Mackerel contains mercury, which is not cleared from but accumulated in the human body, causing various diseases. The Atlantic mackerel is the least dangerous in this regard, and you can eat it as much as you like.

Tuna contains lots of mercury, especially blackfin and bluefin tuna. Additionally, there’s precious little free-grown tuna in the stores because it’s nearly extinct. All the fish comes from farms where it’s fed with antibiotics and hormones.

Eels have lots of fat, and thus easily absorb industrial and farm waste in the water. American species have the highest level of such intoxication. European eels are also known for being contaminated with large amounts of mercury.