Could be contamination of off species fish from your fish supplier. It happens more often than it should. I had some off species sold to me from a fish delivery truck. Luckily I identified them and did not put them in a forage pond that would have been a disaster for my plans.
They also could be hybrids instead of pure green sunfish. I purchased BG for my main pond but it turned out that maybe about 5% were hybrids instead of pure BG. They show most of the characteristics of GSF but they get larger and their mouth is not quite as large as a pure GSF. Hybrids tend to be mostly males.
That is two possibilities.
Whether it is a serious problem depends on the numbers of the GSF, how important it is to you to have pure BG strain, and who you talk to. If the GSF numbers are low in relation to the number of BG, the BG should out spawn the GSF. If the GSF numbers are high, their aggressive nature could cause major problems for the recruitment of your other fish. If they are hybrids it is most likely the majority are males, so reproduction would not be a serious problem.
I like the few hybrids that are in my pond. Caught one today. They will tend to bite a hook even when all the other fish are not biting.
I'm no expert. Just what I have learned reading and trying to manage around GSF in my old pond. They were bad enough in my old pond I was not getting recruitment of BG or RES (likely the GSF were eating all the eggs and/or fry). So I have been removing GSF and adding BG big enough the GSF can't eat them (taking 4-5" BG from my main pond and putting them in this old pond). Many people don't have the luxury of another pond next door to pull larger BG for stocking to help overwhelm the GSF.
The first step is to determine exactly what you have. If it is really GSF you could have a problem. If they are hybrids probably not very serious as far as your other fish reproducing adequately.
Just my opinion. Like I said, I'm no expert.
Last edited by snrub; 10/11/14 09:53 PM.
