Koyun Island (Turkish: Koyun Adası, literally "Sheep Island") is a Turkish island in the Sea of Marmara. At 40°30′33″N 27°34′40″E / 40.50917°N 27.57778°E it is a part of Erdek ilçe (district) of Balıkesir Province. It is situated between the better known islands of Avşa Island and Paşalimanı Island.
The island is a long island from north to south. Its total area is about 1.7 square kilometres (0.66 sq mi). There are only a few summer houses on the otherwise uninhabited island.
Erdek Gulf is a gulf of Marmara Sea, Turkey. It is administratively a part of Balıkesir Province.In fact, the gulf is named after Erdek, an ilçe (district) of the Balıkesir Province which is situated at the north coast of the gulf .The midpoint of the gulf is at about 40°22′N 27°40′E / 40.367°N 27.667°E. Kapıdağ Peninsula, Paşalimanı Island and Avşa Island are to the north, Belkıs Tombolo (connecting Kapıdağ Peninsula to the Anatolia mainland) is to the east, Karabiga ilçe is to the west and the Marmara coast of Anatolia is to the south. The maximum depth of the gulf is 47 metres (154 ft).
There are ferry services in the gulf. One service is to Barbaros in the Thracen side of Turkey, close to Tekirdağ. There are also ferry services to the nearby islands such as Marmara Island (Saraylar terminal), Avşa Island and the Paşalimanı Island.
Paşalimanı Island (Turkish: Paşalimanı Adası), formerly Halone (Greek: Ἁλώνη), is an island in the southern Sea of Marmara in Turkey. The island is the fifth largest island of Turkey (26.2 km (10.1 sq mi)) and administratively belongs to the Erdek town of Balikesir Province in northwestern Turkey. The island has five small villages and has 962 population in total. Paşalimanı Island is also part of the Marmara Archipelago along with three other islands Marmara, Avşa and Ekinlik.