Unfortunately all tuna has mercury contamination, some types more than others. The same is true of most other large fish. Depending on the type and quantity of tuna eaten and what other fish is normally eaten in a person's diet, two portions per week can exceed recommended limits.

There's good news in that wild (not farmed) salmon is relatively uncontaminated and so can be eaten without restriction. It's also available in canned form.

Here is a link that might be useful: Tuna Limits