Canned mackerel generally has mercury at levels near that of salmon.

King Mackerel is known to contain a lot of mercury, but that is a different species of Mackerel which is quite large (in general, larger predatory fish have greater concentrations of mercury).

As long as you are eating the tinned Mackerels, the kind which are small like sardines, I don't think you have significant mercury risk. Canned/tinned mackerel and sardines are considered some of the safest and most sustainable fish you can consume.

https://www.csu.edu/cerc/researchreports/documents/MercuryFa...