Multifactorial indeed. We are burning the critter's candle at both ends, with incessant assaults on every life stage of its natural history. The older/larger phenotype is simply made to endure those assaults for a longer period of time.

That's not to say the specific impact on adult spawners in the river isn't significant. On the contrary, large mesh gillnets have been definitively implicated in the demise of older/larger chinook in the Yukon River. They act like an in-river sieve, taking out all the large breeders ( esp large hens) and allowing only the smaller fish to wiggle thru.