It's actually common in startup founders who sell their startups and then start eating expensive sushi all the time. My understanding is that several billionaires in silicon valley have mild Parkinson's-like tremors because of this.
The amount of mercury in the type of fatty tuna used for sushi is several times more mercury than in light canned tuna, which is normally what you'd use for a tuna melt or whatever.
Sushi grade tuna (unlike eel) isn't one of the types of fish you should never eat, but I definitely wouldn't eat it more than once a month.
Chunk "light" tuna on the other hand tends to come from smaller species and pretty much as a direct consequence has lower mercury levels.
Based on the numbers in https://en.wikipedia.org/wiki/Mercury_in_fish, canned light tuna and skipjack have 1/3 to 1/2 the mercury found in albacore and yellowfin tuna and 1/4 the levels found in bigeye. Notably, those 3 all run 2-10x the size of the smaller species used in "chunk light."