There are many places to go for fishing in Istanbul.
Here is a list of best places to go fishing according to the expert fisherman:
Asian Side
Moda Burnu (burun=nose, point) in Kadikoy
Semsipasa in Uskudar,
Pasalimani in Kuzguncuk, near Beylerbeyi Camii, near Kuleli Askeri Lisesi in Cengelkoy, Kandilli, Kanlica, Cubuklu, Anadolu Kavagi, Kalyon Koyu (koy=cove) and Kadirga Koyu in Agva
European Side
Galata Koprusu, Sarayburnu, Ahirkapi, Yenikoy, Arnavutkoy, Asiyan, Rumelihisari, Near Tarabya Oteli, Buyukdere, Rumelifeneri
Remember Tahtakale is always the best place where you can find anything to buy including fishing equipments.
