Wild salmon and wild steelhead still do that today, but today's total fish runs are lucky to be 10 percent the size of historical runs and today's fish runs are made up of at least 80 percent hatchery fish, which are in many cases taken from the streams before they can give their last gift to the next generation--the nutrients that will feed the system, nourishing the very early food sources of their progeny.
Marine phosphorus and nitrogen has a distinct signature, an extra isotope or a heavier isotope or stable isotope (I've read all three), which means we can track their presence. And we've been able to determine a lot by using this method, such as salmon contributed 33-90 percent of metabolized carbon and nitrogen in grizzly bears in the Columbia River Basin prior to 1931 and something like 94 percent in bears in Alaska. Scientists can drill a core sample out of a tree and find evidence that the tree they sampled was at one time provided nutrients that arrived in the watershed from anadromous fish such as wild salmon and wild steelhead. Wild salmon consume food while they are in the ocean, but when they make their return trip up the rivers to spawn they do not consume food, which means when they die and decompose the nutrients their bodies deliver are almost completely or completely marine in nature. These nutrients help grow the very food their progeny still incubating in an egg on the river bed will eat such as phytoplankton and zooplankton, algae, fungi, and bacteria, which is then consumed by insects and fish and so on up the food chain.
Phosphorus is an essential element of all life, it is the backbone of DNA and RNA and it is a vital cell component and important in chemical energy transfers. Nitrogen is necessary for the formation of amino acids, which in turn are the building blocks for proteins and nucleic acids.
Wild salmon transport marine phosphorus and marine nitrogen from the ocean to mountain watershed. They release these elements when their bodies die and decay. The nitrogen is broken down by bacteria and other micro-organisms and turn the nitrogen into ammonium salts and nitrates which can be used by plants and in turn animals get these nutrients from eating the plants and other animals that ate the plants. Animals, such as bears, wolves, humans and many others then transport these marine nutrients further and further in the watershed or into other watersheds when their bodies turn the food they consumed into waste product and they expel it (to put as nice a term as I can put on it). The process repeats itself as micro-organisms break down the waste product and the plants in the area draw up those nutrients. Nutrients those plants won't get without salmon transporting them all the way from the ocean.
With hatchery fish, the fish are taken from the stream. I've watched the McCall Fish Hatchery do their work at their weir and trap on the South Fork of the Salmon River and they do throw the spent carcasses they used to artificially spawn a new generation of hatchery fish upstream of their facility on the South Fork. So, some nutrients are placed back in the stream, but salmon also deliver some 30 percent of their nutrients during the act of spawning, which is lost with hatchery fish. More importantly, while a lot of hatcheries discard their carcasses, hatcheries don't need anything close to the entire run of fish to get to their set goal and thus many, many hatchery fish are lost to the watersheds and rivers to anglers--commercial, sport and tribal, and many hatcheries, if there is a surplus and fishing season has closed will donate the fish they won't use to area food banks. Again, this practice, while generous and well intentioned, deprives the stream, riparian border and plant and animal life throughout the watershed of vital nutrients they can get from no other source. That leaves the ecosystem with the small number of wild/natural origin fish to provide the nutrients.
Today, the Snake River only receives about half a million pounds of nitrogen and phosphorus according to a study in 2000. Historically, wild salmon transported 6.5 million pounds of marine nitrogen and phosphorus back to watershed and steams linked to the Snake River.
This means, and we've had more hatchery fish and less and less wild fish for at least the better part of 25 years, probably much more and definitely much more in certain watersheds and streams where runs are endangered or extinct, that we are impoverishing the food chain throughout the areas of the world where salmon once were wild and abundant.
The implications of this loss of nutrients that aren't being replaced by any other process has several ramifications and we don't really know just how far it reaches, how much has been altered beyond recovery or even how many layers we need to peel back on this onion. One obvious problem is that riparian borders are less fertile, thereby creating warmer streams due to a lack of shade that could lead to once good stream habitat for spawning and rearing of wild and natural/origin fish becoming streams too warm for salmon and steelhead and other coldwater fishes. That situation will lead to two possible future outcomes, one being occupation of former salmon spawning and rearing grounds by warmwater and likely invasive fish or sterile streams completely devoid of fish. Some natural processes can speed this scenario along, such as stand replacing forest fires, which can obliterate the shade from a riparian corridor, destabilize soils leading to huge sediment loads that make the stream shallower. That shallower stream coupled with no shade will mean a sudden spike in water temperature that can lead to fatal conditions for both adult salmon, fry, parr, smolt and even eyed-eggs. The problem is then compounded by the non fertile soil due to a lack of marine nutrients that either slows the growth of native plants that provide shade along the riparian border or the colonization of riparian areas by invasive species that choke out shade bearing plants and leave the river that flows by under the warming influence of sunlight.
That's just the tip of the iceberg so to speak, but you should get my drift. We have to recover wild salmon and wild steelhead in the Snake River and the only action we can take that more than 85 percent of fisheries biologists in the Pacific Northwest agree has a 100 percent chance of success is the breaching of the four lower Snake River dams. Salmon are a keystone species providing vital nutrients to at least 137 species in the Pacific Northwest, you and I being members of one of those species. If we save the salmon, we are saving not only wild salmon and wild steelhead, not only the beautiful natal streams in which they are hatched, reared and then return to for spawning and death, not only the riparian vegetation, not only the bears and wolves and eagles and osprey and other animals, but in a way we are saving ourselves.
If that isn't persuasive, think of it this way. The dams allow domestic and foreign companies to ship natural resources by barge out of the area (impoverishing the area) while they also have killed off 99 percent of the wild fish that bring nutrients, the building blocks of life, back to the area (making the area wealthier). I hate to bring some economic argument into this, because if there aren't dollar signs attached to it then most people don't make the connection, but salmon enrich us and dams and barges (since these companies mostly take the wealth out of the area and the product) impoverish us and they do so while taxpayers and ratepayers subsidize their costs associated with robbing us. It is just another unaccounted cost of having these salmon killing dams in place. One no one will ever account for.