I saw your post and did a quick search to find where exactly were in SE Idaho Onieda was and the first link I hit I read this:
Oneida Reservoir:, (Franklin County), Walleye, perch and occasional smallmouth bass. Fair fishing for 12-14 inch walleye with occasional fish larger than 20 inches. Oneida Narrows Reservoir is used to generate electricity, it is open to the public for boating, fishing, swimming etc. There are 2 boat ramps, and camping available as well as a day use picnic area. Below the dam another 6 miles of Bear River provide great trout fishing in the beautiful Oneida Narrows.
I would assume the Walleye were introduced before the May release?