The Food and Drug Administration advises pregnant women, women of childbearing age who may become pregnant, nursing mothers, and young children to avoid consumption of shark, king mackerel, tilefish and swordfish. These fish have much higher levels of mercury. None of these species are farm raised. Farm-raised fish tend to have a shorter lifespan, so mercury does not have the opportunity to accumulate in their systems. In addition, formulated feed ingredients used in aquaculture are regularly monitored to avoid possible contamination of feed with methyl mercury. You can read more here.