Most people are not at the pond all the time. A 3 acre pond can go from fish (name your kind) heavy to dang near empty when the cormorant hoard dines. How do I know this ? I have friends who farm catfish commercially who have had their ponds of adult CC (market size) pummeled when the horde descended. Quite common for them to lose $10,000 in catfish in a day.

Double-crested Cormorants
eat about 1 pound of fish per day
and are the most destructive birds
to inland aquaculture, especially
catfish production. A recent study
estimated that the damage they
cause to catfish production is $10
million to $13 million annually in
Mississippi..

Additionally,
potential losses may be higher than
one would estimate by consumption
alone because birds often
wound fish that they do not eat.
The importance of birds as vectors
of fish diseases has yet to be determined.