Fresh Tuna

Fresh tuna keeps 1–3 days refrigerated and only 2–3 months frozen, since it is a fatty fish. Of everything at the seafood counter, this is the one where cold storage discipline matters most — keep it on ice, in the coldest part of the fridge, and cook it sooner rather than later.

The reason is not ordinary spoilage. Tuna is a scombroid fish, and the specific risk it carries is driven directly by temperature abuse at any point in its history. Every hour it spends warm is cumulative and irreversible, including the hours before you bought it.

Buy tuna from a counter you trust, get it home cold, and refrigerate it immediately. If you are driving more than a few minutes, use a cooler bag.

Much of the tuna sold as fresh has been treated with carbon monoxide or "tasteless smoke" to fix its red color. This is legal in the US and does not make the fish unsafe, but it does disable your best visual freshness cue — treated tuna stays bright red long after it would otherwise have browned.

Tuna carries a hazard that most fish do not, and it is worth understanding because it breaks the usual rules about cooking making things safe.

Tuna, mackerel, mahi-mahi, and related species have unusually high levels of the amino acid histidine in their muscle. When these fish are held above about 40°F, bacteria convert histidine into histamine. Eating fish with high histamine causes scombroid poisoning — flushing, headache, palpitations, rash, and nausea within minutes to an hour, resembling a severe allergic reaction.

The critical facts: histamine is heat-stable. Cooking, canning, freezing, and smoking do not destroy it. And it produces no reliable smell or taste, though affected fish is sometimes described as peppery or metallic. There is no way to fix or detect fish that has already accumulated histamine, which is why the entire control strategy is keeping the fish cold from the moment it leaves the water.

This is why tuna's handling advice is stricter than its 1–3 day window alone would suggest, and why a fillet that was left warm at any stage is not rescued by refrigerating it afterwards.

Ordinary spoilage signs — sour smell, slimy surface, mushy flesh — still apply, but they are not the main risk.