Mesoscale eddy structure drives spatial variability in mesopelagic fishes

Abstract Mesoscale eddies are ubiquitous features in the world's oceans that create dynamic habitats supporting diverse biological communities. Yet, most studies focus on primary producers and top predators, leaving a key gap at the mid‐trophic level. In this study, we tracked a mode‐water anticyclonic eddy in the North Atlantic for a month during its decay phase using a Lagrangian framework, integrating ship‐based acoustic observations and net sampling with autonomous hydrographic measurements. We found that deep scattering layers, composed primarily of non‐migratory bristlemouths, exhibited temperature‐dependent vertical distributions, avoiding the warmer core waters. In contrast, midwater scattering layers—formed by vertically migrating hatchetfishes and myctophids—were concentrated at the eddy core, with shifts in community composition and size across the eddy. These patterns likely reflect a combination of passive transport and active behavioral responses to eddy‐driven physical forcing. Together, our results demonstrate that eddy life stage and internal structure—not just polarity—shape mesopelagic community organization, offering new mechanistic insight into how mesoscale eddies shape biological structure in the ocean.