From the Super Mario Wiki
Barnacle's Island is the home of Barnacle in Donkey Kong Country 3: Dixie Kong's Double Trouble!. A small island with little more than Barnacle's residence, Barnacle's island is located in the center of Lake Orangatanga in the Super Nintendo version of the game, and in Pacifica in the Game Boy Advance version.
When Dixie Kong and Kiddy Kong pay a visit to Barnacle's Island, the old Brothers Bear will inform them that he is looking for a Shell to add to his collection. By bringing Barnacle the shell that's being sold at Bazaar's General Store, he will give the Kongs his Banana Bird in exchange.