Slash the skin of the whole fish, if using, on each side with a sharp knife. Mix the ginger and garlic, season with salt, then rub all over the fish.
Mix the yogurt with the oil, spices and seasoning. Use to coat the fish inside and out, then chill until ready to cook.
TO BARBECUE: Cook straight on the rack (or on foil if you are afraid of it sticking) for 6-8 mins each side for the whole fish, or 3-4 mins for the tuna steaks. Cooking time will depend on how hot your barbecue is when you start.