If it is the calorie count of regular mayo that bothers you, you could try Canola mayonnaise (Hellmann's makes it) which has 40 calories in 14g (I think that's a tablespoon). Light Hellmann's has 35 calories in the same amount. My own preference is the Hellman's Canola because I've been trimming saturated fat from my diet.
This is an option if you really crave tuna salad. I've noticed that if you use a lot less mayo than you normally use, but mix it very VERY well, you get the benefits of taste without the calories.