Wind-driven surface currents interact with these gyres and the underwater topography, such as seamounts, fishing banks, and the edge of continental shelves, to produce downwellings and upwellings.
The fishing banks were divided into areas belonging to the nearest fishing base on land and further subdivided into fields where the boats were allowed to fish.
The islands' greatest natural resource, apart from their fabulously biodiverse coral colonies and fishing banks, may be multi-millennium accumulations of guano.