Like other people, I've had this problem also. Here's my count down on ways I think this could happen.
4) Birds carrying fish eggs from pond to pond. So unlikely that the chances of this occuring approach zero. I hope that some biologist jumps in and explains why this is true.
3) Other species mixed in with the fish we buy. Unfortunately this is more common than we would like to believe. There is a wide difference in the care different fish dealers take in making sure all of their fish are what they say they are. I have often bought minnows with small bullheads mixed in with them.
2) Fish swimming upstream and getting into our lakes. Very common. All it takes is one big rain or snow melt.
1) Someone has done a favor unto us. Happens all the time. Sometimes to hurt us, sometimes without thinking and sometimes to "help" improve our fishing. When we first started our lake in 2000, we started to catch 8-10" bullheads. Obviously, I didn't put them in.
The issue isn't how they get into our lakes but what we do about it. Bob Lusk wrote an outstanding comment about how he was able to manage green sunfish populations. I think it applies to bullheads, crappie and about anything else.
This is the method he used. First, he established a good bluegill population. Then he over stocked larger LMB. LMB will eat green sunfish before bluegills. Because of their shape, green sunfish go down easier. LMB can easily eat a mass of young bullheads also. A person can combine this with removal of adults.
In our case, we are overstock with predators. Although we initially stocked as many green sunfish as bluegills, we seldom see a green sunfish now. Bullheads have never become established.
I hope these comments help.
