Yes seers have a high level of mercury
Thanks for choosing health care magic for posting your query.
I have gone through your question in detail and I can understand what you are going through.
Seer fish also popularly known as Meckerel have a high mercury level and should not be taken regularly. Its ok if you have fed your son this fish uptil now but now on you can switch to other options.
Infact a fish is said to be good if it has good amount of omega 3 fatty acids and its mercury contaminant level is below 216PPB mercury.
Following are certain healthy fish options which yiu may try according to your availability
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Wish you good health,
Dr. Srikanth Reddy M.D.