NOAA Protocols for Fisheries Acoustics Surveys and Related ...

factors such as orientation are not taken into account, it might appear that there are fewer or
smaller fish. Thresholding the S v data could compound the error in abundance estimates.
Considerations
Remediation
The survey design (timing and location) should consider potential systematic changes in
detection probability. If systematic changes in detection probability are discovered, either a
change in the survey design is required or analyses should be conducted to determine a
correction factor. If significant changes are made to the survey vessel that are expected to affect
vessel-generated noise (e.g. major modifications to propeller, generators, or main engine), noiserange
measurements should be conducted.
Improvements
The first of four low-noise NOAA survey vessels is currently under construction and will
begin survey work for the AFSC in 2005. The NEFSC, NWFSC, and SWFSC will each receive
one of the other vessels. All four vessels will meet ICES noise standards, which will greatly
reduce the potential for vessel noise affecting fish behavior.
Classification
Definition & Importance
Classification is the discrimination and identification of organism type or species during an
acoustic survey. Discrimination refers to the process of separating targets of interest from other
targets and noise.
Fisheries acoustics surveys are designed to provide density and abundance estimates, usually
age or length-based, for one or more target species. A critical requirement, therefore, is to
separate backscatter of the target species from all other backscatter. This is done in two steps.
First, noise from unwanted sources such as plankton (if not the object of the survey), air bubbles,
bottom echoes, and electrical interference is removed. Then, the remaining targets are
apportioned between species or groups of species of fish or plankton. The main technique used
for this purpose is inspection of echograms. Partitioning of echo integration data is aided by
application of an appropriate S v threshold (S v Detection Probability section) and a survey design
chosen so that the target species is favorably distributed away from boundaries and in schools or
layers that are mono-specific and easily identified. The main source of information used to
validate interpretation of echograms is net catches. Other potential sources of validation data are
underwater video and use of multiple frequency acoustic data.
Techniques
Single Frequency
The standard frequency for estimating density and abundance of marine fish species is 38
kHz; and for krill, 120 kHz. Echo sounders operating at 38 kHz are able to detect fish with
swimbladders, and aggregations of macrozooplankton such as krill. Fish without swim bladders
are more difficult to detect with either of these two frequencies. However, the ability to measure
backscatter from a diversity of species at reasonable ranges make both of these frequencies
useful. A disadvantage is that separating backscatter from target species can be difficult. Thus,
multiple frequency data are often utilized in conjunction with biological sampling as an aid in
classification of the acoustic returns (Reid 2000).
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