Apples and oranges...trout streams vs warmwater/cool water fish populations. Reservoir fish communities are generally much more complex and have an assortment of predator-prey relationships and behavioral mechanisms, Cannibalism and breeding controls exerted by bigger fish to name two, that in many instances result in quality fish populations. Those populations may be somewhat predator crowded at times, producing large predators and large panfish, but substantial to moderate densities overall. In much less complex systems, however, some of the same species will stunt.
Consider the under fished farm pond, a simple system in comparison to a reservoir, occupied by only largemouth bass and bluegill. Things are great for a while and the pond manager thinks that if harvest isn't allowed the population will stay this way forever. But then in typical cases the bass overpopulate, wipe out the bluegill or nearly so, and stunt, often with bass in the 8.5-11.5 inch range and a rare large one with only a very few large, older adult bluegill remaining. Thank too little fishing or too much C&R fishing. Appropriate amounts of bass harvest in the pond, without going overboard, would have prevented this.
