@Testy - Congrats on your pregnancy! I hope it goes well for you and your baby is born beautiful and healthy. You are wise to want to limit your mayo intake, and other fatty foods in general.
I have found that most recipes for tuna that call for mayo are just as delicious with out it. So, don't be afraid to leave it out, it won't compromise on taste.
Mashed up, hard boiled eggs can be a good substitute for mayo. Just eat them with caution, as they are high in cholesterol.
Hope this helps!