Mackerel

Mackerel keeps 1–3 days refrigerated and 2–3 months frozen, and it is the most demanding fish on this site to store well. It is both extremely oily and a scombroid species, which means it fails fast in two independent ways.

Keep it on ice, not merely in the fridge. Traditional fishmongers pack mackerel in ice for a reason, and the difference between 32°F and 40°F is much larger for this fish than for cod.

Cook it the day you buy it if you possibly can. Mackerel is the classic example of a fish that is superb fresh and unpleasant a day or two later, and the decline is not subtle.

For freezing, wrap airtight and use it within 2–3 months. The oil that makes mackerel worth eating oxidizes readily even at 0°F.

Mackerel spoils quickly for two compounding reasons.

First, it is among the oiliest fish commonly sold — often over ten percent fat, much of it highly unsaturated omega-3s. Those fats react with oxygen readily, so mackerel develops rancid, painty off-flavors faster than any lean whitefish, in the refrigerator and in the freezer alike.

Second, and more seriously, mackerel is a scombroid fish with very high levels of the amino acid histidine. Held above 40°F, bacteria convert that histidine to histamine, and eating high-histamine fish causes scombroid poisoning — flushing, headache, rash, palpitations, and nausea beginning within minutes to an hour of eating.

Histamine is heat-stable. Cooking, smoking, canning, and freezing all leave it intact. It cannot be reliably smelled or tasted. This is why mackerel's handling is a matter of unbroken cold from catch to pan, and why a fish that was allowed to warm at any point cannot be made safe by chilling it afterwards.

Mackerel does at least look like what it is. Fresh fish is iridescent and stiff with clear, bulging eyes; aging mackerel goes dull, limp, and soft, with a sour rather than briny smell.