Population

Ecosystems are an integrated process that requires each part to be present to allow the whole system to operate correctly. This is similar to a car engine that requires all of its parts to work - "over removing" the crankshaft affects the whole engine.In the case of fish they are both consumers and consumed. Fish eat smaller plants and animals that would overpopulate their niche in the ecology if not removed. This ould cause a depletion of the smaller plants and animals they feed on crating a barren base ecology. As fish are consumed, the animals they feed would be starved for food and either die out or move to other locations. They might also switch to less nutritious food or to species that are not as numerous and deplete their numbers.

It doesn't stop there. Bears, as an example, eat salmon. They then wander through the woods where their faces provide nutrients for trees. The trees provide ecologies for a wide range of animals and have other functions like soil building with their leaves or needles and soil stabilization with their roots. Therefor - no salmon, no bears, no healthy trees, no birds and squirrels, no control of erosion, eroded soil fills the streams and the hydrogeology of the ecosystem changes.