According to my research contacts, you don't see the malnourished Chinook as they die. What seems to be happening is that the larger Chinook are doing just fine, get up to adult size and head home. When they encounter warmer water the metabolic rate goes up. There are not enough calories in the food per kg eaten, so they essentially starve.

Plus, the salmon are getting smaller at age. In the old paradigm, where fisheries took the larger fish, the fish returning got younger but were essentially the correct size for age. Just more age-3 and less age-5 but the age-3 fish were properly sized for age. Now, the age-3 fish is smaller because they have lower quality/quantity of food to eat.

Perhaps we can put the salmon size into antler terms. A buck will get to be a 4x4 if it has the right genes and, most importantly, if it lives long enough. Now, in addition to killing off the bucks so they don't get old, the food no longer has the calcium supply, so what does grow is smaller.