Fiedler et a I.: Dolphin prey abundance determined from acoustic backscatter data 
239 
tion 2 for the on-axis sensitivity gave ( SL+RR ) = 
74.2 dB. 
Operationally, the ADA program squared V Q , which 
is proportional to pressure at the face of the trans- 
ducer, to give an echo intensity value (Eq. 1) propor- 
tional to power or energy flux per unit of area. The 
intensity values were then binned in 50 10-m bins 
between 10-m and 5 10-m depth. The binned intensi- 
ties were averaged at 10-sec intervals over ten min- 
utes (60 pings), equivalent to an interval of 3 km at 
the typical ship speed of 18 km/h. Recorded intensi- 
ties were converted to units of decibels (dB=10 log// 
I , where I r represents the standard reference of IpPa 
at a 1-m distance from the face of the transducer). 
Then, a correction to the TVG function was formu- 
lated by using depth-dependent sound velocities and 
attenuation coefficients calculated from CTD data 
taken once or twice daily with the algorithms of 
Francois and Garrison (1982). Finally, a constant 
53.4 dB was subtracted to account for the combined 
source level, transducer response, receiver gain, and 
beam pattern terms (-SL - RR - 10 log Vj/ - 10 log t(/ 
2) in Equation 1. The equivalent beam angle is very 
difficult to measure directly without special appara- 
tus; therefore we used the manufacturer’s nominal 
value (10 log vj/=-19.6 dB; Simrad EK400 Scientific 
Sounder Instruction Manual). 
Acoustic Doppler current profilers have been used 
since the early 1980’s to estimate current velocity from 
the Doppler shift of acoustic signals backscattered 
from suspended matter (plankton and sediments) in 
the water column. Echo intensity is calculated and 
recorded during data-processing. RD Instruments 
has developed instrument modifications and an al- 
gorithm to calibrate these values and to estimate vol- 
ume backscattering strength (S v , dB). The algorithm 
is based on the following working version of a sonar 
equation (RD Instruments, 1990): 
S v = 10 log 
4.47 x 10~ 20 K 2 K s (T t +273) 
( 10 
k(E—E r )/ 10 -j\^2 / 
(cPK 1 10 
■2aR/10\ 
(3) 
where K 0 = system noise factor = 4.3; 
K s = system constant = 4.17 x 10 5 ; 
T f = transducer temperature (°C); 
k = conversion factor = 0.435 dB/count; 
E = echo intensity (counts); 
E r = reference level for echo intensity (counts); 
R = slant range (m); 
c = sound velocity (m/s); 
P = transmit pulse length = 8 m; 
ATj = power into the water, W; 
a - sound absorption coefficient (dB/m). 
Although this equation was formulated to be ap- 
plied to individual beams of the ADCP, we used av- 
erage amplitude data for E because we were using 
an empirical reference level (E r ). The reference level 
for echo intensity, E r , represents thermal noise in the 
system electronics plus ship noise. E r was assumed 
equal to the minimum value of E (<40 counts) ob- 
served in each average profile or ensemble (-740 
pings in 10 minutes). The manufacturer states that 
this in-situ method of estimating E r is preferable to 
calculating E r from measured system temperatures. 
Because ship temperature was relatively constant 
and water temperature ranged over only 4°C in 1992 
and 10°C in 1993, E r varied by only (±7-8 counts. 
Values of E < 10 counts were not used (RD Instru- 
ments, 1990) and appear as blank areas at depth in 
150 kHz time-depth sections (Fig. 2). 
Values for c and a were calculated from CTD data 
taken once or twice daily according to Francois and 
Garrison (1982). Transducer temperature, T t , was 
assumed equal to observed surface temperature. 
Slant range, R, was corrected for estimated sound 
velocity in relation to the value assumed by the ADCP 
software (1,475 m/s). The conversion factor, k (dB/ 
count), depends on the temperature of the system 
electronics. However, the deck unit was located in a 
temperature-controlled room (18-21°C) so that the 
range of k was 0.433 to 0.437 (<1 dB). Power into the 
water, K v was assumed constant and estimated to 
be 53.4 W (RD Instruments, 1990). The ADCP was 
set up with 50 bins of length 8 m, starting at 4 m 
from the transducers. 
In both the 38- and 150-kHz data sets, the depth 
bin representing the bottom was identified initially 
by an algorithm searching for strong gradients of 
S v (z). These results were modified by visual inspec- 
tion of time-depth sections. Data from the bottom 
depth bin and below were excluded from subsequent 
analysis. Some 1992 and 1993 profiles of S appeared 
to be biased low on account of sound attenuation due 
to bubbles beneath the ship during stationkeeping 
and during very rough weather (cf. Fig. 2, 20 August, 
at the first tick marking the morning CTD station). 
We identified biased profiles from anomalously low 
S y in bins below the top depth bin. The problem was 
more prevalent in the 150-kHz data (2.5% of 1992 
profiles and 1.0% of 1993 profiles) than in the 38- 
kHz data (0.5% of 1992 profiles and 0.1% of 1993 
profiles). We excluded the biased profiles from sta- 
tistical analyses. 
Dolphin sightings were recorded during daylight 
hours from the flying bridge of the vessel by a team 
