They did a show awhile back on the Wels Catfish. The reports, as they are usually come out, were that there was a "monster" fish that was responsible for a number of unexplained deaths. They thought that there may be a huge catfish that was eating unwary swimmers.

During the show they were able to prove how big these fish get, and how aggressive they can be.

I just recently saw a series of video clips on the same species of fish, now named the "european catfish" in a waterway in France.
The fish were stalking and attacking pigeons that would come to the rivers edge for a drink. Not all attacks were successful, but it does show the crocodile-like cunning they use to approach the birds.
I can only imagine what could happen in out waterways if we had such a fish here in the USA?