I'm 9 weeks pregnant and craving tuna like crazy. What do I do?

I know tuna has high levels of mercury and that's really bad for babies in gestation but I can't get the thought of a giant tuna and pickle sandwich with extra mayo out of my head. What is wrong with me? Do I only want it because I'm not supposed to have it? Is there some sort of tuna substitute I've never heard of? Is it okay to have just a little? Am I just supposed to deal with it? Any advice or suggestions are appreciated.