Idaho wildlife officials say someone illegally stocked walleye in Lake Cascade in west-central Idaho.
The Idaho Department of Fish and Game in a news release Friday says a fish caught earlier this week was positively identified as a walleye on Wednesday.
Fish and Game says the walleye measured more than 19 inches (48 centimeters) long.
Wildlife officials say walleye threaten native species.
The state does offer walleye fishing at isolated reservoirs where the fish can't escape to other waters.
Citizens Against Poaching is offering a reward for information in the criminal case at Lake Cascade.