It doesn't matter if the flow out of the pond is very narrow. Muskies, smallmouth bass and channel catfish has escaped from the ponds at the hatchery where I works at. The flow at the hatchery is very narrow and it has a grate, but they still escaped into the streams and into the lakes where they are not native to. The residents were not very happy with muskies and smallmouth bass in their lakes.....so it is very risky to stocking non-native species in a stream-fed pond but the decision is up to you as long as you are take responsibility for let non-native species escaped or introduced exotic pathogens to the area.
There is another similar case where an owner has a private stream fed pond full of koi and goldfish....When the owner no longer wanted them, we tried to remove all fish in that pond but somehow few of them escaped into a nearby lake as we sampled that lake and found 27 YOY goldfish and two yearling koi carps. A small adult koi carp was reported in same lake last month.
There is another similar case where an owner has a private stream fed pond full of koi and goldfish....When the owner no longer wanted them, we tried to remove all fish in that pond but somehow few of them escaped into a nearby lake as we sampled that lake and found 27 YOY goldfish and two yearling koi carps. A small adult koi carp was reported in same lake last month.
