I think that Tuna is a member of the Mackerel family. Years ago it was called "Horse Mackerel" but it was hard to sell. Then some genius named it Tuna and Charlie became a famous TV cartoon! Our local VFW used to have Mackerel Breakfasts once a month. It was the good old Salted Mackerel which you had to soak 3 times over 2 days to make it edible. It was great with eggs and home fries. If you didn't like it you could also get S.O.S. It was worth the mercurial risk!
Cordially,
Bob