They can be responsive to very small flies.
And that doesn't necessarily mean fished on fly gear. My only experience with chums was using standard drift gear on a Northern Oregon coast river about 30 years ago. Wasn't having any luck with various corky/yarn combo's. Didn't have a fly rod or flies, so just used strips of egg yarn to wrap the shank of the same hooks I was using, leaving a short tag tucked under the egg loop. Basically a chartreuse Teeny nymph. Other than cutting the lead back for a more natural drift, didn't change anything else and started hooking fish. And no I wasn't flossing, I could see fish move to take the fly instead of moving out of the way of the corky.