I bet old age. I have read that perch will live pellet fed 5-8 years and natural 10-15 max.

16" I am guessing old age if no holes in them. I have seen the great blue heron stab my perch when spawning in close to shore and not be able to eat them, but there is two big holes in them from the beak.

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7/8th of an acre, Perch only pond, Ontario, Canada.