Tim,
Tuna is something I have not tried but it is on my list. I did find a recipe on smokingmeatforums.com.
http://www.smokingmeatforums.com/t/111829/smoked-tuna-lots-of-tuna-with-qview
Here is one using the bradley smoker.
https://www.youtube.com/watch?v=fJmL_Hk93f4
Here is a recipe that looks really good, I am going to see if our local fish store has any tuna. I think this is a recipe to try and then reverse sear them on the grill.
http://bbqguam.blogspot.com/2011/06/smoked-yellow-fin-tuna.html
Keep us posted on what you end up doing, I would love to hear how they turn out.
Thanks Greg