Hamsi, or Black Sea anchovies, season is in full swing. Dusted in cornmeal and skillet-fried in a tiny bit of oil is a great preparation. "Hamsi kusu" are two fillets sandwiched and grilled. That's a good way to go as well. Hamsili pilav is also excellent. You'll see these fish everywhere, particularly in Black Sea restaurants.
If you are in the Beyoglu area around lunch time find Hayvore for a full flight of hamsi. It's near Galatasaray.
Pafuli was a big name in Black Sea cuisine but they've slipped in recent years.
I had an excellent meal of fried hamsi at Mohti, also in Beyoglu, recently. Beware, they allow smoking in there, which is great if you smoke. Down on the Golden Horn there are little hamsi fry shacks set up. Most are pretty good but they are battered in flour and deep-fried. Not my favorite preparation.
We were in Istanbul in late Sept/Oct and ate at some wonderful restaurants. Ciya Sofrasi is a gem- they make food from all regions and you point at what you want from the steam table. We also ate at Antiochia in Beyoglu and Balika Sabahattin (seafood) in Sultanmehmet. For more details on these restaurants go to my post in "Where are the best eats in Krakow, Vienna, Prague, Budapest and Istanbul". You will have a great eating experience.
Istanbul is one of those places where it's very easy to eat nothing but mediocre food, but with a little effort you will be richly rewarded.
For our last several trips I read through the following blog for hints and suggestions of where to go. It's well worth the effort. http://istanbuleats.com/
Our overall favorite restaurant in Istanbu is Ciya Sofrasi, which is on the Asian side of the Bosphorus and requires a ferry ride if you're staying in the Sultanahmet/Beyoglu side of the Bosphorus. But the trip itself is a lovely experience and the restaurant serves fabulous "modern Turkish" food.