Natural selection for smaller fish is another factor, as smaller bodied salmon have a better chance of survival against angling pressure, sea lion predation at chokepoints, have lower caloric requirements, and are better able to spawn in lower rivers. Thus, an average smaller body size is being selected for, whether that be caused by a shortened life cycle or lowered metabolism/growth rate.
its hard to really know why there are smaller don't think its the food plenty of sardines and anchovies the ocean runs in cycles people thought the big blue fin tuna would never come back to so calif waters but they did just like the zane grey days so its hard to say what will happen in the future
Actually the sardines and anchovies have not been wiped out. It turns out that both species tend to reflect water temperature cycles. Unfortunately the huge catches by the Monterey sardine fleets (and others) in the '30s also happened to come when the natural cycle was also starting to see a decline so the sardine had a double whammy that severly limited the population and it took decades for a recovery. But, they were back in force by around the '90s. Anchovies too go up and down reflecting warm water/cold water cycles. Usually you will not see big numbers of anchovies the same year you see big numbers of sardine even though you can catch both some years. More harm seems to have been done (in my opinion) by bad management by the Fish and Wildlife Department. Too many of each, in my opinion, have been allowed to be harvested over the years and there never seems to be a prohibition until it is somewhat late.
It's hard to say on the large salmon since they do tend to be fish that only live a certain (fairly short) number of years. I imagine the overall numbers are down and thus few of the truly huge fish are ever seen. Perhaps too many commercial and sport fishermen, increasing numbers of pinnipeds, increasing numbers of cetacea, and continued water issues. Perhaps we will never know.