Large differences in the distribution of pelagic biomass as a result of sonar frequency choice.

Recent efforts to understand the global distribution of pelagic fauna have primarily relied on 38 kHz sonar observations, using water-column backscatter as a proxy of biomass. However, backscatter gradients across ocean provinces are not always consistent with biomass observations from net sampling. This mismatch is particularly evident in temperate to polar transition zones due to changes in the resonance of pelagic fauna, which depends not only on the sonar frequency but also on the size of resonant organs such as fish swimbladders. Here, we investigate how sonar frequency choice changes our vision of pelagic ecosystems across latitudes. We analyse sonar observations at 38 and 18 kHz along with size distributions of swimbladdered fish species across the Indian Ocean Subantarctic Front. Our results show a shift from 38 to 18 kHz dominance towards the poles. More interestingly, backscatter differences across the Subantarctic Front are four times larger at 38 kHz compared to 18 kHz. Size distribution of fish suggests an increase of swimbladder volumes in subantarctic waters, which may explain the observed shift in frequency response. This study highlights the need to address swimbladder resonance variability across latitudes in order to correctly interpret large-scale sonar observations of pelagic fauna.