Nice pics FH. Over winter morts for late born yoy fish up north is very large. Not saying that may have happened but is possible. They just don't have enough time to store enough reserves before the cold sets in and they starve. This should be offset somewhat because of the new pond effect (new ponds generally have high growth rates). Even if that batch is gone I am not sure it�s a bad thing based on your stocking.

Some beaver burrow , all of them chew trees and all the added wood increases somewhat the likelihood of DO problems during winter over time. Lead or steel is a good remedy.