NOAA Protocols for Fisheries Acoustics Surveys and Related ...

Detection Probability
Thresholding
The AFSC does not set a data collection S v threshold. The post-processing s v threshold is –
70 dB. This threshold eliminates most of the backscattering attributed to smaller non-walleye
pollock organisms while accounting for most of the echo sign in regions identified as walleye
pollock. When decreasing the s v threshold from –70 dB to –80 dB for the echo sign shown in
Figure 2, the s A of the dense schools of juvenile walleye pollock increased by 1% and the s A of
the dispersed individual adult walleye pollock increased by 9%, whereas the s A of the
unidentified zooplankton increased by 68% (Figure 3). Most of the increase in s A within the
pollock regions can be attributed to the increased detection of smaller non-walleye pollock
scatterers, seen as amorphous stippling throughout the water column seen in Figure 2B. The
post-processing s v threshold is included as a field in the survey database MACEBASE.
Juvenile pollock
Unidentified
zooplankton
Adult pollock
A
B
Figure 2.--Example of thresholding on pollock echo sign using (A) the standard s V threshold
of -70 dB and (B) a decreased s V threshold of -80 dB. Data were collected during August
2001 off Kodiak Island, Alaska.
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