Eat fish which are on bottom of the food chain - sardines and sprat are low in organic mercury but high in omega3 fatty acids.

https://www.foodwatch.org/fileadmin/Themen/Quecksilber/Queck...

This says the tolerable weekly intake for methylmercury is 1.6 μg per kg body weight per week. For a 200 lb man that is 80 milligrams of shark meat (p95 1835 mg/kg) or 11 lbs of sprat (p95 0.029 mg/kg) or 2.9 lbs of sardines (p95 0.112 mg/kg).