The biggest fish in the world is the whale shark (Rhincodon typus), which can grow over 40 feet long and 21 tons in weight, and is found in tropical oceans across the world. Despite its large size, whale sharks are considered harmless to humans, as they are filter-feeders whose diets compose mainly of plankton.
The largest freshwater fish in the world is the beluga sturgeon (Huso huso), which can grow to 20 feet long and 3,400 pounds. Primarily found in the Adriatic Sea, the Black Sea and the Caspian Sea, beluga sturgeons generally live in brackish environments, but migrate up freshwater rivers to spawn.
Since the beluga sturgeon only travels into freshwater to spawn and does not live in a freshwater environment, the Mekong giant catfish (Pangasianodon gigas) could also be considered the largest freshwater fish, as it spends its life exclusively in freshwater. The Mekong giant catfish can grow to 10 feet long and weight over 600 pounds, and as its name suggests, lives in the Mekong river.