The Trout guts were filled with crawdads and small krill I'm not sure where they found krill in a lake but I'm sure they were krill.
my thoughts are as follows-
think of the kokanee in Oregon, they are filter feeders and develop orange flesh from the food in their lakes, so astaxanthin exists in some foods in lakes...
as for Montana, I don't see anything in their regs about clipped fish, so I don't think that they clip any of their hatchery fish and they appear to plant all the species, so if they are using food with astaxanthin at the hatcheries (not sure why they would spend the extra money on that fancy food but who knows), so you could have been catching planted fish that got the color at the hatchery.
cheers, roger