which fish has the least calories
It will depend of how much fish you eat in one sitting, but per serving pickled Herring has the least amount of calories in it. Go here to see a list of other fish: http://dietbites.com/CalorieIndexFish.html.
Cod is low in calories. So is tuna steak. For a list of foods with the least amount of calories, check out http://www.livestrong.com/article/82690-list-foods-least-amount-calories/.