CM -
The size of Chinook salmon across their range has been shrinking for at least a century. A major driver has been selectivity of fishing methods.

A couple examples -
The historical information shows us during the early 1900s the typical gear used in the Skagit in river spring Chinook gill net fishery (a non-treaty fishery) had a stretch 9.5 inches. Such gear would be very efficient in capturing the larger fish (say over 30#s) but fish smaller than 15 #s could potentially pass through the mesh. The very definition of selective against the larger fish.

In a 1970s Sam Wright (WDF) paper he made the case that the minimum size limit in the troll fishery was selecting against the faster growing fish. He based his argument on the fact that the faster growing fish exceeded the minimum size during their second year at sea while the slower growing fish would not enter the fishery (exceed the min. size) until their third year. The result would greater exploitation on the faster growing segment of the population. Over time this was expected to lead to smaller size at a given age.

While the forage abundance is important, we can not overlook the impacts from our fishing and in this case the impacts from fishing on the feeding grounds. Those impacts are more easily (if the managers had the will) than changing ocean conditions.

Curt