If you are talking about fish - and definitely don't mean to include whales - then the answer is easy as there is only one contender, the Whale Shark - Rhincodon typus.
Originating from about 60 million years ago, they feed mainly, though not exclusively, on plankton, microscopic plants and animals. However, the BBC program Planet Earth filmed a whale shark feeding on a school of small fish.
Largest freshwater fish
The biggest one ever captured and measured was caught in Thailand in May of 2005. It was 9 feet long and weighed 646 pounds.
Unfortunately the Mekong catfish is in decline and although research projects are currently ongoing, relatively little is known about this species. Historically, the fish had a natural range that reached from the lower Mekong in Vietnam (above the tidally influenced brackish water of the river’s delta) all the way to the northern reaches of the river in the Yunnan province of China, spanning almost the entire 4,800 km length of the river. Due to threats, this species no longer inhabits the majority of its original habitat. It is now believed to only exist in small, isolated populations in the middle Mekong region.
But what about the sturgeon?
The trouble is that it doesn't count as sturgeons are not truly freshwater fish as they also spend a great deal of time in brackish water and as well as switching between saltwater and freshwater during their life cycle. Unless of course you want to know what the world's largest brackish fish is, but then where do you stop?
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