Jacks are part of the whole tapestry of the run. One thing they provide is genes from "other" broods. Take coho, which are primarily a 3 year fish. Without jacks there would be very little genetic interchange between brood years.

The way they learned about jacks and sneaker males being successful in spawning was by genetic examination of the resultant fry, fingerlings, and smolts.

We had big salmon returning to spawn when we didn't kill them as juveniles in the ocean.