Aside from harvest selectivity and the loss of genetic material from older spawners, there is also increasing evidence that environmental and habitat factors also play in to the decreasing size picture. The majority of the older fish, especially for Chinook, have longer juvenile freshwater residence times. Most of the Chinook that return as 5 year old fish reside in freshwater for a year, and outmigrate as yearling smolts to the marine environment whereas the 3-4 year olds are typically fry-parr migrant type only rearing in freshwater and estuary environments for a comparatively short amount of time. The yearling life history strategy for chinook is shrinking not only due to those genetics being harvested out, but also due to unfavorable habitat conditions and environmental factors such as water temperature and poor food availability that make freshwater rearing habitats unfavorable, especially for over summering yearling type Chinook. Chinook as compared to Coho have much lower temperature thresholds they can withstand for survival. In the lower river and estuaries I have studied, we consistently see temperature values too high to sustain Chinook life during summer low flows particularly as summer flow values decrease and ambient air temps increase.

I feel strongly there is an important habitat/environmental factor playing in to the decreasing size story.