Uncertainty and Acoustic Estimates

Fisheries acoustics is the use of underwater sound to detect, enumerate, and measure the distribution of fish and other living marine and freshwater resources. Uncertainty enters acoustic surveys from a number of sources and here we attempt to identify and describe these sources so that they may be better understood and accounted for. A list of these sources will serve as a starting point from which we can build our understanding.

 

1. Acoustics / Physics components reflect the within beam effects of the acoustic device and will include assessment of target strength, absorption and beam angle.
2. Behavior accounts for uncertainty associated with fish orientation, diurnal movements, migratory effects and vessel avoidance.
3. Classification in association with biological sampling takes into consideration identification of targets, sample size of the biological collections and timing and allocation of samples.
4. Dead Zones exist in reference to the placement of the acoustic device relative to the environment. The area above the transducer is out of detection range of the device, while areas near bottom have a buffer zone of noise making these regions inaccessible to sound detection.
5. Estimation of the mean and variance of abundance from backscattered signal may be derived using classical or geostatistical methods. Understanding the basics of acoustic sampling design (in addition to the biological sampling design discussed above) is important for attaining precise and unbiased estimates.
6. Final Expansion is used to estimate total abundance from mean estimates. Typically assumptions of area sampled by the elementary sampling unit and total area of the population distribution are needed to expand mean estimates up to total abundances estimates.
7. Global Variance Estimation is derived by combining all the sources of uncertainty discussed above.