Oceanic mesoscale eddies, ubiquitous swirling structures, are often considered 'oceanic oases' that aggregate marine life. Previous research, primarily focusing on predator distribution via fishing catches and satellite tracking, suggests a preference for anticyclonic eddies (AEs) due to increased food resources attracting predators that feed on forage fauna (small fish, crustaceans, and molluscs). However, studies on forage fauna aggregation within eddies have yielded mixed results: some show aggregation in AEs, others in cyclonic eddies (CEs), and some show no discernible effect. The mechanisms proposed for forage fauna aggregation include a bottom-up structuring effect (increased chlorophyll attracting zooplankton, boosting forage fauna), a trapping effect (physical trapping of micronekton), and physical convergence of water masses in AEs. The lack of comprehensive observations of forage fauna communities, typically requiring expensive and labor-intensive oceanographic cruises with net tows or ship-borne acoustic echosounders, has hindered a full understanding of these processes. Previous studies with limited sample sizes (13 and 4 eddies respectively) have reported an oasis effect, but these are insufficient for global conclusions. This study aimed to systematically evaluate the influence of eddies on forage fauna using a vast acoustic database to test the widely held 'eddy-enhanced forage biomass' hypothesis.

Existing literature demonstrates conflicting findings regarding the effect of oceanic eddies on forage fauna. While some studies suggest an aggregation of forage fauna in anticyclonic eddies (AEs) due to increased food resources or physical trapping, others reveal no significant effect or even a negative correlation in cyclonic eddies (CEs). These discrepancies could stem from differences in study locations, eddy characteristics (age, size, amplitude, lifespan), methodologies, and limited sample sizes. The lack of comprehensive, global-scale datasets on forage fauna distribution within eddies has made it difficult to draw firm conclusions. This study addresses this gap by leveraging a large global dataset of acoustic data to provide a more comprehensive assessment of the phenomenon.

This study combined an extensive database of acoustic vertical profiles (covering the upper 750 m of the ocean) from various global oceanic regions (2001–2020) with a global eddy database derived from satellite altimetry. The acoustic data, processed to derive the Nautical Area Scattering Coefficient (NASC) as a proxy for forage fauna biomass, covered >350,000 km² and encompassed 999 eddies (approximately evenly split between AEs and CEs). The 38 kHz frequency used primarily detects gas-filled organisms, potentially overlooking some components of the forage fauna community. For each eddy, acoustic observations were categorized as 'Inside' and 'Outside' based on eddy contours. Acoustic anomaly values were calculated by comparing the mean inside and outside profiles, controlling for diel vertical migration by comparing only day to day and night to night. Remotely sensed sea surface temperature (SST) and surface chlorophyll-a concentration (Chl) data were also extracted for comparison. Statistical analyses, including Wilcoxon tests, were used to compare NASC values inside and outside eddies and to assess the relationship between forage fauna response and various eddy characteristics (amplitude, SST anomaly, Chl anomaly, trapping metric, effective area, lifespan).

The study found that while eddies exhibit consistent and significant impacts on SST and surface Chl, with CEs generally showing colder temperatures and increased Chl and AEs the opposite, the effect on forage fauna is surprisingly subdued. Only a small percentage of eddies (around 13%) showed a significant effect on forage fauna, and only 6% demonstrated a clear 'oasis effect' (significant increase in forage fauna density inside the eddy). This lack of effect was observed across both AE and CE, and in both epipelagic (0-200m) and mesopelagic (200-750m) layers. Further analysis revealed that eddy amplitude and trapping capacity were the only characteristics significantly associated with increased or decreased forage fauna density. Stronger eddies with higher trapping capacity, whether AE or CE, were more likely to exhibit either increased or decreased forage fauna biomass. This suggests a physical trapping mechanism rather than a simple bottom-up control via phytoplankton increase. Regional analysis indicated some areas with higher proportions of eddies impacting forage fauna (e.g., subtropical regions and southern latitudes with strong currents), but even in these areas the null effect remained dominant. Even among the strongest eddies, a significant portion showed no effect on forage fauna.

The findings challenge the widely held belief that oceanic eddies consistently act as oases for forage fauna. The limited influence observed, particularly the dominance of null effects, suggests that the commonly cited 'bottom-up' mechanisms (increased chlorophyll leading to increased zooplankton and forage fauna) may not be as prevalent as previously thought. The importance of physical trapping in the few eddies affecting forage fauna highlights the role of hydrodynamic processes in shaping pelagic ecosystems. The study's focus on open-ocean eddies, in contrast to previous studies often near coastlines or strong currents, might explain some discrepancies, as strong currents and coastal effects might enhance the aggregation effect. The spatial distribution of prior studies and the dataset used in this study may potentially lead to some overestimation of null-effect eddies. Future research is needed to specifically examine the role of the bottom-up effect and other mechanisms in areas not sufficiently sampled by this study.

This study, using a comprehensive global dataset of acoustic data, demonstrates that the impact of oceanic eddies on forage fauna is more nuanced than previously thought. While strong eddies with high trapping capacity can significantly influence forage fauna density, the 'oasis effect' is far from ubiquitous. This suggests that mechanisms beyond simple forage fauna aggregation may contribute to the observed abundance of top predators in eddies. Future research should focus on expanding data coverage in under-sampled regions, incorporating additional eddy characteristics and environmental parameters, and directly observing prey-predator interactions within eddies to improve understanding of the complex interplay of physical and biological processes.

The study's reliance on a single acoustic frequency (38 kHz) limited the identification of forage fauna species and potentially overlooked some components of the community. The acoustic data did not cover the first 20 m of the water column due to technical limitations. The dataset had uneven spatial coverage, with some high-amplitude eddy regions being under-sampled compared to those regions that have been previously sampled by other studies, potentially impacting the observed distribution of null effect eddies. The depth range of the acoustic data (up to 750 m) may have missed some aspects of diel vertical migration, hindering a complete assessment of mesopelagic organism response to eddies. Furthermore, the study focused on open ocean eddies, which might differ from coastal or strongly current-influenced eddies.