Geoff,
Have had this debate with a few people, but sturgeon aren't strictly freshwater as they return to the sea - can't remember the scientific name for it off of the top of my head but there is a word that covers fish like this that live in both fresh and salt water, eels would be another one.
Nothing else that really comes close to this, although Bung San Ram is supposed to have an arapaima in excess of 600 lb, and Siamese carp are rumoured to go to huge sizes, certainly in excess of 400 lb, and Mekong catfish also go pretty big - the common theme here being that they all are from Thailand (I know arapaima are native to South America but they don't grow as big in the wild), so not a bad place to go if you want to catch a huge fish.