I didn't see it, but the remaining evidence pointed strongly towards cat. Rebuilding the first pond, I had my goldfish in a kiddie pool on my covered front porch. The kiddie pool wasn't covered, I had floating fake plants in the pool. Came out one morning and only had one fish left alive. The kiddie pool also had a lily and some other pond plant in it, only the fish were damaged. Claw/tooth marks looked most like cat. We have semi-domesticated cats previously feral kittens in the 'hood. I blame them. We had one older cat, regular visitor, neighbors very spoiled cat who would come to visit to drink the water from the fountain. He had no interest in the fish. Straight sided pond sides probably do the most for keeping cats away from the fish, not many cats willingly go swimming. Think my kiddie pool was shallow enough the suspected cat could just walk around shoulder deep.