Spatial distribution and resource status of fish in the Nanchang section of the Ganjiang River based on hydroacoustic assessment

To evaluate the spatial distribution and resource status of fish in the Nanchang section of the Ganjiang River, three research areas—upper, middle and lower reaches—were selected for a hydroacoustic survey from August 14 to 16, 2020. The results showed that a total of 171 684 fish signals were detected. The mean target strength in the upper, middle, and lower reaches was (−61.9±4.9) dB, (−58.4±3.2) dB, and (−61.2±3.7) dB, respectively, indicating stratified signal concentrating in the middle and bottom water layers. The average fish density in the upstream section was 3.84 ind.·m⁻³, ranging from 0.01 to 575.42 ind.·m⁻³, with a mean total length of approximately 4.4 cm. In the midstream, the average fish density was 5.91 ind.·m⁻³, ranging from 0.04 to 877.81 ind.·m⁻³, with a mean total length of approximately 5.2 cm. In the downstream section, the average fish density was 6.37 ind.·m⁻³, ranging from 0.04 to 607.03 ind.·m⁻³, with a mean total length of approximately 3.9 cm. There was no significant difference in the horizontal distribution of fish density among the three regions. However, vertically, fish density exhibited significant stratification, with over 80% of fish distributed in the middle and bottom layers across all three regions. Additionally, the estimated total fish biomass in the upper, middle, and lower reaches was 8.52×10⁷ ind., 16.56×10⁷ ind., and 1.255×10⁷ ind., respectively. These results can support the sustainable utilization and conservation of fish resources in the Ganjiang River.