The term “upwelling” sounds familiar, right? We’ve all heard of upwelling and even cite it as a primary factor in finding good billfishing grounds, but few of us really understand it.

Simply put, an upwelling is the process by which deep water is brought to the surface layers, where phytoplankton (microscopic plant life) can get at it and multiply, forming the foundation for rich fishing grounds.

This phenomenon takes place along most coasts where currents clash and also along the Equator, where huge currents traveling in opposite directions meet. This equatorial upwelling is not quite as intense as the better-known coastal upwelling regions, but it covers a lot more area and also supports huge populations of tropical offshore game fish.

You don’t have to be a physicist to understand the basic mechanism involved. Large-scale upwelling is due to something called the Coriolis effect. Simply put, the spin of the Earth imparts rotation to the fluids and air on or near its surface, clockwise north of the Equator and counterclockwise south of the Equator.

This effect, combined with surface friction from prevailing winds, controls ocean circulation, which ultimately determines where you’ll get the most bites when you’re trolling.

Where two major currents run by each other in opposite directions, the adjacent, opposite-spiraling action draws nutrient-rich seawater from the dark depths to the sunlit surface layers and voila! The phytoplankton populations boom, supporting elevated levels of biological production through the upper-echelon predators like tuna, mahi, wahoo and billfish.

The surface currents of the Eastern Pacific include three major western flows: North Equatorial Current from 20-10 degrees, which runs just south of the Hawaiian Islands; Equatorial Current (Galapagos Islands); and South Equatorial Current from 20-10 degrees (Cook Islands, Samoa), with two eastern countercurrents between them: the North Equatorial Counter Current from 10-5 degrees (Palmyra, Fanning, Islands), and South Equatorial Counter Current from 10-5 degrees, (Marquesas Islands).

Islands and seamounts within this equatorial upwelling zone concentrate the already enhanced levels of game fish, creating the potential for some prolific offshore tropical fishing hot spots.

The divergent action of the spiraling currents can be enhanced by higher wind flow, such as during a La Nina condition in the equatorial Pacific. It can also be suppressed by the decreased equatorial wind flow associated with an El Nino condition.

Much of the equatorial Pacific upwelling zone is unavailable to the vast majority of sport anglers, since it lies within one of the largest, least-traveled expanses of water on Earth. Fortunately, after that long stretch of open ocean between the Galapagos and the Marquesas, the zone is spanned by a complete variety of tropical islands that have regular air service and accommodations to this extremely productive area.