1. Catfish have a shape that is probably not best for constant swimming. Therefore they are prone to rest on the bottom. Bur far from being sluggish bottom feeders, most catfish and especially those found in North American rivers are aggressive predators. There are numerous accounts by fishermen of catfish leaping out of the water when hooked. Likewise there are accounts of catfish leaving the water in pursuit of prey (insects, pigeons etc.)/

2. Jumping catfish are found in a number of other songs:
"I'm Going Fishing" by   Doc Watson, Merle Watson
"King Creole" Elvis Presley and The Jordanaires,
"Outta Here"   Kenny Chesney
"Night in the Wilderness"   Jethro Tull
"Black Water"   The Doobie Brothers
"Down Home"    Marty Stuart
"Mandaddy"    Trout Fishing in America

3. Because "Green Leaves of Summer" was in a popular movie soundtrack, it has been recorded by many diverse people and groups. Possibly a majority of the recordings are instrumental versions where the jumping catfish would not be so obvious. The following list is not meant to all inclusive.

The Gatemen,Eddy Arnold, Hank Snow , Sarah Vaughan, Billy Vaughn, Wes Montgomery, Ennio Morricone, Asleep at the Wheel, Frankie Laine, Johnny Ventura, Lawrence Welk & His Orchestra, 101 Strings, Royal Philharmonic Orchestra. Nelson Riddle, Ray Conniff, Hampton Hawes, Mahalia Jackson, The Ventures, Patti Page, Phil Coulter, Roger Williams, King's Singers, Xavier Cugat, Herb Alpert, Tex Ritter, Johnny Smith, The Springfields, Peter & Gordon, Fausto Papetti, Manny Albam, Kenny Ball, Matt Monro, Jimmy Justice, Crouch End Festival Chorus, Shelby Flint, Sil Austin, Klaus Wunderlich, Nick Perito ( on the soundtrack of the movie "Inglorious Bastards"), Dice of Dixie Crew, Gregg Galbraith, The 50 Guitars of Tommy Garrett, Tak Shindo, Karen Stachel, London Starlight Orchestra, Bonney & Buzz, John Modlin, Spencer Day, Sil Austin, Gil Goldstein, DuO, John Gregory, The Brothers Four (of course)and finally Terry Gilkyson and the Easy Riders. I find it interesting that Terry Gkilkyson wrote and recorded "Greenfields" and also recorded "Green Leaves of Summer".