Parker is talking about Ichthophyonus, a disease that has been discussed on this board before. It is a big problem in chinook on the Yukon but probably not the primary cause of decreasing size. Chum and other chinook runs in western Alaska exhibit the same problems as the Yukon chinook but don't have the disease. More likely the disease is a symptom of the problem, climate change. The severity of the disease seems to track the increasing temperature of the Yukon over the past 30 years. The study that Parker is referring to is well done and interesting to read. Look at the final report for study no. 01-200 at the link below to read it.

http://alaska.fws.gov/asm/fisreportdetail.cfm?fisrep=22

I realized that the link I gave in my earlier post was incomplete. It is now correct. That report notes the disease problem in Yukon chinook.

Finally this is an interesting news article that mentions the climate change problem on the Yukon.

http://www.ctnow.com/outdoors/hc-finalriversyukon.artoct24,0,3335887.story?coll=hce-headlines-outdoors