Complementary integrated assessment of zooplankton abundance, size structure, and biomass using hydroacoustic and...

Accurate estimation of zooplankton biomass remains challenging due to species-specific acoustic signatures and sampling biases. This study evaluated zooplankton abundance and biomass across acoustically defined layers and assessed the consistency between biomass estimates derived from net sampling and acoustic methods. Zooplankton samples were collected using the Multiple Opening/Closing Net and Environmental Sampling System (MOCNESS) in higher signal scattering layers (HSLs) and lower signal scattering layers (LSLs), with acoustic signals verified using a scientific echosounder (38 and 200 kHz). Although zooplankton abundance was higher in the LSLs, the difference was not statistically significant (p > 0.05). However, significant differences in biomass were observed between HSLs and LSLs (p < 0.05). Notably, the biomass of Euphausia pacifica exhibited strong agreement between net- and acoustic-based estimates (1.7 % overestimation), while small copepods (Calanus sinicus and Paracalanus spp.) displayed significant discrepancies due to their small size and low acoustic reflectivity. These results underscore the complementary strengths of net and acoustic methods and highlight the need for species-specific correction models to enhance biomass estimation in marine ecosystems.