What would cause mushy cat fish?

so today i went fishing at our pond and caught a few catfish ,which seemed tobe skinny in compairison to last year. ( keep in mind it is a man made pond and fish were raised on actual cat fish food for 10 yrs. ) when i went to filet them it was almost impossible , the meat was so mushy it almost had a geliten textire to it. I fried it up tonight and well it was nasty and mushy not even edible , to the trash they went, what would cause this.?