I'd guess the YP are a big food source for the LMB. Much easier to swallow a fusiform shape like that compared to a larger disk shaped PS. There is a post on the forum that I think ewest posted on
Trophy Bass Concepts - Bill Cody (Fantastic Read full of great info) that mentions YP as a possible source of food for the LMB. I think I've read that post half a dozen times and still go back to find some of info on growth rates that I will be using to see how we are doing on our pond.
Do you have YP in the lake or are you PS, RBT, and Brookies?
Not sure on the RES or PS eating zerba. If they are, there should be something in the stomach contents if you clean any of the larger PS (guessing).