Keith Magnuson now says he thinks the roughly 8-foot-long creature is a sturgeon.
The Seattle Times reports Magnuson found the fish Saturday.
The University of Washington School of Fisheries tied the fish up to a post and the Washington state Fish and Wildlife Department plans to send a biologist to look at it.
Fish and Wildlife biologist Annette Hoffman, in Mill Creek, says a sturgeon "is not a common fish to find in the lake."
She says one found in 1987 was 11 feet long and weighed 640 pounds.
Hoffman says the deep-water bottom-feeders commonly migrate up and down the Columbia River.
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