Streams in W WA are naturally nutrient poor due to the underlying geology and relatively high rainfall. Under a heavily forested canopy, streams receive relatively little solar energy and are also often very cold and dark. Low nutrient avilability and low solar inputs to streams both can severely limit primary production, invertebrate production, and fish production. Increase solar inputs and nutrients in those streams, and productivity typically goes up.

In addition nutrients that increase production through indirect trophic pathways, spawning salmon bring food for fish and invertebrates directly (eggs, fry, flesh) - So even a cold, dark, nutrient poor stream can still be very productive for fish IF sufficent spawning salmon are present to offset the lack of production from other trophic pathways. Pretty simple really.