Using active acoustics from an underwater glider to investigate the relationship between acoustic target locations and coastal ocean fronts off the Pacific Northwest coast

Abstract

Traditional ship-based sampling has provided valuable information about marine ecosystems. However, repeated high-resolution measurements are increasingly necessary to capture dynamic biophysical processes. In this study, we used active, multi-frequency acoustics from an autonomous underwater ocean glider to examine diel vertical migration and the relationship between zooplankton and fish aggregations to coastal fronts off the coast of Washington in the Pacific Northwest. The objectives of this study were: (1) to examine the reliability of glider acoustics by comparing it with acoustic data from a ship; (2) to assess diel migration patterns over the continental shelf; (3) to relate the spatial and temporal distributions of zooplankton and fish to frontal positions. Sampling occurred in late spring/early summer across both coastal upwelling and buoyancy-driven freshwater fronts. A Slocum glider, equipped with sensors for conductivity, temperature, depth, bio-optical properties, including chlorophyll and colored dissolved organic matter (CDOM) fluorescence, dissolved oxygen, and multi-frequency acoustics (67, 120, 200 kHz), completed 10 cross-front transects over 1–2 weeks. One cross-shelf transect was sampled concurrently with the NOAA Ship Bell M. Shimada to enable direct comparison of active acoustic profiles. The glider- and ship-based acoustic observations were consistent (R 2 = 0.87), confirming the glider system’s reliability. Diel vertical migration was not clearly resolved, mainly due to limitations of frequency differentiation; however, distinct vertical partitioning was evident, with “fish” targets occupying the upper water column and “zooplankton” the deeper layers. Both groups were predominantly distributed offshore of the upwelling front. This study demonstrates the potential and challenges of using multi-sensor autonomous underwater vehicles in upwelling regions to resolve fine-scale biophysical patterns, highlighting their value as a complementary tool for traditional ship surveys.