Not quite. Different mackerel are at different levels in the food chain, often in different geographical areas. The king mackerel is "at the top" of that genus. If you look here at the EDP advisory (Health Alerts - Seafood Selector - Environmental Defense Fund) you'll see that spanish & atlantic mackerel are at significantly different mercury levels than king. (Atlantic mackerel are at the lowest end of the "risk meter" for all mackerel.)The cans that we buy say "jack mackarel" so? But considering all mackarel are all from the same genus (very closely related) that they have a similar amount of mercury in them because of a similar diet. All the fish listed above are somewhat listed in the order or top of the food chain to bottom of the food chain. The ones on top eat the fishes towards the bottom.
Same story with Tuna: different sub-species, different mercury levels.