Some of us believe that the Great Blue Herons are trying to 'kill' larger fish as those larger fish compete with the herons for eating smaller fish in the pond.

However, I don't think the herons would limit the size of your LMB.

You might have some stunting of LMB, and perhaps a lack of a good forage base for them also.

All of that is just guessing based on the limited info available right now.

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."