# Editorial for Special Issue: Fishery Acoustics, Applied Sciences, and Practical Applications
## 2 Acoustic Biomass Estimation  
The first interest in the fishery science and applications is what amount of biomass we can estimate in a limited space of ocean under a realistic situation. The practical approach is quite straightforward, (1) each target fish is quantified as Target Strength amount, (2) which can be described as a reflected amount of acoustic backscattering from the target, (3) using an underwater acoustic transceiver, and (4) the total integrated backscattering represents the amount of the biomass or the number of biomass once normalized by the Target Strength. The practical approach still has many pitfalls to be filled. There are many fish species in the real world, and we need constant evaluation of Target Strength specific to each fish at each environment. Backscattering is a hypothetical concept, for which real measurements need analytic investigation on multiple scattering theory especially for the dense biomass. Practical instruments are still evolving for measuring physical acoustic strength. Finally, counting is another difficulty, while the distribution of fish length is another issue to be resolved concurrently.  
In the paper \"Acoustic Target Strength of the Endangered Chinese Sturgeon (Aciepenser sinensis) by Ex Situ Measurements and Theoretical Calculations\", [PERSON] et al. [5] presented the Target Strength of Chinese Sturgeon with 250 cm for the first time, using a 199-kHz split echo sounder. The sturgeon breeding in the Yangtze River is critically important for effective management and population preservation, and this research is expected to provide a good basis for future hydroacoustic studies on the critically endangered Chinese sturgeon.  
Another Target Strength measurement on Redlip mullet (Chelon haematocheilus) distributed in coastal waters of the North-Western Pacific Ocean is reported in [6], which is popularly aquacultured in Korea. In [6], [PERSON] et al. used the split-beam scientific echo sounder comprised of 38, 120, 200, and 420 kHz frequencies.  
[PERSON] et al. [7] explored and reported acceptable differences of fish density and Target Strength distribution estimates between two ingle beam systems, EK15 and EY-M, in the paper \"Comparing Fish Density and Echo Strength Distribution Recorded by Two Generations of Single Beam Echo Sounders\". They showed that the measurement site, the oligotrophic Lake Storsjeen (48 km\({}^{2}\)), has nurtured the introduced smelt population since 2013, by using the dramatic increase of density of TS = \(-\)46 to \(-\)44 dB echoes (>10 cm) between 2013 and 2016.  
In the paper \"Variation of Zooplankton Mean Volume Backscattering Strength from Moored (750 KHz) and Mobile (307.2 KHz) ADCP Instruments for Diel Vertical Migration Observation\", [PERSON] et al. [8] used acoustic Doppler current profiler (ADCP) instruments to measure the mean volume backscattering strength (MVBS), with proper deletion of the Zooplankton migration effect. The results include analysis of the MVBS and vertical velocity from ADCPs at the same time and location for zooplankton's daily vertical migration (DVM) observation.