In the short time frame, it's very simple: 5 sub 2 chinook and 4 sub 2 chinook are chinook that return to the river at a large size. We have been told, and I believe it makes sense, that southern resident killer whales like to target larger chinook.
The bulk of the hatchery closures in the Fraser have taken place on hatcheries such as Dome Creek, Willow River, and Spius Creek that focus on those larger chinook.