Pacific salmon return to the freshwater rivers once, to spawn and die; the Atlantic salmon make several returns.
Industrial pollution, silting, and damming of rivers for hydroelectric power have seriously threatened the salmon.
In the photic zone, growth rate depends on light intensity and available nutrients.
Nutrients are constantly depleted by the slow sinking toward the bottom of dead plankton, the floating and mainly miniature plant and animal life, which forms the primary link in the ocean food chain.
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.
However, under certain conditions in some regions of the tropics and subtropics, currents and winds induce a sustained upwelling of mineral nutrition from lower strata, producing spectacular results.
These fishes travel in immense schools several miles long and wide, containing thousands of millions of individuals.
Herring feed on small marine animals and other plankton; in turn, such predators as cod, mackerel, tuna, and sharks, as well as certain kinds of whales and birds, eat freely from the enormous schools.All marine life is directly or indirectly tied to the photic zone, on which both recycling and decomposition, also in other spheres of the ocean, depend.Those few microorganisms deriving their energy from sources other than light have relatively little significance in the overall productive balance of the oceans.Simultaneously, fertility is constantly restored as the nutrient-rich deeper waters are brought to the surface.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.This effectively restricts most fishing operations on the continental shelves to national vessels or to craft licensed by that country.