What makes them think that interbreeding (which is not a good term because, genetically, they are the same species) will result in weaker fish being born? It's been my experience that when a domestic breed mixes with a wild one, as in hogs, you get a very strong, and mean feral hog. Many times, domestic cattle have been bred to longhorns, to make the breed stronger.

I think the bad part of the this deal is the financial loss to the owners of the domestic fish. They will never recover those lost fish.