Why do salmon go upstream in fresh water from oceans: Evolution of the global recycling of nutrients through higher organisms

The quasi‐homogeneous cold water mass lying deeper than 1000 m below sea level serves as a sink for, and hence is rich in, inorganic nutrients, including nitrates and above all phosphates, which, directly or through numerous food chains, have found their way to the ocean bottom. This cold water mass surfaces during the cold season in temperate zones and permanently in arctic/antarctic zones, which could support extremely rich marine life permitted by relatively low temperatures. Such fruits de mer are predated by rich marine faunas of local and migrating mammals and birds. As for the migrating birds, they breed during the short summer in both circumpolar zones of the earth, to bring the newly acquired biomaterial including inorganic matter to their overwintering sites of the temperate/ subtropical region. Through food chains mainly involving birds and insects, the nutrients accumulated in the deep sea waters would thus be brought back to the land, upwards even to the high mountains. A similar role may be attributed to salmonids and their southern hemisphere counterparts.

In the case of woodland birds, an estimate made in Japan indicates that ca. 10 tons per km2 of insects support the total insectivorous bird lives of the area. This figure may approximately be used as an index of the inorganic nutrients to be returned per annum to the soil through bird droppings and corpses. Using the number of salmonids caught during the peak period of the highly exploitative fishery perpetrated by Japanese fishing boats around 1910 near the mouth of Amur River, I estimate possible biomass contribution to the land from the sea by salmon returning to the upstream of the Amur in wilderness (ca. 6000 tons per annum). According to Arsen'ev (1930), a school of salmons swimming upstream became a victim of active predation by mammals and birds in the mountains. Whereas the total length of the main path of the Amur beyond the junction of Shilka and Onon Rivers is ca. 4000 km, the total contribution of biomass (and hence inorganic material) by salmonid fish to land would amount to, using the above index for birds’ annual predation of insects per unit area, an equivalent of 100 m‐wide zone along streams of a total length of at least 60,000 km, a distance which would cover a good part of smaller tributary currents of the Amur.

The coevolution of nutrient recycling through organism's migration throughout the global ecosystem would be difficult to explain by natural selection, because there cannot be competition for improvement with the unique system of global redistribution of nutrient materials. Also the time scale needed to bring about selection would by far exceed the generation time of organisms involved.