , dB) in corresponding 10-and 8-m depth bins (cen- 
tered at 25, 65, 105, 145, 185, 225, 265, and 305 m) at all times of day or night during PODS92 (rc=24,193) and PODS93 (n=26,963) 
surveys. 
-100 
-100 
-100 
-100 
Table 1 
Mean day (0800-1600) and night (2000-0400) volume scattering strengths 
(S,,,dB) in the surface layer (0-100 m) in 1992 and 1993. Values are cruise 
means, with range in parentheses. PODS92 = Population of Delphi nus Stocks 
1992 cruise; PODS93 = Population of Delphinus Stocks 1993 cruise. 
Day 
Night 
PODS92 38 kHz -75.1 (-82.2 to -70.7) 
150 kHz -73.9 (-83.7 to -69.0) 
-68.6 (-77.1 to -62.0) 
-67.5 (-76.8 to -57.3) 
PODS93 38 kHz -76.3 (-85.8 to -69.4) 
150 kHz -79.2 (-90.2 to -70.1) 
-65.3 (-73.5 to -60.2) 
-68.2 (-80.3 to -59.7 ) 
pared with 0.008 dB/m at 38 kHz) 
and 2) transducer orientation 30° off 
the vertical, resulting in a slant range 
15% greater than depth. 
Volume scattering strengths at 38 
and 150 kHz were significantly cor- 
related (Fig. 3, r= 0.58 in 1992 and 
r=0.77 in 1993). Some error in this 
relation is due to the different sizes 
and inexact depth matching of bins 
between frequencies. The ADCP 
samples echo intensity only in the 
last quarter of each depth bin, so that 
scattering is averaged over only 2 m, 
compared with 10 m for the 38-kHz echo sounder. Cor- 
relation decreased at depth (e.g. in 1993, r=0.83 at 25 
m and r=0.53 at 185 m). This could have been due sim- 
ply to the low signal to noise ratio at depth in the 150- 
kHz data. However, it probably also reflects changes 
in species and size composition, and thus spectral re- 
sponse, of scatterers at depth. The mean difference be- 
tween 38- and 150-kHz volume scattering strength in 
Figure 3 was -2.9 dB in 1992 and 0.0 dB in 1993. 
Spatial patterns of mean day and night volume 
scattering strength ( S v ) in the surface layer (0-100 m) 
were very similar at the two frequencies; therefore 
only the 38-kHz patterns are illustrated here (Figs. 
4 and 5). On average, night values exceeded day val- 
ues by 6.4 dB in 1992 and 11.0 dB in 1993 (Table 1). 
Thus, near-surface S v was 4 and 13 times greater at 
night than during the day, indicating that 77-92% 
of the scatterersjeft the surface layer during the day. 
Mean surface S v values (daily or nightly, 38 or 150 
kHz) varied by 11.5 to 20.6 dB during the cruises, 
representing ranges of 14 to 115 times. 
In 1992, low S v was_pbserved along 8-9°N both 
night and day (Fig. 4). S v increased to the south to- 
wards the equator and to the north and east towards 
