Clipping the fins on the GSF is a good way to get rid of them and at the same time feed the LMB. I have done that a lot in the past. When you feed, the GSF are likely numerous eating the feed along with the BG. They are definitely feed hogs and will readily take to being fed.
I get quite a few natural GSF/BG hybrids in my pond and I really like fishing for them because they grow fast on feed and are aggressive to take a hook when almost nothing else wants to bite. We have filleted a bunch this spring that have fattened up on fish food from our feeder. But they are not everyone's cup of tea, As your LMB get more established and your BG spawn multiple times a year, most of the GSF will go by the wayside naturally. They just get out competed. But in the mean time their mouth gape is the same size as a LMB inch for inch in length of fish so they definitely take up a lot of food sources that could be going to your smaller LMB and BG. So clipping their tails so the LMB can easily eat them is a good use for them.
A couple years ago I actually had some LMB and CC trained to come to the dock when they heard my footsteps. I would take a bunch of 2-3" HBG I had trapped in a minnow trap out of my forage pond with a minnow trap, clip their tails, then toss them in the water and watch the LMB and CC compete for them. They would come right up in front of me to take them as soon as they hit the water.
A baited minnow trap will catch a bunch of them. The GSF seem to be especially attracted to a baited minnow trap. You can expand the opening slightly to get slightly larger fish in the trap. Most minnow traps have a 1" opening (many states have laws forbidding anything bigger in public waters) but if you open it up to about 1 1/4 it is surprising how much larger fish you can catch.
Old thread on minnow trapsMy GSF journey in my old pond back in 2014GSF management thread
