Mackerel The blood and viscera of mackerel contain a substance called "histidine". If it is not handled properly, it will cause histidine in it to decompose and produce histamine, which will cause food poisoning. Generally, mackerel's blood, viscera and head contain a lot of histidine, so the above parts must be handled carefully before eating, so as not to cause food poisoning. And Spanish mackerel is a kind of freshwater fish, which generally does not cause poisoning.