Belle Isle was privately owned in 1879, as it had been since June 5, 1768, when Lieutenant George McDougall (with the blessing of King George III) purchased the island from local Ottowa and Ojibwa chiefs for, famously, five barrels of rum, three rolls of tobacco, three pounds of vermilion and a wampum belt. The McDougalls sold the island to the Macomb family (they also owned Grosse Ile! Own all the islands!) in 1793, and the Macomb family sold the island to Barnabas Campau in 1817. Descendants of Barney Campau lived and operated fisheries on the island until its sale to the city. But people continued to use the island more or less publicly, to picnic, wander, resort, and to fight duels.
Source : Amy Elliott Bragg, “Belle Isle and The Park Question: 1879-1881 (Part II)”, The Night Train Blog, February 15, 2013.