Mystery

[Priory99]

I live in S Oxford and have a wildlife/Koi pond, kidney shaped- 7m x 2m (max) x 1.5m deep. About three years ago it was stocked with small koi and three green carp. The latter are now 30 cm long and live at the bottom, the koi have disappeared probablt due to a heron that is a regular visitor despite the grid of fishing lines I have strung across the pond. Anyway with a new grand-daughter I was pressed to make the pond secure, so I covered 90% of it with a heavy steel grid (7cm square) , with a small area in the shallow end (<10cm of water) open. Next day I found one of carp on grass about 2m from pond, with no marks on it. I assumed a heron had chased it into the shallow end but then been unable to carry it off and dropped it on the lawn. But next day same thing happened, in same spot! This time with a few puncture marks on underside of fish. I'm now beginning to think it could have been an underwater predator? But why wouldn't they have eaten their kill? I guess carp could have jumped through the grill then slithered across the grass, but twice? Any ideas? Thanks

[Guppy]

Regarding the grandchild, I would consider a redisign to the pond by making a semi raised edgeing, keeping the original pond and putting a outer collar of around 30-45 cm and a,in effect, a new build.with pond liner etc. This will give a shelf around the pond and, what every pond keepers wants, a bigger pond. Sleepers would do the trick and a decking edge on top of them.
A sort of fruit cage type thing on top using 50x100mm garden fence green plastic coated wire(Screwfix) with feeding cut outs on a timber 47x47mm frame, secures the pond with netting roof.
Just an idea.

[Priory99]

Thanks Guppy,
I suspect the garden police won't allow anything like that. The steel grille is a temporary measure while I see whether I can put a framework in for a sub-surface plastic grid. But either way I'd like to be certain what's killing the fish first.

[Guppy]

A CCTV camera could be the answer. If within reach of wi fi, an Amazon Blink system is ideal, got this myself.

[Guppy]

Another way us to put a sand trap around the circumference of the pond. A foot wide edge of sand would show marks, footprints, of anything entering the edge of the pond. Worth a try.