Really true. That was one of the reasons that the Europeans brought them to the New World.
They got rid of 'bad' fish, so they could restock with 'good' fish.
Mullein seeds have a very high level of rotenone. It paralyses the fish.
Not a good choice around your
pond if you are trying to raise fish.
EDITED to add: As a fish poison, the seeds were normally ground up (to release more of the toxin quickly). I don't know if whole seeds would work as quickly, or as well. I have no intention of experimenting.