Keith Magnuson of Seattle was waterskiing on Lake Washington Saturday when he happened upon the massive fish. He and friends called state wildlife officials who said they plan to inspect the curious finding, which Magnuson said he left tied to a post near shore.
Magnuson told The Seattle Times he first thought the fish was a shark but they were later able to identify it as a sturgeon.
"It is not a common fish to find in the lake and rather unique," said Annette Hoffman, a state Fish and Wildlife biologist.
Hoffman said sturgeon are deep water bottom-feeders and usually difficult to spot.
Though not common in Lake Washington, it isn't the first time a massive sturgeon was found there. Hoffman said an 11-foot monster weighing 640 pounds was hauled out of the lake in 1987.