Acoustic Estimation of Blue Mackerel (Scomber australasicus) Spawning Biomass in Yilan Bay, Taiwan: Integrating Depth Compensation and Fishery Data (2021–2024)

The mackerel fishery is Taiwan’s most productive coastal fishery sector, with the blue mackerel (Scomber australasicus) being its primary target species. Given the economic and ecological significance of this fishery, considerable attention has been devoted to assessing stock status and promoting sustainable use. Between 2021 and 2024, acoustic transect surveys were conducted in Yilan Bay during the blue mackerel spawning season, supplemented by hook-and-line sampling to confirm the identity of single-target acoustic signals. Acoustic detections within ±10 m of capture depth and ±10 min of capture time were used to establish a depth-compensated regression model linking target strength (TS) to fork length (FL). Validation revealed that over 80% of the hook-and-line samples were blue mackerel. After careful noise filtering, a depth-compensated regression model was established to relate TS to FL and sampling depth. The model incorporated both logarithmic body length and depth terms, effectively accounting for vertical variations in TS. The model improved alignment with biological sampling data by effectively accounting for depth-related variations in TS, thereby enhancing biomass estimation accuracy. Cross-validation with auction records from Nan-Fang-Ao Fishing Harbor confirmed that the acoustic biomass estimates closely mirrored commercial catch trends. These findings highlight the effectiveness of depth-compensated acoustic methodologies for obtaining reliable, fishery-independent spawning biomass estimates, supporting their continued application in long-term monitoring and spatial resource management.