Hence survey vessel with acoustic detector emits sound waves to estimate the density of plankton and fish shoal.
Generally, the transducer is put under water, which is linked to an echo sounder in the vessel which records the shoals of fish as "marks" on a screen or paper trace.
[1] One real case of acoustic survey was conducted In July 1993 to examine pre-spawning concentration of autumn-spawning herring using a Simrad EK 500 38-kHz sounder and echo-integrator in the fisheries research vessel in FRV Scotia.
The echo sound provided approximately 2.5*10,000,000 measurements of fish density during acoustic survey of herring population.
Acoustic survey has an advantage since it can be conducted over greater spatial extents in many habitats with little cost, but drawbacks are species identification difficulty and population size counting unavailability.