That area is the Middle Bass ice fishing hole. The ice gets locked in the "circle" of PIB, MB, Rattle and Green so normally people can fish late into the season.
A lot of the people on the upper islands have hovercrafts so they can still fish and get around while the ice is in the between stage of too thick to boat, not safe enough to 4-wheel on. Some times they ride around in a group...Hells Angels the island way. In the summer you see the dingy beach full of rafts and wave runners and then in the early spring you see it full of hovercrafts. Kind of a funny thing to see.