The available spawning area is just one of the parameters to consider in determining the "carrying capacity" of a basin. For species like Chinook and coho it is rare that the amount of spawning area is a limiting factor. More typically it is the stability of the gravel used for spawning or usually it is the amount and quality of the rearing habitat that determines the "carrying capacity". That rearing habitat for species like Chinook, coho, steelhead, etc. often includes more than one of the following; early juvenile rearing, over-wintering, parr rearing, and estuarian habitats.

BTW -The idea of carrying capacity is not a single value but that varies (sometimes by a lot) depending on survival factors. The carrying capacity during good conditions (freshwater or marine) is higher than during poor conditions. Further consistently fully seeding the habitat (achieving carry capacity) would require that there be no fishing expect for those years that the run sizes are lager than the habitat conditions their offspring are likely to find; which of course can only be know after the fact.

Curt