It's been a busy day of sampling so I didnt get to reply fully earlier. One of the reasons that scientists dont believe they are eating fish is because those kinds of adaptations take several generations and their has to be a reason for them to be adapting. Currently these fish are very well adapted to eating plankton. Everything about them is designed to eat plankton efficiently. This design has made them into extremely good invaders who easily outcompete our native planktivores in most situations. A shift to eating fish would happen very slowly and might work something like this.

Hypothetically,

genetic variation in our bighead carp might mean that some of them have slightly bigger throat openings or slightly wider intestines. Those with that larger opening might be able to ingest some larval fish or eggs. Those fish that eat the larval fish would have to be more likely to survive than the other fish with smaller throats and intestines . Those large throated fish would then be more likely to pass on their genes to their progeny. Within the next generation, suppose that the ones with the largest throats were able to eat slightly larger larval fish and again survived longer and spawned more successfully than their smaller throated brothers and sisters. This trend would have to continue quite a while (years and years and years) for there to be any fish capable of eating a d20 or any of the baits Ive caught them on. All of this hinges on a need for them to adapt.

All of this being said, I have always been curious about observing for myself whether they might ingest some larval fish when the densities of larval fish are really high. During about a 3 week span each spring their are millions of small (<10mm) shad which might fit in their throats. No one has ever observed any fish in their guts and I believe the research, but seeing is believing so I may cut a few open myself! This would still be a far cry from them eating an adult or juvenile shad, but theres always a chance.