Well lets see, you have 3 agresive predators and 1 prey species. Thats prety simple math. You need more prey and less predators. Crappie and catfish are just as bad as your bass. They are all eating your gills and competing for food. You need to get your pond balanced. You need to harvest some fish. You should do this even when your pond is healthy but now its even more important. I would start eating some fish. I would eat all the crappie I could catch and catfish too. I might leave a few big cats because they are cool though. Kill off some of those little stunted bass too. Any bass that under 2lbs gets eaten. I am not a biologist but I have managed a couple private pond and they were very succseful.
Adding structure and cover is a good idea to but removing predators is the key. Adding prey species is also a great idea but in the condition your pond is in now you would most likeley just be buying them dinner and that food wouldnt last too long.
#1 reduce predators
#2 stock some prey
#3 give the prey some cover to hide in

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