I love sushi and sashimi, and actually I like it without the rice and associated vegetables... What shimmering posted actually made me look in the Food Explorer with Fitday, and I was pretty surprised at what I found for their description of "Sushi, with vegetables, rolled in seaweed"...
Eating twelve pieces actually comes out to over 350 calories and 80+ grams of carbs. (Not really that low in calories, all things considered.) I couldn't find what just fish would rate, but I certainly hope that it would be a lot less than that.
Of course, 12 pieces of sushi are certainly better than 12 Chicken McNuggets any day!