I'm not arguing.. fish go through the pipe.. I noticed hundreds yoy bass and bluegill in a big area behind my pond where the culvert drains. Thing is does it matter? In your case where there's another lake downstream with a separate fish stock, obviously it does matter and you should definately try to screen everything in to separate your too ponds.. In my case mine wash's into a creek and and losing 100's of yoy bluegill don't effect my pond at all, bass loss prolly does but 30 yrs of no management and a good quality stock of fish I can't arque with mother nature..