I am pretty sure it was herons. There are a lot of them here. This post is quite old and since then we have replaced the fish and managed to keep them by using a sort of raised cover for the small pond. Plenty of sir gets in but herons can't see its a pond from above. It's worked so far.
Another possibility is the fish are hibernating now the weather is colder. I am in Cornwall which is comparatively mild but my fish disappear each year from Oct/ Nov. until around April. Cats are also adept fishers of goldfish as well as ducks! I had to ask my neighbors to keep their ducks under control after they visited my pond, ripped the pondweed to pieces, ate several of the fish, left one dying on the surround and smashed a decorative piece of statuary which I had made during pottery classes. This after I had restocked after a visit from a heron which ate most of my fish. I have now put a fine black net over the pond, supported by granite sets that blend into the slabs around the edge of the pond. Cats do not like it because it catches on their claws, I like it because it does not show, frogs and newts can crawl under the edge.
I thought a heron was eating my koi so I put the heron netting over the pond. I was surprised when I caught two very large water snakes in the netting. They were both over 6ft in length.
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Cats are also adept fishers of goldfish as well as ducks! I had to ask my neighbors to keep their ducks under control after they visited my pond, ripped the pondweed to pieces, ate several of the fish, left one dying on the surround and smashed a decorative piece of statuary which I had made during pottery classes. This after I had restocked after a visit from a heron which ate most of my fish. I have now put a fine black net over the pond, supported by granite sets that blend into the slabs around the edge of the pond. Cats do not like it because it catches on their claws, I like it because it does not show, frogs and newts can crawl under the edge.