Pond fish predators?

Okay, so I know about the heron.........and indeed a few years back one regularly breakfasted on fish from our pond until we covered it with wire mesh. It doesn't look pretty, but it keeps the fish safe, or it did.

Three days ago I noticed that the fish were very agitated, and kept darting down to the bottom or under lily pads, they still came up for their food though. Then the next day I only spotted one fish darting for cover, and none came to feed.
Today I haven't seen hide nor scale of any of them (about 15 in there, and although some are black the others are brightly coloured). This to me suggests that whatever it is that is spooking/eating them, is coming back each day - for all I know, there are no fish left in the pond

The thing is, what is it?? A heron cannot access the water, but it could conceivably be standing on the mesh - however would it then come back time after time, and would the fish be so terrified of something that had not hurt any of them. A cat cannot dip a paw in there either, and in any case Jimmi would let me know if a cat had been in his garden. There is a small gap all the way around between the wire mesh and the ground.........and the only other predator that I can think of is a grass snake, but he would have to be pretty big to take these fish.

Any ideas anyone?

No ideas sorry I was going to say a cat but like you say Jimmi would let you know unless of course it is coming through the night.

My fish had been getting spooked and we realised we had cat visiting it has stayed away since having our little visitor this week.

My Dad has on and off problems with a heron in his pond. He covered his with mesh years ago but the heron still used to land on the mesh and bounce up and down which frightened the fish.

Any sign of the fish today?

I honestly don't think that it is a cat, because the merest hint of one having been in the garden - even hours ago - makes Jimmi go ballistic, and he hasn't been doing his "cat thing".

I wonder if that's what is happening here then. They are still too frightened to come out from wherever they are hiding, but I did spot two tails sticking out from beneath a large lilly pad, so I DO still have two fish at least. I would dearly love to set up a CCTV, but failing that will just have to hope that whatever it is gives up and goes away.