I once saw a brookie of large size swim up to a bank and take a frog. They are predators and will eat anything. A survey of the Schuylkill R. wild brookies showed that 90% of their diet was aphids, which are a dry fly of about a #99 that's how small they are. The same survey area showed another time frame they were eating millipedes. Those little brown 1 inch long segmented worms that have been on earth since before the dinosaurs.
One opening weekend, about ten years ago, one of the guys in our camp caught a brookie with a large hellbender in its guy.