NOAA Protocols for Fisheries Acoustics Surveys and Related ...

Mid-water juvenile pollock (max sa=1,554)
Dispersed near-bottom pollock (max sa=158)
Non-pollock (max sa=25)
-70 dB
100%
Normalized mean sa
75%
50%
25%
0%
-90 -85 -80 -75 -70 -65 -60 -55 -50
sV threshold (dB)
Figure 3. Contrast in sA as a function of sV threshold for a
dense juvenile pollock mid-water layer, dispersed nearbottom
adult pollock, and non-pollock echo sign. Data
were collected during August 2001 off Kodiak Island,
Alaska.
Range
The vast majority of the areas surveyed by the AFSC are located over the continental shelf
(i.e. bottom depth 200 m) although some effort occurs over the shelf break and upper continental
shelf break slope where depths can exceed 1500 m. In these latter situations, EIT data to
describe pollock distribution and biomass are generally restricted to depths less than 750 m
below the ocean surface.
The theoretical return from a 50-cm walleye pollock (based on the 20Log L–66 target
strength to length relationship) can be detected to a depth of 550 m before falling beneath the
noise threshold.
Acoustic Dead Zones
A fixed depth of 14 m from the surface is used as the surface offset. This value is derived
from the location of the transducer on the centerboard 9 m below the water surface plus a 5 m
buffer zone for the transducer’s near field. An offset of 0.5 m above the sounder-detected
88