Mesoscale fronts and eddies shape neon flying squid distribution through effective transport

Mesoscale oceanic fronts and eddies form coherent structures that regulate transport, retention, and mixing in the upper ocean, yet how their internal physical and biogeochemical structure shapes the distribution of mobile predators remains poorly understood. Here we adopt an active Lagrangian perspective to investigate the distribution of neon flying squid (Ommastrephes bartramii) using a decade-long fisheries dataset from the Northwest Pacific, combined with mesoscale diagnostics and Biogeochemical Argo observations.

Across multiple frontal systems, squid catches exhibit a robust cross-frontal asymmetry: catches are on average 1.6-fold higher on the warm side, with an optimal fishing offset of ~10 km toward warmer waters. This pattern arises from behaviorally mediated effective transport across a sloping frontal interface. Squid undergo diel vertical migration, occupying colder subsurface layers during daytime and ascending toward frontal zones at night. Because frontal surfaces tilt downward toward the warm side, subsurface squid habitats are systematically displaced relative to surface frontal indicators and fishing locations, producing a persistent warm-side bias without invoking passive advection.

In mesoscale eddies, squid distributions display a contrasting but complementary structure. Squid preferentially aggregate near the cores of warm-core eddies, whereas in cold-core eddies they are predominantly distributed along the outer periphery. Biogeochemical Argo float observations reveal that these patterns are closely linked to differences in the vertical structure of temperature and dissolved oxygen, which modulate habitat depth and suitability. Warm-core eddies provide vertically expanded, oxygen-rich habitats conducive to retention near the eddy center, while cold-core eddies constrain suitable habitat to peripheral regions.

Together, these results demonstrate how mesoscale coherent structures—fronts acting as transport barriers and eddies acting as retentive or exclusionary features—interact with active predator behavior to shape asymmetric spatial distributions. This study highlights how effective transport and mixing of mobile marine organisms can be interpreted within a Lagrangian framework integrating physical structure, biogeochemical environment, and behavioral dynamics.