In order to change the age and size there need to be significant changes in how the ocean is managed. NO harvest on immature fish. That's the easy part. Then you have to stop destroying the food base.

Plus, there are many fisheries where the actual catch of Chinook is minuscule compared to the target stock. Gonna close a trawl fishery that catches tens of thousand (millions?) of pounds of the target and incidentally takes a few thousand Chinook? Nah.

The productivity of our fisheries in the 16-17- and maybe 1800s had to be due, at least in part, to the fact that the high seas were pretty much left alone. How much damage are we doing to the ecosystem by taking, even at "sustainable" rates, resources that have not been harvested until now? They did not exist in an ecological vacuum.

Too many mouths to feed.