Well I just found out that Utricularia gibba, though it has very small traps, has a very small chance of catching a fish. I dropepd a piece into a small cup containing a few belly sliding fry(fry which cannot swim properly...if you want you can read about the rest of them in the FW aquariums section). When I got back one of them was dead with its tail caught neatly inside one of the traps.
I took a few photos but they may not be good...will post them after I am back from a trip.
Seeing this I have no trouble believing a larger utricularia could do similar without having to be confined to a small space.