Two distinct species that look very different. What people call wild carp are not a different species to bog standard common carp, they just look a little undernourished compared to the gut buckets that we've got used to seeing. It's also debateable as to whether there really are any genuine wild carp in the uk. They've been selectively bred for centuries.
A grass carp has a totally different mouth and fins. There should be no mistaking them.
This makes me question whether or not I've caught grass carp. Look at the dorsal fin. Very small compared to a Wild Carp which is closer to a common. And the mouth is smaller/downturned a bit in this picture.
The wild carp I was catching years ago were just as long and thin as grass carp, but I can't remember them being so distinct.
Genuine OLD strains of Carp Wildies are slow growing, rarely hit mid double figures but can get big if small numbers present. Long, lean, slightly more golden than toadys commons and have the very big paddles.
Oddly they can be fickle feeders prefering naturals or very soft baits.
My best two went 21 and 22lbs respectively, I thought I had caught same fish twice in half an hour, the 22 was an inch longer.
Both caught on paste, drennen super speialist specimen rod and small waggler, 8lb line on a 4000 shimano aspire 9bb fighting drag reel straight through to a size 6 super specialist hook.
During matches I saw a few poles get broken from a certain area, a friend then told me he had found a clean spot when plumbing with pole down a margin.
I just plumbed up, tipped in a load of pellets then fished paste over the top.
Unfortunately a fish kill saw the strain die off at that venue - old estate waters in a valley.
Both fish tested me and my tackle to the limit taking 8lb line off a fully engaged fighting drag hooping the rod through its 1.5lb test curve into the 3lb but section to the handle - proper Wilson stuff!
Same water had grass carp- they only fought once in the net!
Anyone who has had a proper OLD slow growing strain will remember it.
I've got Grass carp in my pond, i've also got Mirrors and Ghost carp, all are part of the cypriniform family which is what a wild carp is (basically it's a common carp) only because it isn't over fed in a commercial pond/lake it remains torpedo like, the grass carp is similar in shape but not quite the same.
I have no idea what that fish is in the picture you have posted but it looks nothing like my grass carp or any of the grass carp i've ever seen..... if anything i would say it looks more like a silver carp than a grass carp!
These Wild Carp I was catching were probably stocked by Cadbury at some point so were very old and wild. Probably all about 4/5lb but very long and thin and the best thing was in 2 foot of water they had nowhere to go but run, so fought very hard in a very snaggy and overgrown pool. Half was covered with lillies. Dog biscuits and floating bread was how we used to catch them.
Grass carp - I'm sure I've caught one, with a different head, but do not remember where...