NOAA Protocols for Fisheries Acoustics Surveys and Related ...

elative to the transducer. In general, backscatter in the geometric scattering region is strongest
when the major axis of the organism is perpendicular to the transducer’s acoustic axis and
weakest when the organism’s major axis is parallel to the acoustic axis. Activities such as
swimming, feeding, or vertical migrations potentially affect Sv measurements by increasing or
decreasing detection probabilities. Variability in backscattering strength as a function of
organism orientation and behavior combined with Sv thresholding may affect the detection
probability. For example, when the distribution of organism orientations is centered near
perpendicular to the transducer, the detection probability may be high. When these organisms
orient at angles near parallel to the acoustic axis, the S v decreases and potentially below the S v
threshold.
Vertical and horizontal migrations can affect detection probabilities. Migration into or out of
the near-surface or bottom zones during the survey will nearly eliminate the detection probability
for these organisms. Horizontal migrations may take the organisms out of the survey area or
outside of the acoustic beam.
Vessel Noise and Avoidance
All vessels radiate underwater noise (Urick 1983). Fish species are able to detect this vessel
noise over a range of frequencies from tens to at least several hundred Hz (Mitson 1995).
Whether the fish react to the vessel noise, thereby altering their behavior and detection
probability has been the subject of much research (Ona and Godo 1990, Mitson 1995, Handegard
et al. 2003). Vessel avoidance is defined as a zero detection probability resulting from a change
in behavior due to the vessel noise, but even less severe changes in behavior may affect S v
measurements and the detection probability. The ICES Cooperative Research Report No. 209
(Mitson 1995) provides guidelines for making noise range measurements and gives
recommendations for dealing with vessel noise and avoidance, including, in particular, proposed
standards for underwater radiated noise levels in the design of new vessels. Underwater noise
levels should be determined for vessels to be used in fishery research that do not conform to the
ICES standard. Mitson (1995) recommends that ‘observations should be carried out whenever
possible to relate the known (measured) characteristics of a vessel to any observed avoidance
behavior of fish, or to noise affecting acoustic survey equipment.’
Multiple scattering and shadowing
The theory of the linear summation of individual echo strengths within a sampling volume is
valid for a wide range of organism densities (Foote, 1983b). However, when organism densities
are high, multiple scattering (echoes have scattered off multiple individuals before returning to
the transducer) and shadowing (similar to bubble attenuation, the pressure wave amplitude
decreases at a greater rate than explained by transmission loss) have non-linear affects on the
summation of echoes within a sampling volume (MacLennan 1990, Toresen 1991).
Error
Uncertainty in detection probabilities of target and non-target species affects interpretation of
S v measurements and the efficacy of post-processing techniques. Systematic and random
changes in detection probabilities during the survey will have linear and non-linear effects on S v
measurements. A systematic change in fish orientation, for example, from a horizontal to a more
vertical position during vertical migration, will cause a decrease in the volume backscattering. If
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