NOAA Protocols for Fisheries Acoustics Surveys and Related ...

Figure 1. Normalized water column S A and Atlantic herring S A as a function of S v
threshold. Data were collected during September 1999 on Georges Bank.
Range
Currently we do not have protocols to account for range or signal to noise affects.
Acoustic Dead Zones: Near surface and near bottom
No S v data are eliminated during data collection.
When post-processing the S v data, a constant depth below the surface is chosen where data
above this depth (i.e., near-surface data) are eliminated from analyses (this depth is commonly
called the ‘bubble layer’). The minimum depth is set to 10 m for the 18, 38, and 120-kHz echo
sounders. The minimum depth was set to 32 m for the 12-kHz echo sounder. The depth for the
18, 38, and 120-kHz systems was chosen based on: 1) the hull-mounted transducers are located
approximately 3 m below the surface, 2) the near field of the 38 kHz transducer is approximately
7 m, 3) a consistent depth is desired to compare data among the frequencies, and 4) under normal
survey conditions, surface noise (e.g., bubbles) do not penetrate deeper than 10 m. The 32 m
depth was chosen for the 12 kHz due to significant ‘ring-down’ of the 12-kHz transducer in the
top 30 m.
During data collection and post-processing of the S v data, a constant distance above the
bottom where data below this depth are eliminated from analyses (this distance is commonly
called the ‘backstep’) is selected. The ‘backstep’ is set to 0.5 m for all frequencies. This
distance was chosen based on observations of the S v data and the EK500 and Echoview bottomdetection
algorithms. EK500 bottom-detection parameters are:
i. Minimum Depth: 5.0 m
ii. Maximum Depth: 500 m (dependent on survey area)
iii. Min. and Max. Depth Alarm: 0.0 m
iv. Bottom Lost Alarm: Off
v.Minimum Level: -50 dB
For post-processing the S v data, we use Echoview’s bottom-detection algorithm to select the
echoes from the seabed. Echoview bottom-detection parameters are:
i. Bottom detection algorithm: Maximum Sv with backstep
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