The huge difference in catches between the 1980s and the later 1992 to 2016 is driven by several factors. During the late 1970s and early 1980s there was a dramatic increase in the release of sub-yearling hatchery Chinook. The early period also was fortunate enough to have experience a much productive ocean; from the mid- 1990s average smolt adult survivals has been about 1/2 of that seen during the earlier period.

In addition following the ESA listing of Puget Sound Chinook the overall exploitation rates on wild Chinook populations have been reduced approximately 1/2.

Regarding the potential decreasing size of the Puget Sound Chinook. As discussed on earlier threads there is no doubt that the hatchery Chinook are getting smaller; fewer older fish, earlier maturing, and slower growing. The traits such as size, run timing, spawn timing, etc. for wild Chinook are shaped by natural selection process in their environment. Those forces operate in both the salt and freshwater habitats and include many factors including selection forces from fishing as well as spawning gravel size, flows, etc. in freshwater. Those forces often operate in contradicting ways. In the case those forces have shaped the current day Skagit Chinook which with the exception of lost of the oldest fish appears to have remain much the same the latest few decades. That however doesn't mean that the Skagit Chinook have not gotten smaller.

An old report for the 1920s (Smith and Anderson) report that a century ago an in-river commercial fishery on the north fork of the Skagit during the month of May caught approximately 18,000 Chinook (spring Chinook?). The interesting note was that the gill net used in that fishery had a 9 1/2 inch mess; in today's Chinook fisheries the standard gear uses 7 1/2 inch nets for Chinook.

Even if today's Skagit Chinook the last few decades have had stable fecundities that does not mean that the have retained the same productivity (ability to produce future adults) that they once had. In fact in the same report referred to by Brian the current productivity of Skagit Chinook is only a fraction (less than 1/4) they once were. Undoubtedly that productivity loss is a combination of both changes in productivity of both the habitat and the fish. I don't know the contribution of both factors to the over all changes.

Curt