Hatchery trout are dumb as a bag of bricks when it comes to them mistaking other items for brown pellets. The stream behind our cabin is lined in hemlocks. The trout gorge themselves on the fallen hemlock cones that fall into the water and are small and brown. Rainbow trout seem to be even more stupid than brown trout in this regard. In those rare years where we get the trout to hold over the summer, they seem to get brighter and hold over trout rarely eat junk. I am betting your rainbows were thinking the snails were pellets...
