IMHO my first guess would be oxygen. Cats are generally not the first to pop up when DO levels get low, but adult BG are a different story. Do you have any form of aeration in place? This wicked weather pattern we have all seen this year has thrown some unusual twists and turns into the pot. Tell us some more about your pond in regards to size (water loss), # of fish, feeding plan in place, blooms, ...
Cody will probably be able to guide you in a more correct manner than I.
Deb
