All the other wildlife or plant communities have, in some way, some dependency. Working on the Snoqualmie River in Washington, Bilby found that as much as 40 percent of the nitrogen in the bodies of young coho salmon and 60 percent of the nitrogen in young steelhead came from marine sources. When they dumped salmon carcasses on streams running into Willapa Bay, they found higher densities of young fish around the carcasses. The young fish were feeding on the carcasses and eggs laid in the river. The extra food meant juvenile fish grew bigger before migrating to the ocean.
Bigger fish survive better, so more fish come back to the river.
And so on. It appears that salmon evolved this as a survival strategy because the streams in the Northwest were generally low in nutrients. So they brought their own lunch. You'll still have aquatic life occurring in those streams, but there won't be much of it. The Oregon Plan for restoring dwindling salmon populations recognized this research, and for the past two years volunteers have been tossing salmon carcasses from five hatcheries along coho rivers on the northern Oregon Coast.