My neighbour had a similar problem. He went back to the garden centre where he bought the fish and asked them for advice. They sold him a life-size plastic heron which he duly stationed by the side of his pond. Next day the (real) heron came back and gobbled a few more fish. Neighbour returned to garden centre with plastic heron and complained that it wasn't "fit for purpose". The garden centre bloke asked, "Did the real heron try to jump it?" "Actually, yes" replied neighbour. "Oh sorry, pal. I should've sold you a male. Try this one!"
Neighbour, ever gullible, took the replacement and set it up by his pond. No further trouble from visiting herons helping themselves to a free lunch!
I should perhaps mention that by this time there weren't any fish left in the pond anyway.