Evalution of the ability to infer tilt angle and size distributions of fish using a broadband scientific echosounder based on simulation

The biological information, such as species, size, and tilt angle, is crucial for converting the echo data into biomass information in acoustic surveys. Typically, the information can be obtained through trawl net sampling or underwater camera observations. However, both methods have some limitations. To overcome these limitations, scientists have utilized inversion methods with multi-frequency and broadband echosounders to derive biological information about fish, plankton, and krill. However, evaluating the reliability and accuracy of these inversion methods has been challenging due to the difficulty in obtaining accurate biological information. In this study, a numerical simulation method was used to generate fish school echoes with custom biological parameters, which were then used to infer biological information. The results showed that a reasonable distribution of fish tilt angles could be obtained through inversion using the mean target strength (TS) spectra. However, the cost function did not converge when the mean relative frequency response of volume backscattering (Sv" role="presentation" style="-webkit-user-drag: none; -webkit-tap-highlight-color: transparent; margin: 0px; padding: 0px; user-select: text; display: inline-table; line-height: normal; font-size: 14.6667px; font-size-adjust: none; word-spacing: normal; overflow-wrap: normal; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;">SvSv) spectra was used. Furthermore, the error in inferring fish size using the mean TS spectra (assuming a broadband echosounder) was lower (5.7%) compared to the utilization of the mean TS at 120 kHz (assuming a traditional narrowband echosounder) with a higher error rate of 8.6%. This study particularly highlights the advantages of using broadband echosounders and the measured mean TS spectra for inferring biological information.