Just after the Civil War catfish weighing 125-200 pounds were reported as commonly caught in the Missouri.
In 1879 a 150 pound blue was shipped to the U.S. National Museum that was caught in the Mississippi.A large catfish had been requested and the day the letter was received by a Dr.Steedman,he went to the St.Louis fish market,found a 144 and a 150 pounder for sale and shipped the bigger one back East to the museum.These two fish were just the two largest that happened to be there that day.
The book "Steamboating Sixty-five Years on Missouri's Rivers" mentions a 315 pound blue caught in the Missouri near Morrison in Gasconade County.
Yes,multi-hundred pound fish are easily believable. :)