Always thought the act of spawning darkened the fish no matter how close the lake is.
The fish do not activly spawn until their eggs and milt ripen and loosen up. During this wait the fish darken and basically rot to death. The protective slime coat disappears and the skin gets leather like, exposing them to all the nasties that live in the water.
A chrome spawned out nook would be a thing to see, hopefully I will see one soon. The odds have to be in my favor, since I must have seen close to 5000 Chinook in the past 24 years. I'll be sure to take some pix to share.