Bottom-moored acoustic monitoring reveals seasonal, diel, and vertical variability of fish acoustic backscatter in Jinhae Bay, Korea

Fine-scale temporal variability in fish distribution is critical for ecosystem monitoring in dynamic coastal environments; however, continuous observations across multiple temporal and spatial scales remain limited. A bottom-moored acoustic observation system equipped with a wideband autonomous transceiver echosounder, underwater cameras, and a temperature logger was deployed in Jinhae Bay, South Korea, from April to September 2024. Acoustic observations were conducted at 70 and 120 kHz, and fish-associated acoustic backscatter was classified using frequency differences in mean volume backscattering strength. Seasonal, diel, and vertical variability in acoustic backscatter were quantified, and generalized additive models (GAMs) were applied to examine the effects of depth and bottom temperature on acoustic backscatter patterns. Individual acoustic targets were further characterized using target tracking to provide additional evidence supporting the interpretation of the backscatter as fish-associated. Fish-associated acoustic backscatter peaked in spring, declined markedly in summer, and partially recovered in autumn, with the highest backscatter occurring in upper 5 m. Diel variability was gradual, with no pronounced day–night transitions. Warmer bottom-water conditions (≥16 °C) were linked to greater short-term variability in acoustic backscatter. GAM analysis indicated season-dependent associations, with both depth and bottom temperature significantly associated with backscatter in spring, whereas summer patterns were mainly associated with depth. These results demonstrate that bottom-moored acoustic monitoring provides high-resolution observations of fish-associated acoustic backscatter across multiple temporal scales and can support ecosystem monitoring and fisheries management in dynamic coastal systems.