This post made me wonder... I found a site that had sushi and sashimi calories (can't vouch for it's accuracy) and the interesting thing was that a piece of tuna SUSHI only had 10 calories more than a piece of tuna SASHIMI, although it also indicated that a piece of sashimi is usually 2x the fish than the sushi.
So for their tuna sushi, you were getting 20 calories from the fish and 30 for the rice, where as with the sashimi, you were getting 40 calories from the fish.
The eye opener is if you get the tuna ROLL (ie, the hand roll thats cut up into the circles). That has 184 calories for 6 large circles. So I think when you are sticking with plain fish, not the fancy sushi that has other stuff in it or is deep fried, etc, the danger comes from eating too many rolls, I guess due to all the rice.
Here is the site I was looking at. http://www.sushifaq.com/sushi-health/calories-in-sushi/