I believe that YP do just fine with LMB. It's just the timing. The original YP have to get over 10" or so, and bigger, and then they are just another fish in the pond that are too big for the LMB to eat.

Keep in mind though, that YP are both a forage and a predator fish, so you'll need smaller fish for the bigger YP to eat, as well as the LMB.

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."