I think I've found the disconnect WRT people discounting Sturgeon as the largest freshwater fish. In the article linked below they make the statement that Sturgeon are not purely freshwater fish.
"Sturgeon spend most of their lives at sea, or in brackish water, and only swim into freshwater rivers to spawn."
SOURCE There is one place where the Sturgeon don't move into salt water, but that isn't the natural order. They don't return to the ocean like most Sturgeon because man-made dams have blocked their way. Apparently they are surviving and even multiplying, but who can say if they will remain viable in the long run?
"Dams constructed along the Columbia and Snake Rivers has also become a threat to sturgeon. The dams block passage to and from the ocean leaving landlocked populations. The higher water levels have, as a result, destroyed areas that used to have rapid currents, eliminating spawning grounds. At this point in time, the only healthy population of White Sturgeon on the Columbia River and its tributaries is the lower Columbia River population. These fish are still able to move freely between marine, estuarine, and freshwater environments. All other populations have become landlocked because of the dams. These populations have suffered from the inaccessibility of many food sources, inability to migrate, and destruction of their habitat. As a result, none of the populations above Bonneville Dam are considered to be abundant but some do support fisheries. The Oregon Department of Fish and Wildlife has worked together with The Washington Department of Fish and Wildlife and the Columbia River Treaty Tribes to impose size restrictions as well as season and quota restrictions on all sturgeon caught in the Columbia River. The objective of size restrictions is to minimize fishing impacts on large sturgeon that comprise the spawning population."
SAUCE So it looks like the title of largest freshwater fish has to go to the Mekong cat for now.
