In the anadromous zone the "resident" and "anadromous" rainbow are the same stock. A pair of residents can produce a smolt with the converse being true, too.

Anadromy in rainbow seems to have a strong environmental component. It actually seems that really good freshwater conditions-high rearing flows and low rearing temperatures-are the choice of residents. Being anadromous appears to be a response to problems in FW-land.

If, on the other hand, you subscribe to the idea that the residents and anadromous are genetically different, then going to sea and/or growing large are not stock-sustaining choices.

As with teenaged males, the goal and purpose of life is to reproduce. What survives to spawn is what is successful. If the only rainbow that can successfully spawn is a resident then that's what we'll get.