The ocean is ploughed by the action of winds drifting surface waters away from coastal areas, by nutrient-rich waters welling up from the depths, and during the winter season of the temperate regions by cooled surface waters becoming heavier and sinking downward, forcing nutrient-rich waters to rise.As a rule tropical surface waters do not interchange with the mineral-supplying waters below as much as those of colder regions and are therefore less productive.
Salmon are anadromous, migrating to ocean waters for growth and returning to fresh water for spawning.
Pacific salmon return to the freshwater rivers once, to spawn and die; the Atlantic salmon make several returns.
Within the EEZ, fresh water and coastal waters are often demarcated by law, with fishing within, for example, three miles of the coast allocated only to small-scale, non-trawling fishermen and larger industrial vessels required to remain farther offshore.
Small-scale fishermen are usually not restricted to the three-mile zone, and they often may be found well offshore or along the coast from their home ports as they follow the fish.
For example, West African canoe fishermen traditionally migrate hundreds of miles coastwise in open canoes, frequently fishing out of sight of land.