Who says jacks aren't fit? Size isn't everything, despite what some here will say.

The purpose of life is to reproduce. There is no other purpose. A jack, if successful, gets his genes in the mix. Provides genes from another brood year which offers the eggs more genetic options.

There are even some Chinook that spawn before going to sea. The more options you have for getting spawners the higher the probability that some will survive.

The best example of this that I know of is coho. Coho are (or were when I was involved) managed top optimize the yearling age-1 stream reared smolts. Allow only enough fish to escape to produce those smolts. We no know that coho have fry that smolt, fingerlings that smolt to estuaries in summer, fall smolts, spring smolts, lake-rearing smolts, lake and beaver pond over wintering smolts, and fish called nomads that smolt, then overwinter in FW (maybe in two successive winters in different areas) before going to sea. All of this results in more adults from different brood years, different sizes of adults, etc.

The stream I worked on had a capacity of about 15K spring smolts. The tributary lake produced, after it was cleaned out of bass, pike, and other stuff, 30K spring smolts. Among those spring smolts were age-0, age-1, and age-2. Lots of stability when we maximize the life history options rather than maximize the catch.