Most years I would say you would be too late by Mothers Day, this year there could still be fish here or there. Try near Grand Marais. Maybe get a coaster around the mouth of the Flute Reed (C&R, please).
Slinky fishing is more of a bait technique and really better suited to a spinning rod or center pin.
Rubber legs stone patterns and streamers that immatate smelt can get you a few fish in the lake by the river mouths.
Check in with Great Lakes Fly Shop in Duluth or the Superior Fly Angler in Superior for local patterns and good advise/ conversation.