Fisherman Isle actually consists of 3 islands. The first one hosting the trading outpost and the shop, the second one hosting a decorative house and the third one hosting the fortress and the only trader located on the island.
It's located near Jolly Island, so it is usually one of the first islands to be conquered. The island is also almost completely surrounded by fish, as It's name implies.
A 'Potted Bonsai' is also able to be bought on the island.