What Is an Ocean Eddy, and How Do You Fish It?

An eddy is a rotating loop of water that has broken away from a larger current, spinning as an independent, often long-lived feature. NOAA describes eddies as spanning 10 to 100 kilometers and lasting for months, long enough to plan a trip around one that's sitting on a chart.

Warm-core vs. cold-core

Eddies come in two flavors. A warm-core eddy traps and carries warmer water (and the species that came with it) away from its source current. A Gulf Stream warm-core ring, for example, spins off and drifts independently, carrying warm water and its associated marine life into cooler surrounding water. A cold-core eddy does the opposite: it pulls colder, deeper water toward the surface at its center, similarly to upwelling but in a rotating, offshore feature rather than a coastal band.

Why eddies concentrate predators

NOAA Fisheries' own research on open-ocean eddies notes that for top predators like tuna, eddies are potential foraging hotspots: the rotation concentrates plankton and bait at the eddy's edge, and predators follow. This is the same underlying mechanism as a temperature break: a boundary that concentrates the food web, just organized in a rotating loop instead of a straight line.

Finding an eddy on a satellite chart

An eddy typically shows up as a closed, roughly circular temperature contour: a warm or cold blob with its own distinct edge, separate from the surrounding water's general gradient. The Gulf Stream is the classic source of eddies visible on U.S. East Coast charts; see the Gulf Stream / Outer Banks region for a working example.

Check today's Gulf Stream structure on the SST map. Definition only? Glossary entry. Back to Learn.