Yes a seine survey to see what you have. Then go from there depending on your goals. What you have , if we are guessing right, is not an unusual situation and can be managed - depending on your goals. The GSF can be managed by adding adult CNBG and a predator (LMB) if you want a BG/LMB/CC pond. It will take some time but the CNBG will out reproduce the GSF and the LMB will target the more fusion formed (shaped like a oval)GSF. Assuming that is your goal.