In a mixed population, I don't know how you'd selectively feed.

I do agree with Poppy that the bluegill will shutdown at some point.

Do you have any idea in how low of a water temp your HSB will still feed?

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."