The research, led by Otavio A. P. Mendes and colleagues at Oregon State University together with NOAA fisheries scientists, deployed a Teledyne Webb Slocum electric glider off the Washington coast from late May to early June 2022. The torpedo-shaped vehicle flew a zigzag course between 46.7 and 47 degrees north latitude, completing ten cross-shelf transects over one to two weeks. Each transect sliced through two of the region’s defining oceanographic features: a wind-driven upwelling front, where cold, nutrient-rich water rises toward the surface, and a buoyancy-driven freshwater front created by the Columbia River plume.

The glider carried an impressive sensor suite for a vehicle that weighs little more than a person. A conductivity-temperature-depth profiler measured the water’s physical structure, while a WET Labs ECO Puck Triplet tracked chlorophyll-a fluorescence, optical backscatter, and colored dissolved organic matter. An Aanderaa optode recorded dissolved oxygen. But the centerpiece was an Acoustic Zooplankton and Fish Profiler, or AZFP, pinging sequentially at 67, 120, and 200 kilohertz every five seconds during dives, with transducers angled 26 degrees downward so the beams pointed straight into the water column below.

Active acoustics work by transmitting sound pulses and listening for echoes bounced off organisms. Because gas-bearing animals such as fish with swim bladders scatter sound differently across frequencies than soft-bodied zooplankton, comparing backscatter at multiple frequencies allows researchers to sort echoes into broad biological categories. The team quantified this difference as the change in mean volume backscattering strength between frequencies, then classified targets with a difference below 5.25 decibels as fish-sized and above it as zooplankton-sized. When that fixed threshold proved ambiguous, with overlapping distributions spanning roughly three decibels on either side, the researchers turned to a Gaussian mixture model, a statistical technique that fits two overlapping bell curves to the histogram and assigns each acoustic target a posterior probability of belonging to each class. Only targets with at least 92 percent confidence were retained.

Validation came from an unusual partnership. On the evening of May 28, 2022, the NOAA Ship Bell M. Shimada steamed the same cross-shelf line at 47 degrees north while the glider worked below. The ship covered the roughly 20-kilometer transect in about two hours; the glider needed nearly 29. The team restricted their comparison to the subsection with the closest temporal overlap, averaged the data into depth profiles, and fit a linear regression between the two platforms’ backscatter measurements. The result was striking: a coefficient of determination of 0.87, confirming that the pint-sized glider system could reproduce the vertical structure seen from a research vessel, including subsurface backscatter maxima near 20 and 40 meters depth and a zone of reduced scattering between 40 and 45 meters. Glider returns were somewhat weaker in magnitude, likely reflecting its smaller transducers and lower acoustic power, along with the hours of temporal separation between the two sampling platforms.

Independent support came from the ship’s nets. A midwater trawl close to shore, at the 50-meter isobath about 16 kilometers offshore, was dominated by whitebait smelt, which made up roughly 95 percent of the catch. Farther out, at the 115-meter isobath, the trawl was dominated by invertebrates: euphausiids accounted for 55.6 percent of the catch and oceanic pink shrimp another 44 percent. Those catches aligned with the acoustic picture of fish-sized targets near the coast and zooplankton-sized scatterers offshore.

One of the study’s most intriguing findings concerns diel vertical migration, the planet’s largest synchronized animal movement, in which plankton and fish rise toward the surface at night and sink during the day, traditionally attributed to the trade-off between feeding in productive surface waters and hiding from visual predators in the dark depths. Off Washington, the classic pattern failed to appear. Zooplankton-sized targets sat at about 50 meters during the day and, counterintuitively, deeper at about 62 meters at night, a difference that was not statistically significant. Fish-sized targets stayed shallow throughout, hovering between roughly 16 and 20 meters with no meaningful day-night shift. When the researchers computed the weighted mean depth of backscatter hour by hour across the entire survey, zooplankton showed a gradual, continuous drift through the diel cycle, but in the opposite direction from textbook expectations.

The authors attribute this reversed and muted migration signal to the region’s violent physical dynamics. Shifting front positions, predator-prey interactions, and variable wind forcing likely scrambled any circadian-driven pattern. The survey itself caught the ocean in transition: alongshore winds swung from 7 meters per second southward, favoring upwelling, to 10 meters per second northward, favoring downwelling, as the system moved from spring downwelling toward summer upwelling. In the ten days before the survey, cumulative upwelling-favorable wind stress reached minus 48.56 newtons per square meter, with peaks near minus 0.376, meaning the cold, salty water the glider encountered had been set in motion by winds that had already relaxed. Recent work has shown that diel migrators respond to short-term upwelling events, and that wind relaxation can launch buoyant, northward-propagating plumes that reshape shelf stratification, both processes that could override or reverse conventional migration behavior.

The spatial story was clearer. Fitting exponential functions to isopycnal depths, the researchers located the upwelling front within about 25 kilometers of shore on average, and the freshwater front farther out at a more variable mean of roughly 29 kilometers. Across all ten transects, both fish and zooplankton acoustic maxima sat predominantly offshore of the upwelling front, typically within a few kilometers of the front itself rather than at the far edge of the survey. Along the validation transect, chlorophyll-a peaked near the surface offshore of the frontal intersection, with a secondary maximum of 16.3 micrograms per liter at about 30 meters depth, coinciding with elevated CDOM. Fish targets showed a weak, non-significant positive correlation with chlorophyll, and the whole pattern reinforced the idea that upwelling fronts act less as symmetric aggregation zones and more as boundaries that fish and plankton press up against, consistent with earlier findings that planktivorous fish distributions stop at these fronts.

The study demonstrates both the promise and the growing pains of autonomous bioacoustic monitoring. Gliders offer continuous, high-resolution, three-dimensional sampling at a fraction of ship costs, and previous work has shown glider-based biomass estimates within 4 to 7 percent of ship surveys. But a single glider moves slowly and samples different places at different times, producing snapshots rather than simultaneous community portraits, which complicates the detection of rapid behaviors like diel migration. The 67-to-200-kilohertz frequency pair also proved suboptimal; prior studies suggest 38 and 200 kilohertz separates zooplankton from swim-bladder-bearing fish more cleanly, but a functioning 38-kilohertz glider transducer was not yet available. The authors suggest that future missions pair gliders with net tows or low-cost cameras for direct validation, deploy multiple vehicles simultaneously to shrink sampling gaps, and target periods of strong upwelling. If those refinements succeed, robot oceanographers could soon keep constant watch over the living boundaries of coastal seas, complementing ships and extending ecosystem monitoring into places and times vessels rarely reach.

Subject of Research: Glider-based multi-frequency active acoustics for mapping fish and zooplankton distributions relative to coastal upwelling and river-plume fronts off the Pacific Northwest

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

Article References: Mendes, O. A. P., Barth, J. A., Erofeev, A., Marquardt, J., Wells, B. K., Huff, D. D., & Cossio, A. M. (2026). Using active acoustics from an underwater glider to investigate the relationship between acoustic target locations and coastal ocean fronts off the Pacific Northwest coast. Frontiers in Marine Science, 13, Article 1944587. https://doi.org/10.3389/fmars.2026.1944587

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.1944587

Keywords: underwater glider, active acoustics, zooplankton, diel vertical migration, coastal upwelling, ocean fronts, Columbia River plume, California Current System, fisheries survey, echosounder, Washington coast, autonomous platforms