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Thanks for all of the answers everyone. We live in the middle of a town and we have badgers in the garden every single night.
We went away for a week last week, returned and discovered the carcass of our biggest koi at the top of the garden, the majority of it eaten (head, spine and fin remaining). Overnight, it disappeared so assume the badgers took and ate the rest of it.
We assumed that particular fish had jumped out of the pond (it did last year and I saw it so threw it back in and it recovered), or as you say a heron had got it out and dropped it (though strange as it was the biggest one, talking about a foot long koi here, would have thought a heron would try to take the others). We have seen herons try their luck a lot, but boulders and steep sides on the pond seem generally to deter them.
Anyway last night, we saw a badger actually standing in the waterfall, by the ponds edge, as if it was trying to get the fish out of the pond. I’m quite panicked because if it is that situation, the fish don’t stand a chance as these badgers are determined little things (still coming back even after one of our dogs had a big scrap with one a few weeks back).
We have a camera set up on the pond to try and catch them in the act but like everyone here, it seems badgers haven’t been recorded as trying to catch and eat fish - though the evidence here is suggesting otherwise.
Evolution?!?