I raise yellow perch in one of my ponds and are the only fish I eat out of them. GBs, snails and yellow perch are hosts for the yellow grub parasites. Not very appetizing seeing grubs in your fillets, at least for me. Also, they will spear fish bigger than they can eat an discard them on the bank. i really mis my G Shepherd..... he loved to eradicate them from my ponds!
Ann, not sure if any native raptors would take on a GB. I have witnessed coyotes scoring on a few nesting Canadian Geese.