It is important to remember that critters have a way
of getting out of even a 'closed' system. It doesn't have
to connect to a waterway. Fish eggs are sticky and leave
attached to duck feet and fur and paws. I've heard of kingfishers
flying off with their prize and dropping it, still alive, with many
anguished
kingfisher cries, into another watery environment. Some seeds
will actually do better going thru someone's digestive track
and lots of parasites and vermin go from host to host, land to
air to water, to get around.
My favorite is this parasite that causes an ant to climb to the
tippy top of the grass to be eaten by a passing cow.

Best to err on the side of caution even without the village
council breathing down your neck.

k :-)