Adequate in sparse totals of fish, its analysis requires a comprehension of the size distribution and the species type of the surveyed population. Another process is echo integration, which measures the total energy returned from an area in the water column. It provides relative abundance and must be combined with some knowledge of acoustic size to obtain absolute abundance estimates. Measures of individual target strength must rely on individual echoes and can be accomplished by direct or indirect methods. Direct methods use multiple beams to adjust for fish location in the beam while indirect methods use a single transducer and the mean target strength of a collection of echoes to estimate the individual target strengths.
Underwater acoustics has several advantages over the more traditional fish sampling methods.
With the use of sound, an entire water column can be sampled and detailed maps of fish densities can be acquired over large bodies of water without disturbing the aquatic environment. Rapid and continuous sampling of the water column also provides resource managers with uninterrupted fish distributions to better evaluate current fish trends. Fish acoustics does however, suffer from the inability to readily discriminate species in mixed-species situations. Time is required to deploy midwater trawls to identify species of fish within the water column which decreases the efficiency of acoustics surveys. Acoustics, when coupled with midwater trawling and a sound, geographically-stratified sampling design, provides reliable lakewide biomass estimates for the major pelagic prey species within the Great Lakes at an acceptable level of certainty.