A colleague from ADFG, since retired, was looking at salmon growth in the Alaska Gyre. Conventional wisdom is that the decline in size of adult salmon (and increase in a few stocks) was the result of fishing. Depending on the species, it was either hook and line or net. This was the situation for decades.

What he was seeing, and was scaring him, was that size at age was declining, as mentioned above. This is more likely ascribed to food supply, or at least that is what appears to be happening in AK.

Further, they were seeing that the salmon have been forced to with to lower calorie dense foods. As such, a full stomach provides less energy. This seems to hit when a large adult heads to the spawning grounds, encounters warmer water (raises metabolic rate just to survive) and the essentially starves as it can't take in enough calories. Like a human trying to live on celery. So, the big adults just disappear.

Being weakened, they may be easier prey for whales or they may just sink to the bottom and feed the crabs.