I can't speak for wild populations, but I still struggle to get my pickeringii to eat anything other than silversides. He has taken worms and mice in the past, but he's also refused them more often than not. Seems to be doing just fine on a 90% fish diet.

I know that bass, sunfish, trout, and perch generally do not contain thiaminase, so there are many places in North America where a thiaminase-free fish diet would be possible.