Boil potatoes in salted water until crisp-tender. Drain and combine with 3/4 cup olive oil and next 4 ingredients.
Rub fish with 1 tablespoon olive oil; sprinkle with salt and pepper. Cover one side of fish with grape leaves to resemble scales. Turn and cover the other side.
Place wrapped fish in a shallow roasting pan, and spoon potatoes and olives around fish.
Bake at 450° for 20 to 25 minutes or until fish is opaque and potatoes are lightly browned. (To check for doneness, make an incision in the center of the thickest part of the fish.) Serve with lemon halves.